geoenvironmental enginnering

NPTEL – Civil – Geoenvironmental Engineering

Joint initiative of IITs and IISc – Funded by MHRD of 14

Module 1

FUNDAMENTALS OF GEOENVIRONMENTAL

ENGINEERING

A) Scope of geoenvironmental engineering

Any project that deals with the interrelationship among environment,

ground surface and subsurface (soil, rock and groundwater) falls under the

purview of geoenvironmental engineering (Fang and Daniels 2006). The scope is

vast and requires the knowledge of different branches of engineering and

science put together to solve the multi-disciplinary problems. A geoenvironmental

engineer should work in an open domain of knowledge and should be willing to

use any concepts of engineering and science to effectively solve the problem at

hand. The most challenging aspect is to identify the unconventional nature of the

problem, which may have its bearing on multiple factors. For example, an

underground pipe leakage may not be due to the faulty construction of the pipe

but caused due to the highly corrosive soil surrounding it. The reason for high

corrosiveness may be attributed to single or multiple manmade factors, which

need to be clearly identified for the holistic solution of the problem. The

conventional approach of assessing the material strength of the pipe alone will

not solve the problem at hand.

A lot of emphasis has been laid for achieving a “green environment”.

Despite a lot of effort, it is very difficult to cut off the harmful effects of pollutants

disposed off into the geoenvironment. The damage has already been done to the

subsurface and ground water resources, which is precious. An effective waste

containment system is one of the solutions to this problem. However, such a

project has different socio-economic and technical perspectives. The realization

of such projects require the contribution of environmentalist, remote sensing

experts, decision makers, common public during its planning stage, hydrologists,

geotechnical engineers for its execution stage and several experts for

management and monitoring of the project. The totality of the problem can be



visualized under the umbrella of geoenvironmental engineering. Therefore, the

real challenge for a geoenvironmental engineer is how well he can integrate the

multi-disciplinary knowledge for achieving an efficient waste containment.

As mentioned earlier, in most parts of the world, damage has already

been done to the geoenvironment and groundwater reserves due to

indiscriminate disposal of industrial and other hazardous wastes. Owing to the

excessive demand, it becomes important to remediate and revive the already

polluted geoenvironment and groundwater. A geoenvironmental engineer has a

great role to play for deciding the scheme of such remediation practice. A lot of

concepts from soil physics, soil chemistry, soil biology, multi-phase flow, material

science and mathematical modelling, need to be taken for planning and

execution of an efficient remediation strategy. Therefore, it is essential for the

geoenvironmental engineer to think out of the box, to an extent that the

knowledge can help him visualize the problem better and suggest efficient

solution. Else, the solution to such problems becomes a trial and error process or

rather, learn from mistakes and rectify. Since such projects are cost intensive

one cannot afford to take too much of chances.

Another important issue is the reuse and recycling of waste materials,

which reduces the burden on our environment manifold. A very good example is

exploring the possibility of mass utilization of fly ash for geotechnical

applications. However, while using waste materials for meaningful applications

there are issues such as short term and long term impact, which is a governing

factor for deciding its selection as a viable material. Although, short term

behavior can be assessed using planned laboratory evaluations it often becomes

difficult and complex for understanding the long term behavior. The scope of

geoenvironmental engineering is to simplify the process of understanding the

behavior and resort to reliable predictions and estimations. This would require a

thorough knowledge on material science and chemistry and the reaction it

undergoes with time. This is indeed a tough task, but needless to say, such

challenges make this subject quite interesting.



The frequent occurrence of landslides especially during rainy season has

drawn the attention of researchers and practicing engineers. The conventional

slope stability analysis is partially helpful in understanding the problem. A wider

perspective of the problem would be to include factors such as infiltration and

seepage of rain water through the slope. Such factors are going to add on to the

instability of slope. The scope and challenge for the geoenvironmental engineer

is to couple the geotechnical, geological and hydrologic concepts to explain

rainfall induced slope failure. Construction of flood protection works such as

embankments and levees also comes under the purview of geoenvironmental

engineering. Unless a thorough hydraulic study is conducted, any geotechnical

measures for flood protection would prove to be futile. This is specifically true for

large rivers and for meandering sections.

Geoenvironmental engineering is more research oriented and new

concepts and methodologies are still being developed. Therefore, this particular

course intends to introduce different avenues and overall scope of

geoenvironmental engineering to the reader. The course would highlight the

uncertainties and complexities involved and the wide research potential of the

subject. Special emphasis has been laid on the basics of soil-water interaction,

soil-water-contaminant interaction, which are essential for understanding the

impact of geoenvironmental contamination, its minimization and remediation.

B) Multiphase behavior of soil

Conventional or classical soil mechanics assumes soil media to be

completely water or air saturated. This is a typical example of a two phase media

consisting of soil solids and water/air. The assumption of two phases

considerably simplifies the mathematical quantification of the complex

phenomena that take place in porous media. Off late, geotechnical and

geoenvironmental engineering problems require the concept of three or

multiphase behaviour of soil for realistic solution of several field situations. For

example, a partially saturated soil is a three phase porous media consisting of

air, water and soil. The three phases result in transient and complex behaviour of



unsaturated soil. Such cases are encountered while designing waste

containment facility where flow characteristics of unsaturated soil need to be

determined. When it comes to soil-water-contaminant interaction there are multi-

phase interactions involved. The migration of non-aqueous phase liquid (denoted

as NAPL) through porous media is a typical example. Fluidized bed, debris flow,

slurry flow, gas permeation through unsaturated soil media are some problems

where multiphase behaviour becomes important. Such studies are handy while

designing remediation scheme for contaminated soil and groundwater, which are

very important issues for the geoenvironmental engineer to solve. Understanding

the complex interaction of different phases is challenging and has paved way for

the study of multiphase behaviour of porous media. Such a realization has

generated a lot of interest in the research fraternity for developing experimental

and mathematical procedures for clearly delineating the phenomena in

multiphase porous media.

C) Role of soil in geoenvironmental applications

All civil engineering structures are ultimately founded on soil and hence its

stability depends on the geotechnical properties of soil. Conventional

geotechnology is more concerned about rendering soil as an efficient load

bearing stratum and designing foundations that can transfer load efficiently to

subsurface. Apart from this, soil is directly related to a number of environmental

problems, where the approach should be a bit different. Consider the case of

groundwater recharge as shown in Fig. 1.1. The infiltration and permeation

property of homogenous or layered soil mass above water table decides the rate

of recharge. In this case, a geotechnical engineer has to work closely with

hydrogeologists for deciding different schemes of artificial groundwater recharge.



Fig. 1.1 Artificial groundwater recharge

Consider the case of waste dumped on ground surface. During

precipitation, water interacts with these wastes and flow out as leachate. When

the leachate flows down, soil act as buffer in retaining or delaying several harmful

contaminants from reaching groundwater. Such a buffering action obviously

depends on the texture and constituents of soil mass. While designing a waste

containment facility, the role of soil in such projects is enormous. A coarse

grained soil with filter property is required for leachate collection where as a fine

grained soil is required for minimizing flow of leachate. These are two entirely

different functions expected from soil in the same project. The cap provided for

waste dumps also necessitate the use of specific type of soils with the required

properties. The amount of water that infiltrates into the waste below is minimized

by soil used in such caps. Special type of high swelling soils is used as backfills

for storing high level radioactive waste in deep geological repositories. Another

important geoenvironmental problem, namely, carbon sequestration uses the

geological storage capacity for disposal of anthropogenic CO2 to mitigate the

global warming. Therefore, soil plays a very vital role in geoenvironmental

projects and the property by which it becomes important is problem-specific.

Precipitation

Artificial recharge

Aquifer

Groundwater

Bed rock



D) Importance of soil physics, soil chemistry,

hydrogeology and biological process

Soil physics is the study of the physical properties and physical processes

occurring in soil and its relation to agriculture, engineering and environment. It

deals with physical, physico-chemical and physico-biological relationship among

solid, liquid and gaseous phase of soil as they are affected by temperature,

pressure and other forms of energy. Hence, the knowledge of soil physics

becomes important for solving geoenvironmental problems. The concepts of soil

physics is used for determining the transport of water, solute and heat (matter

and energy) through porous media, which is important to solve the problems

related to subsurface hydrology, groundwater pollution, water retention

characteristics of soil, improving crop production, rainfall induced landslides etc.

Soil physics is mostly quantitative and mathematical in nature and requires the

knowledge of soil physical properties. The important soil physical properties

include soil texture which deals with the particle gradation; soil water which

include mechanisms such as retention, infiltration, run off, permeation,

evaporation, transpiration, irrigation scheduling etc; soil aeration to take into

account exchange of gases such as oxygen and carbondioxide by plant roots

and microorganisms present in the soil. While defining these physical properties

of soil, it is very important to consider representative elementary volume (REV)

which is required to describe or lump the physical properties at a geometrical

point (Scott 2000). REV therefore describes mean property of the volume under

consideration.

Soil chemistry is the study of chemical characteristics of the soil and is

one of the important information required for many of the geoenvironmental

problems. The emergence of discipline “soil chemistry” began when J. T. Way

(father of soil chemistry) realized that soil could retain cations such as NH4+, K+ in

exchange for equivalent amounts of Ca+2 (Thomas 1977). This means that soils

act as ion exchangers. This aspect is vital for using soil in waste management

application. The contaminants leaching out of the waste dumps find its way to



groundwater flowing past the soil porous media. The concentration of

contaminant at a distance away from the source for a given time is fully governed

by the chemical interaction of contaminant and the soil. There are several simple

and complex chemical reactions that may take place in soil-water system

depending upon the prevailing favourable condition. An example is the

phenomenon of solubility and precipitation as governed by the pH of the soil-

water-contaminant system. The knowledge of soil chemistry is important to

understand interactions between soil solids, precipitates and pore water,

including ion exchange, adsorption, weathering, buffering, soil colloidal

behaviour, acidic and basic soils, salinity etc. There is an interesting story which

resulted in the effects of soil acidity and alkalinity. The investigation on poor crop

productivity in eastern United States in early 1800’s lead to the understanding of

high soil acidity, which was regulated by the addition of lime. This resulted in high

yield of crops. Similarly the deleterious condition of soil due to high alkalinity was

realized and investigated in detail. After 1920’s the understanding on structural

soil chemistry and soil organic chemistry improved a lot. The acidity and

complexation potential of organic matter was appraised. A lot of chemists

researched on the structure and reactivity of water on soil mineral surface. These

and many other findings lead to the development of soil chemistry and today it is

one of the important branches of science required to explain several phenomena

in geoenvironmental engineering.

Understanding subsurface for geoenvironmental problems requires

extensive knowledge of hydrogeology. Hydrogeologic parameters influence a lot

on how a waste containment facility performs over its design life. Therefore, while

deciding the location for such facility it is important that the subsurface

hydrogeology condition is fully explored and studied. Different in-situ

methodologies are used for remediation of a contaminated site. For effective

functioning of such methods one has to study the hydrogeological aspects of the

site. Hydrogeologists play a vital role in locating groundwater aquifer, its

management and optimal extraction. Efficient watershed management by

artificial recharge is possible only if the hydrogeology of a particular area is



known. The knowledge of hydrogeology is also required for understanding the

direction of groundwater flow. This is often required for assessing the extent of

contamination occurring due to a particular source of pollution and for risk

assessment.

Off late a lot of emphasis is laid on biological processes occurring in soils.

Initially, agriculturists were more bothered about this subject. But the subject has

caught the attention of many researchers due to its potential in solving different

geoenvironmental problems. For example, some type of microorganisms such as

Pseudomonas aeruginosa is used for remediation of hydrocarbon contaminated

site. It is very essential to understand the rate of such reaction and the impact of

such remediation. A lot of researchers worldwide are working on this interesting

problem. Biological process in soils is dependent on temperature and climatic

condition of a place, which need to be studied in detail. The soil biological

process is found to influence the exchange of greenhouse gases between soil

and atmosphere and many other soil physical parameters such as water

retention characteristics.

E) Sources and type of ground contamination

Solid, liquid and gaseous waste forms contaminates subsurface and

groundwater due to indiscriminate disposal. Solid wastes come from municipal,

domestic and industrial sources. Municipal wastes amounts to around 50 percent

of the total wastes produced. Household, hospital, agricultural wastes forms part

of municipal wastes. Returning these wastes to soil is considered to be a low

cost option. Abandoned e-waste, batteries, vehicles, furniture, debris from

construction industry is considered as solid waste and is produced from both

urban and rural areas. Large scale industrial development produces huge

quantities of hazardous waste and the sources are iron and steel industries,

packaging factories, paints, dyes, chemicals, glass factories, fertilizer and

pesticide industries, mine excavation waste etc. Coal mining, radioactive fuel

mining, petroleum mining and thermal power plants generate hazardous solid

waste that requires effective management.



The main source and type of hazardous liquid waste include industrial

waste water contained in surface impoundments, lagoons or pits. It is also

produced from municipal solid refuse and sludge that are disposed on land. If not

handled properly sewage becomes an important source of liquid waste that has

undesirable effect on environment. Petroleum exploration leaves waste brine

solution which needs to be managed to prevent groundwater pollution. Liquid

waste emerges due to mining operation which is hazardous. A typical example is

acid mine drainage from dumped mine wastes.

Some of the gaseous waste includes NOx, CO, SO2, volatile

hydrocarbons etc. Chemical reaction may take place in air producing secondary

pollutants. SO2 combines with oxygen to produce SO3, which in turn combines

with suspended water droplets to produce H2SO4 and fall on ground as acid rain.

Natural breakdown of uranium in the geoenvironment emits cancer causing

radon gas into atmosphere.

F) Impact of contamination on geoenvironment

In most of the cases, wastes are disposed off indiscriminately in low-lying

areas without taking adequate engineering measures to effectively contain it.

This results in a highly unhygienic and unhealthy environment leading to

breeding of pests, mosquitoes and several harmful microorganisms. Many of the

emerging diseases found these days are direct impact of geoenvironmental

contamination due to wastes. During precipitation, or groundwater coming in

contact with these wastes generates contaminated water called leachate that can

travel far field and pollute the surface and groundwater resources. Many of the

harmful heavy metals can also travel along with the leachate if it is not contained

properly. Some of the solid waste such as excavation and mining waste, fly ash

(wet and dry) from thermal power plants requires large area of land for its storage

as wastes. This in turn would interact with rain water and can cause

contamination. Several harmful heavy metals well above the contamination limit

can enter the life cycle of organisms living in close proximity with such disposal

sites.



One of the complexities of contamination impact is its long term effects

without a chance for realization. Most of the impacts are realized much later from

rigorous studies, and by the time the damage would have been done. Hence,

remediation becomes a tedious and cost-intensive affair. This makes

geoenvironmental engineering a challenging and much needed subject. There is

a need to focus on research that would help to predict and minimize the long

term impact of indiscriminate and mismanaged waste contamination.

G) Case histories on geoenvironmental problems

Use of readily available local soil instead of expensive

commercial soil (like bentonite) for waste management

Engineered waste management scheme necessitates the construction of

highly impermeable barrier so that waste disposed on it does not find its way to

ground water resources. Mostly these barriers are made of high plastic clays

which are commercially available. This would considerably increase the cost of

such geoenvironmental projects. Exploring the possibility of using local soils for

such applications, therefore, becomes an important geoenvironmental problem.

Any success in this direction would add to the economy of the project. This in

turn would result in sustainable development of such very important project. The

following research paper is an excellent case history of finding solution to one of

the geoenvironmental problems.

Taha and Kabir (2005) have explored the possibility of using tropical

residual soil for waste containment, which is readily available over a considerable

part of peninsular Malaysia. Hydraulic conductivity is used as the criterion for

evaluation of soil suitability for the said application. The soil was compacted at

different water content and compaction effort and then permeated with de-aired

tap water. The results of hydraulic conductivity test indicates that the required

flow of less than 10-9 m/s can be achieved by using a broad range of water

content and compaction effort. The soil has minimum shrinkage potential and

adequate strength to support the load of waste overburden. These properties



discussed would fall under the purview of geotechnical engineering. But the

evaluation of soil suitability is not complete without understanding its chemical

reactivity. In this study, cation exchange capacity (CEC) of soil is used as an

indicator of chemical reactivity. It is desirable that the pollutants released from

the waste disposal site should be effectively attenuated by the liners. This means

that the soil should have high chemical reactivity. A soil with high CEC indicates

high reactivity and hence high attenuation capacity of pollutants.

Bioremediation of oil spills:

The case history is discussed in U. S. Congress, Office of Technology

Assessment, Bioremediation for Marine Oil Spills report. It essentially deals with

a marine oil spill that has occurred on the beaches of Alaska, USA, in late 80s.

The reason was due to the grounding of a ship on the shores. Office of

Technology Assessment (OTA), USA, felt the need of technologies to fight such

calamities. A comprehensive review of the methods for oil spill clean up was

conducted to develop an environmental friendly solution. One of the effective

solutions that came up was bioremediation in which specific species of

microorganisms were used to degrade oil. This is a slow natural process and

hence the major focus was on accelerating and improving the efficiency of this

natural process. Even though, some research has been initiated, it was found

that there is a dearth of data and hence the advantage of bioremediation over

other methods of oil spill clean up is yet to be ascertained. It has been opined

that in case of emergency situation, mechanical process such as using

dispersants and in-situ burning may still be appropriate.

Protecting environment from harmful effects of mine waste

using cover system

O’Kane and Wels (2003) have discussed the performance based design

of covers for mine wastes dumped on ground. The objective of the cover system

is to control harmful contaminant release from the waste dumps, chemical

stabilization of acid forming mine waste, dust and erosion control and provide

growth medium for sustainable vegetation cover. The proposed methodology of

cover design links predicted performance of cover system to the groundwater



and surface water impacts. This method is impact oriented performance criteria.

In this method, a conceptual cover is selected first based on the type of waste,

size and geometry of the waste disposal, climate etc. A detailed cover design

analysis is performed that correlates cover design parameters (for example cover

thickness) to cover performance (net percolation). Third step links cover design

parameters to environmental impact assessment (groundwater quality). Fourth

step is to assess the risk based on the result from third step and the regulatory

law. If unacceptable, then cover design is modified. If acceptable then field trial

with performance monitoring is suggested. The feedback loop between impact

assessment and cover design is crucial for developing efficient cover system

without being overly conservative.

Value addition of waste products: Geopolymers from fly ash

Andini et al. (2008) have discussed about the value addition of fly ash by

converting it to a product called geopolymers. Davidovits first introduced the term

geopolymers for a new class of three dimensional alumino-silicate materials

(Davidovits 1989). Geopolymers are alkali-activated alumino-silicate binders and

its synthesis takes place by polycondensation from a variety of raw materials

such as metakaolin, coal fly ash etc. Polycondenstation reaction was carried out

by mixing fly ash with alkali metal silicate solution and then curing at different

temperature and time. Amorphous geopolymers are obtained at condensation

temperature ranging from 20 to 90 °C. The geopolymers has excellent

mechanical properties, thermal stability, acid resistance and are durable. It has

got a wide application in ceramics, cements, hazardous waste stabilization, fire

resistant materials etc. Environmentally sound recycling of fly ash into

geopolymers by hydro-thermal treatment is an excellent example of value

addition to the waste material.



References

1. Andini, S., Cioffi, R., Colangelo, F., Grieco, T., Montangnaro, F. and Santoro,

L. (2008) “Coal fly ash as raw material for the manufacture of geopolymer-

based products”, Waste management, Vol. 28, pp. 416-423.

2. Davidovits, J. (1989) “Geopolymers and geopolymeric materials”, Journal of

Thermal Analysis, Vol. 35, pp. 429-441.

3. Fang, H-Y. and Daniels, J. L. (2006) “Introductory geotechnical engineering-

An environmental perspective”, Taylor and Francis, London.

4. O’Kane, M. and Wels, C. (2003) “Mine waste cover system design - linking

predicted performance to groundwater and surface water impacts”, Sixth

International Conference, Acid, Rock, Drainage, Cairns, Queensland, Carlton

South: AUSIMM.

5. Scott, H. D. (2000) "Soil physics: agricultural and environmental applications”,

Iowa State /university Press, USA.

6. Taha M. R. and M. H. Kabir (2005) “Tropical residual soil as compacted soil

liners”, Environmental Geology, Vol. 47, pp. 375-381.

7. Thomas, G. W. (1977) “Historical developments in soil chemistry: Ion

exchange”, Soil Science Society of America Journal, Vol. 41, pp. 230-238.

8. U. S. Congress, Office of Technology Assessment, Bioremediation for Marine

Oil Spills-Background Paper, OTA-BP-O-70 (Washington, DC: U.S.

Government Printing Office, May 1991).



Model Questions

1) Explain the importance and scope of geoenvironmental engineering. 2) With examples, discuss the multiphase behavior of soil. 3) Why soil becomes important in geoenvironmental engineering? 4) Discuss the multidisciplinary nature of geoenvironmental engineering.


Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 60

Module 2

SOIL-WATER-CONTAMINANT INTERACTION

Knowledge of soil-water interaction and soil-water-contaminant interaction is very

important for solving several problems encountered in geoenvironmental

engineering projects. The following section introduces soil mineralogy and

various mechanisms governing soil-water-contaminant interaction.

2.1 Soil mineralogy characterization and its significance

in determining soil behaviour

Soil is formed by the process of weathering of rocks which has great variability in

its chemical composition. Therefore, it is expected that soil properties are also

bound to the chemical variability of its constituents. Soil contains almost all type

of elements, the most important being oxygen, silicon, hydrogen, aluminium,

calcium, sodium, potassium, magnesium and carbon (99 percent of solid mass of

soil). Atoms of these elements form different crystalline arrangement to yield the

common minerals with which soil is made up of. Soil in general is made up of

minerals (solids), liquid (water containing dissolved solids and gases), organic

compounds (soluble and immiscible), and gases (air or other gases). This section

deals with the formation of soil minerals, its characterization and its significance

in determining soil behaviour.

2.1.1 Formation of soil minerals

Based on their origin, minerals are classified into two classes: primary and

secondary minerals (Berkowitz et al. 2008). Primary minerals are those which are

not altered chemically since the time of formation and deposition. This group

includes quartz (SiO2), feldspar ((Na,K)AlSi3O8 alumino silicates containing

varying amounts of sodium, potassium), micas (muscovite, chlorite), amphibole

(horneblende: magnesium iron silicates) etc. Secondary minerals are formed by



the decomposition and chemical alteration of primary minerals. Some of these

minerals include kaolinite, smectite, vermiculite, gibbsite, calcite, gypsum etc.

These secondary minerals are mostly layered alumino-silicates, which are made

up of silicon/oxygen tetrahedral sheets and aluminium/oxygen octahedral sheets.

Primary minerals are non-clay minerals with low surface area (silica minerals)

and with low reactivity (Berkowitz et al. 2008). These minerals mainly affect the

physical transport of liquid and vapours (Berkowitz et al. 2008). Secondary

minerals are clay minerals with high surface area and high reactivity that affect

the chemical transport of liquid and vapours (Low 1961).

Silica minerals are classified as tectosilicates formed by SiO4 units in frame like

structure. Quartz, which is one of the most abundant minerals comprises up to

95percent of sand fraction and consists of silica minerals. The amount of silica

mineral is dependent upon parent material and degree of weathering. Quartz is

rounded or angular due to physical attrition. The dense packing of crystal

structure and high activation energy required to alter Si-O-Si bond induce very

high stability of quartz. Therefore, the uncertainty associated with these materials

is minimal. In the subsurface, quartz is present in chemically precipitated forms

associated with carbonates or carbonate-cemented sandstones.

Clay minerals, which can be visualized as natural nanomaterials are of great

importance to geotechnical and geoenvironmental engineers due to the more

complex behaviour it exhibits. Therefore, this chapter emphasise more on

understanding clay mineral formation and its important characteristics. Basic

units of clay minerals include silica tetrahedral unit and octahedral unit depicted

in Fig. 2.1.



Fig. 2.1 Basic units of clay minerals (modified from Mitchell and Soga 2005)

It can be noted from the figure that metallic positive ion is surrounded by non-

metallic outer ions. Fig. 2.2 shows the formation of basic layer from basic units

indicated in Fig. 2.1. There are 3 layers formed such as (a) silicate layer, (b)

gibbsite layer and (c) brucite layer.

Aluminium, Iron or

Magnesium

Oxygen

Oxygen

Oxygen

Silicon

Oxygen

Silica

tetrahedron

Hydrox

yl

Oxygen

Aluminium

octahedron

(Si4O10) -4

(a) Silicate layer

S

Symbol

Al4(OH)12

G

Symbol

(b) Gibbsite layer



Fig. 2.2 Basic layer of mineral formation (modified from Mitchell and Soga 2005)

Gibbsite layer is otherwise termed as dioctahedral structure in which two-third of

central portion is occupied by Al+3. Similarly, brucite layer is termed as

trioctahedral structure in which entire central portion is occupied by Mg+2. These

basic layers stack together to form basic clay mineral structure. Accordingly,

there is two and three layer configuration as indicated in Fig. 2.3. More than

hundreds these fundamental layers join together to form a single clay mineral.

Fig. 2.3 Fundamental layers of clay minerals (modified from Mitchell and Soga 2005)

Description on common clay minerals

Some of the important and common clay minerals are described below in

Table 2.1.

Mg6(OH)12

B

Symbol

(c) Brucite layer

G

S

B

S

G

S

S

B

S

S

Two layer Three layer



Table 2.1 Summary of important clay minerals

Mineral Origin Symbol

Bond Shape Remark

Kaolinite Orthoclase Feldspar (Granitic rocks)

Strong hydrogen

bond

Flaky and platy

Approximately 100 layers in a

regular structure d =7.2A0

Halloysite (Kaolinite

group)

Feldspar Tropical

soil

Less strong bond

Tubular or rod

like structur

e

At 600C it looses water and alter soil

properties

Illite Degradation of mica

under marine

condition Feldspar

K+ provides bond

between adjacent

layers

Thin and

small flaky

material

Bond is weaker than

kaolinite d =10A0

High stability

Montmorillonite

(Smectite group)

Weathering of

plagioclase

H2O molecules

pushes apart mineral

structure causing swelling

Presence of

cations

Very small

platy or flaky

particle

Exhibits high shrinkage and

swelling Weak bond

d >10A0

Vermiculite

Weathering of biotite

and chlorite

Presence of H2O and

Mg+2 predominantl

y Mg+2

Platy or flaky

particle

Shrinkage and swelling less

than montmorillonit

e

G S

G S

G S

G S

H2O

G S

S

G S

S

K+ K

+

d

G S

S

G S

S

H2O H2O

B S

S

B S

S

H2O Mg+2



Kaolinite formation is favoured when there is abundance of alumina and silica is

scarce. The favourable condition for kaolinite formation is low electrolyte content,

low pH and removal of ions that flocculate silica (such as Mg, Ca and Fe by

leaching). Therefore, there is higher probability of kaolinite formation is those

regions with heavy rainfall that facilitate leaching of above cations. Similarly

halloysite is formed by the leaching of feldspar by H2SO4 produce by the

oxidation of pyrite. Halloysite formations are favoured in high-rain volcanic areas.

Smectite group of mineral formation are favoured by high silica availability, high

pH, high electrolyte content, presence of more Mg+2 and Ca+2 than Na+ and K+.

The formation is supported by less rainfall and leaching and where evaporation is

high (such as in arid regions). For illite formation, potassium is essential in

addition to the favourable conditions of smectite.

2.1.2 Important properties of clay minerals

Some of the important properties that influence the behaviour of clay minerals

are presented below:

Specific surface area

Specific surface area (SSA) is defined as the surface area of soil particles per

unit mass (or volume) of dry soil. Its unit is in m2/g or m2/m3. Clay minerals are

characterized by high specific surface area (SSA) as listed in Table 2.2. High

specific surface area is associated with high soil-water-contaminant interaction,

which indicates high reactivity. The reactivity increases in the order Kaolinite <

Illite < Montmorillonite. For the purpose of comparison, SSA of silt and sand has

also been added in the table. There is a broad range of SSA values of soils, the

maximum being for montmorillonite and minimum for sand. As particle size

increases SSA decreases.



Table 2.2 Typical values of SSA for soils (modified from Mitchell and Soga 2005)

Soil SSA (m2/g)

Kaolinite 10-30

Illite 50-100

Montmorillonite 200-800

Vermiculite 20-400

Silt 0.04-1

Sand 0.001-0.04

For smectite type minerals such as montmorillonite, the primary external surface

area amounts to 50 to 120 m2/g. SSA inclusive of both primary and secondary

surface area, (interlayer surface area exposed due to expanding lattice), and

termed as total surface area would be close to 800 m2/g. For kaolinite type

minerals there is possibility of external surface area where in the interlayer

surface area does not contribute much. There are different methods available for

determination of external or total specific surface area of soils (Cerato and

Lutenegger 2002, Arnepalli et al. 2008).

Plasticity and cohesion

Clay attracts dipolar water towards its surface by adsorption. This induces

plasticity in clay. Therefore, plasticity increases with SSA. Water in clays exhibits

negative pressure due to which two particles are held close to each other. Due to

this, apparent cohesion is developed in clays.

Surface charge and adsorption

Clay surface is charged due to following reasons:

Isomorphous substitution (Mitchell and Soga 2005): During the formation of

mineral, the normally found cation is replaced by another due to its abundant

availability. For example, when Al+3 replace Si+4 there is a shortage of one

positive charge, which appears as negative charge on clay surface. Such

substitution is therefore the major reason for net negative charge on clay particle

surface.



O-2 and OH- functional groups at edges and basal surface also induce negative

charge.

Dissociation of hydroxyl ions or broken bonds at the edges is also responsible for

unsatisfied negative or positive charge. Positive charge can occur on the edges

of kaolinite plates due to acceptance of H+ in the acid pH range (Berkowitz et al.

2008). It can be negatively charged under high pH environment.

Absence of cations from the crystal lattice also contributes to charge formation.

In general, clay particle surface are negatively charged and its edges are

positively charged.

Due to the surface charge, it would adsorb or attract cations (+ve charged) and

dipolar molecules like water towards it. As a result, a layer of adsorbed water

exists adjacent to clay surface, the details of which are presented in section

2.2.1.

Exchangeable cations and cation exchange capacity

Due to negative charge, clay surface attracts cations towards it to make

the charge neutral. These cations can be replaced by easily available ions

present in the pore solution, and are termed as exchangeable ions. The total

quantity of exchangeable cations is termed as cation exchange capacity,

expressed in milliequivalents per 100 g of dry clay. Cation exchange capacity

(CEC) is defined as the unbalanced negative charge existing on the clay surface.

Kaolinite exhibits very low cation exchange capacity (CEC) as compared to

montmorillonite. Determination of CEC is done after removing all excess soluble

salts from the soil. The adsorbed cations are then replaced by a known cation

species and the quantity of known cation required to saturate the exchange sites

is determined analytically.

Flocculation and dispersion

When two clay particles come closer to each other it experiences (a)

interparticle attraction due to weak van-der-Waal‟s force (b) repulsion due to –ve

+ +

Typical charged clay

surface



charge. When particles are sufficiently close, attraction becomes dominant active

force and hence there is an edge to face configuration for clay particles as shown

in Fig. 2.4(a). Such a configuration is termed as flocculant structure. When the

separation between clay particles increase, repulsion becomes predominant and

hence the clay particles follows face to face configuration called dispersed

structure (Fig. 2.4b).

Fig. 2.4 Different arrangement of clay particle

A lot of micro and macro level behaviour of clays are associated with these

arrangement of clay particles (Mitchell and Soga 2005).

Swelling and shrinkage

Some clay minerals when exposed to moisture are subjected to excessive

swelling and during drying undergo excessive shrinkage. A lot of engineering

properties of soil is affected by this behaviour and the stability of structures

founded on such soils become detrimental. The swelling of clay minerals

decreases in the order montmorillonite > illite > kaolinite.

2.1.3 Minerals other than silica and clay

Other than silica and clay, subsurface contains a variety of minerals such

as oxides and carbonates that governs the reactivity of soil and its interaction

with the environment. Some of the abundant metal oxide minerals present are

iron oxides (hematite, magnetite, goethite etc.) and aluminium oxides (gibbsite,

boehmite). Other oxide minerals (such as manganese oxides, titanium oxides)

are far less than Fe and Al oxides, but because of small size and large surface

area, they would affect very significantly the geochemical properties of

subsurface. These oxides are mostly present in residual soils of tropical regions.

Other major components include soluble calcium carbonate and calcium

+ + +

+

+

+ (a) Flocculant

+ + + + + + + + +

+ + +

(b) Dispersed



sulphate, which has relatively high surface area. In most soils, quartz is the most

abundant mineral, with small amount of feldspar and mica present. Carbonate

minerals such as calcite and dolomite are found in some soils in the form of bulky

particles, precipitates etc. Sulphate minerals mainly gypsum are found in

semiarid and arid regions.

2.1.4 Soil mineralogy characterization

One of the very well established methods for mineralogy characterization

of fine-grained soils is by using X-ray diffraction (XRD) analysis. Majority of the

soil minerals are crystalline in nature and their structure is defined by a unique

geometry. XRD identifies minerals based on this unique crystal structure. In

XRD, characteristic X-rays of particular wave length are passed through a

crystallographic specimen. When X-ray interacts with crystalline specimen it

gives a particular diffraction pattern, which is unique for a mineral with a

particular crystal structure. The diffraction pattern of the soil specimen (according

to its crystal structure), which is based on powder diffraction or polycrystalline

diffraction, is then analyzed for the qualitative and quantitative (not always)

assessment of minerals. Sample preparation method for XRD should be done

with great care as the XRD reaches only a small layer (nearly 50 µm) from the

surface of the sample. Hence, homogeneity is very important. Soil sample is

initially dried and sieved through 2 mm sieve. Sieved sample is homogenized in a

tumbler mixer for 30 min. A control mix of 30 g was taken and ground in lots of 15

g in a gyratory pulverizer. 15 percent by weight of KIO4 (internal standard) was

added to 5 g of specimen and again homogenized in a mixer. The prepared

specimen is then subjected to analysis. .

X-ray wave of monochromatic radiation (Kα) is commonly obtained from copper

radiation, which is commonly known as Cu- Kα. A typical XRD output is

represented by Fig. 2.5. It can be noted from the figure that ordinate represent

relative intensity of X-ray diffraction and abscissa represents twice of angle at

which a striking X-ray beam of wave length λ makes with parallel atomic planes.

Based on this diffraction pattern, the minerals can be identified by matching the



peak with the data provided by International Centre Diffraction Data (ICDD)

formerly known as Joint Committee on Powder Diffraction Standards (JCPDS).

100

200

300

400

10 20 30 40 50 60 70 80 90 100

0

250

500

750

1000

Q

A

Mo

K

Cu-K (2 Deg.)

Rela

tive Inte

nsity

A=Anorthite I=Illite K=Kaolinite Mo=Montmorillonite Q= Quartz

CS

KKK

KI

K

K

I

WC

Fig. 2.5 A typical XRD pattern with mineral identification for two different soils (modified from Sreedeep 2006)

It is understood that the area under the peak of diffraction pattern gives the

quantity of each phase present in the specimen. However, quantitative

determination of mineral composition in soils based on simple comparison of

diffraction peak height under peak is complex and uncertain because of different

factors such as mineral crystallinity, hydration, surface texture of the specimen,

sample preparation, non-homogeneity of soil samples, particle orientation etc.

The method of quantification will be more precise for those soils with less number

of minerals. Al-Rawas et al. (2001) have discussed about constant mineral

standard method and constant clay method for quantification of clay minerals. In

the first method, increasing quantity of clay are added to the fixed mass of known

standard and the difference in X-ray diffraction intensity when the specimen

changes from 100 percent standard to 100 percent clay is noted. The peak area

ratio for each component is then plotted against percentage of clay, based on



which regression equation is determined. This regression equation is further

used for mineral quantification. In the second method, known weight of pure

standard mineral is added to clay containing the same components, and the

change in the reflection peak-area intensity of each component is measured to

estimate the weight proportion of that component.

The fundamental discussion on the theory of XRD is quite extensive and cannot

be dealt in this course. Interested readers can go through literature available on

XRD in detail (Whittig and Allardice 1986; Moore and Reynolds 1997; Chapuis

and Pouliot 1996; Manhaes et al. 2002).

2.1.5 Applications of soil mineral analysis in geoenvironmental

engineering

As explained above, the soil-water and soil-water-contaminant interaction

and hence reactivity is greatly influenced by the mineralogy.

Chapuis and Pouliot (1996) have demonstrated the use of XRD for determining

bentonite content in soil-bentonite liners employed in waste containment.

Predicting global hydraulic performance of liner is very difficult with small scale

permeability test conducted in the field. There are no methods available for the

prediction of global permeability from small scale permeability test. For this

purpose, the XRD quantified bentonite content is used for understanding the

global hydraulic performance of liners. The soil used in this study was subjected

to heating at 550 °C in order to reduce its tendency for hydration, there by

eliminating the possibility of variation in diffraction intensity due to difference in

hydration. An internal standard was used for controlling X-ray absorption and has

been added to all specimens in equal quantity. In this study, authors also indicate

the usefulness of using XRD for knowing the quality and constancy of bentonite

supplied for the project.

When there are problems associated with expansive soils, the best

method for identifying the problem is by conducting XRD and checking for

expansive clay minerals. Bain and Griffen (2002) highlights that acidification of

soil can be understood by understanding the transformation of minerals. This is



mainly due to the fact that micas get transformed to vermiculite by weathering

process under acidic condition. Velde and Peck (2002) have shown that crops

can affect the clay mineralogy of the soils on which they are grown over periods

of time. The influence of fertilizer addition on cropping can be studied by

analyzing transformation of soil mineral in the field where the cropping has been

done. By analyzing mineralogy, the land use practices can be assessed.

2.2 Soil-water-contaminant interaction

Under normal conditions, water molecules are strongly adsorbed on soil

particle surface. Unbalanced force fields are generated at the interface of soil-

water, which increases soil-water interaction. When particles are finer, magnitude

of these forces are larger than weight of these particles. This is mainly attributed

to low weight and high surface area of fine particles. Before discussing the

concepts of soil-water interaction, a brief discussion is given on forces between

soil solids.

Forces between soil solids

There are essentially two type of bonding: (1) Electrostatic or primary valence

bond and (2) Secondary valence bond. Atoms bonding to atoms forming

molecules are termed as primary valence bond. These are intra-molecular

bonds. When atoms in one molecule bond to atoms in another molecule

(intermolecular bond), secondary valence bonds are formed. What is more

important in terms of soil solids is the secondary valence bonds. van der Waals

force and hydrogen bonds are the two important secondary valence forces.

Secondary valence force existing between molecules is attributed to electrical

moments in the individual molecules. When the centre of action of positive

charge coincide with negative charge, there is no dipole or electric moment for

the system and is termed as non-polar. However, for a neutral molecule there

can be cases where the centre of action of positive and negative charge does not

coincide, resulting in an electric or dipole moment. The system is then termed as

polar. For example, water is dipole. Also, unsymmetrical distribution of electrons

in silicate crystals makes it polar. Non-polar molecules can become polar when



placed in an electric field due to slight displacement of electrons and nuclei. This

is induced effect and the extent to which this effect occurs in molecule

determines its polarisability.

van der Waals force is the force of attraction between all atoms and

molecules of matter. This force comes into effect when the particles are

sufficiently close to each other. Hydrogen bond is formed when a hydrogen atom

is strongly attracted by two other atoms, for example: water molecules. This bond

is stronger than Van der Waals force of attraction and cannot be broken under

stresses that are normally experienced in soil mechanics. These secondary

valence bonds play a vital role in understanding soil-water interactions.

Essentially, the forces in soil mechanics may be grouped as gravitational forces

and surface forces. From classical soil mechanics perspective, gravitational

forces which are proportional to mass are more important. However, in

geoenvironmental engineering surface forces are important. Surface forces are

classified as attractive and repulsive forces. Attractive forces include (a) Van der

Waals London forces (b) hydrogen bond (c) cation linkage (d) dipole cation

linkage (e) water dipole linkage and (f) ionic bond. Van der Waals London force

is the most important in soils and becomes active when soil particles are

sufficiently close to each other. For example, fine soil particles adhere to each

other when dry. Cation linkage acts between two negatively charged particles as

in the case of illite mineral structure. Other types of forces are less important and

will not be explained in this section. Repulsive forces include like charge particle

repulsion and cation-cation repulsion.

2.2.1 Soil-water interaction

Water present in pore spaces of soil is termed as soil water or pore water.

The quantity of water present in the pores will significantly influence its physical,

chemical and engineering properties. It can be classified as (a) free water or

gravitational water and (b) held water or environmental water. As the name

suggests, free water flows freely under gravity under some hydraulic gradient

and are free from the surface forces exerted by the soil particle. This water can



be removed easily from the soil. Environmental water is held under the influence

of surface forces such as electrochemical forces or other physical forces. Both

type of water are important in geoenvironmental engineering. There are many

cases like seepage and infiltration problems whose solution necessitates the

knowledge of free water. However, these concepts are discussed in detail in

classical soil mechanics text books. At the same time, there are several

phenomena, which will be discussed in detail in this course, where the

understanding of held water becomes essential. The mechanism of soil-held

water interaction is complex and influenced by soil type, mineralogy, current and

past environmental conditions, stress history etc.

Held water can be further subdivided into structural water, adsorbed water and

capillary water. Structural water is present within the crystal structure of mineral.

This water is not very important as far as engineering property of soil is

concerned. For finding solution to several problems in geoenvironmental

engineering, it is essential to understand in detail adsorbed water and capillary

water.

Adsorbed water

Adsorbed water is strongly attracted to soil mineral surfaces especially

clays. Dry soil mass can adsorb water from atmosphere even at low relative

humidity and it is known by the name hygroscopic water content. For the same

soil, hygroscopic water content will vary depending on relative humidity and

temperature. Adsorptive forces between soil and water is polar bond and

depends on specific surface area of soil. Adsorbed water or bound water

behaves differently from the normal pore water. It is immobile to normal

hydrodynamic forces and its density, freezing point etc. are different from free

water.

Possible mechanisms for water adsorption (Low 1961)

a) Hydrogen bond and dipole attraction: Soil minerals are essentially made up of

oxygen or hydroxyls, facilitating easy formation of hydrogen bonds. Surface



oxygen can attract positive corner of water molecules (H+) and H+ present in OH-

can attract negative corner (O-2) of water molecules as depicted in Fig. 2.6.

Fig. 2.6 Water adsorption by hydrogen bond in soil minerals

b) Hydration of cations: Every charged soil surface has affinity towards ions,

specifically cations. These cations get hydrated by water dipole due to the

formation of hydrogen bond as shown in Fig. 2.7. Therefore, cations present in

the soil would contribute to the adsorbed water. In dry clays, these cations

occupy in the porous space of clay mineral. During hydration, these cations

engulfs with water molecules and move towards centre space between two clay

particles. The discussion on hydration of cations is very vast and its significance

will be dealt in detail, after this section.

Fig. 2.7 Water adsorption by ion hydration

Clay

surfac

e

Water

dipole

+ - +

+

+

+

+

+ -

+ -

+ - +

-

+ - + -

+ -

+ -

+ -

Cations

H+

present in

outer OH-

of soil

mineral

Oxygen

of water Surface

oxygen of

soil

mineral

H+ of water



b) Osmosis: Concentration of cations increases with proximity to clay surface.

The relatively high concentration would induce osmotic flow of water to neutralize

the high concentration of cations. Such an osmotic phenomenon is true in the

case of clays which act as semi-permeable membranes (Fritz and Marine 1983).

c) Attraction by Van der Waals-London forces causes attraction of water

molecules towards clay surface.

d) Capillary condensation: A range of pore size is possible in soils due to the

different particle size distribution and packing density. For saturation less than

100 percent, water and water vapour can get retained in soil pores by capillary

forces and attraction to particle surfaces.

Properties of adsorbed water

Several studies have been conducted to understand structural, chemical,

thermodynamic and mechanical properties of soil water by using different

techniques such as X-ray diffraction, density measurements, dielectric

measurement, nuclear magnetic resonance etc.

Density: At low water content, less than that needed to form three layers on clay

surfaces, the density of adsorbed water is greater than that for normal water. For

higher water content the density variation with reference to free water is less.

Viscosity: There is not much difference in viscosity between adsorbed and free

water. This is a very important observation relative to analysis of seepage,

consolidation etc. for unsaturated soils.

Dielectric constant: Dielectric property of a material depends on the ease with

which the molecules in the material can be polarized. It is observed that dielectric

constant of adsorbed water is less (50) as against 80 for free water.

Freezing of adsorbed water: Adsorbed water exhibit freezing point depression as

compared to free water. This is mostly attributed to the less molecular order of

adsorbed water as compared to free water.

Other properties: Energy is released when water is adsorbed by clay surface.

There is a time-dependent increase in moisture tension of water after mechanical

disturbance of at-rest structure of clay-water. The thermodynamic, hydrodynamic



and spectroscopic properties of adsorbed water vary exponentially with distance

from particle surface. The surface interaction effect is evident to a distance of 10

nm from the surface. This corresponds to around 800 percent water content in

smectite and 15 percent in kaolinite (Mitchell and Soga 2005).

Diffused double layer (DDL)

Diffused double layer (DDL) is the result of clay-water-electrolyte

interaction. Cations are held strongly on the negatively charged surface of dry

fine-grained soil or clays. These cations are termed as adsorbed cations. Those

cations in excess of those needed to neutralize electronegativity of clay particles

and associated anions are present as salt precipitates. When dry clays come in

contact with water, the precipitates can go into solution. The adsorbed cations

would try to diffuse away from the clay surface and tries to equalize the

concentration throughout pore water. However, this movement of adsorbed

cations are restricted or rather minimized by the negative surface charge of

clays. The diffusion tendency of adsorbed cations and electrostatic attraction

together would result in cation distribution adjacent to each clay particle in

suspension. Fig. 2.8 presents such a distribution of ions adjacent to a single clay

particle. The charged clay surface and the distributed ions adjacent to it are

together termed as diffuse double layer (DDL). Close to the surface there is high

concentration of ions which decreases outwards. Thus there are double layers of

ions (a) compressed layer and (b) diffused layer and hence the name double

layer. The variation in concentration of cations and anions in pore water with

distance from clay surface is also presented in Fig. 2.8. A high concentration of

cations close to clay surface gradually reduces, and reaches equilibrium

concentration at a distance away from clay surface. For anions, concentration

increases with distance from clay surface.



Fig. 2.8 Distribution of ions adjacent to clay surface (modified from Mitchell and Soga 2005)

Several theories have been proposed for defining ion distribution in DDL. Gouy

and Chapman is one of the initial explanations on DDL ion distribution (Mitchell

and Soga 2005). The theory has been further modified by Derjaguin and Landau;

Verwey and Overbeek which is known by the name DLVO theory (Mitchell and

Soga 2005). In addition to ion quantification, DLVO describe the repulsive

energies and forces of interaction between clay particles and prediction of clay

suspension stability. Sposito (1989) observed that the theory predicts ion

distribution reasonably for only smectite particles suspended in monovalent ion

solution at low concentration. However, the theory can still be used for defining

forces of interaction, flocculation, dispersion, clay swelling etc. A much more

refined description of interparticle forces has been proposed by Langmuir (1938)

and extended by Sogami and Ise (1984).

Following are the assumptions which pertain to the formulation of DDL

theory:

a) Ions in the double layer are point charges and there are no interactions

among them.

b) Charge on particle surface is uniformly distributed.

c) Platy particle surface is large relative to the thickness of double layer (to

maintain one dimensional condition).

Cla

y

surf

ace

+

_ +

+

+

+

+

+

+

+

+

+

+ +

+

+

+

+

_

_ _

_ _

_ _

_

_

+

_ _ _

_

_

_

_

Conce

ntr

atio

n

Cations

Anions

Distance from clay

surface



d) Permittivity of medium adjacent to particle surface is independent of position.

Permittivity is the measure of the ease with which a molecule can be

polarized and oriented in an electric field.

Concentration of ions (no of ions/m3) of type i, ni, in force field at equilibrium is

given by Boltzmann equation as follows:

ni =

kT

EEexpn iio

0i (2.1)

E is the potential energy, T is the temperature in Kelvin, k is the Boltzmann

constant (1.38 x 10-23 J/K), subscript 0 represents reference state which is at a

large distance from the surface.

Potential energy of an ion “i” in electric field is given by Eq. 2.2.

Ei = vieψ (2.2)

where vi is the ionic valence, e is the electronic charge (=1.602 x 10-19 C) and ψ

is the electrical potential at a point. ψ is defined as the work done to bring a

positive unit charge from a reference state to the specified point in the electric

field. Potential at the surface is denoted as ψ0. ψ is mostly negative for soils

because of the negative surface charge. As distance from charged surface

increases, ψ decreases from ψ 0 to a negligible value close to reference state.

Since ψ = 0 close to reference state, Ei0 = 0.

Therefore, Ei0 - Ei = -vieψ and Eq. 2.1 can be re-written as

ni =

kT

evexpn i

0i (2.3)

Eq. 2.3 relates ion concentration to potential as shown in Fig. 2.9.



ni

0 ψ ψ -vieψ negative -vieψ positive

Anion distribution

Cation distribution ni0

Fig. 2.9 Ion concentration in a potential field (modified from Mitchell and Soga 2005)

In Fig. 2.9 anion distribution is marked negative due to the reason that vi and ψ

are negative and hence -vieψ will be negative. For cations, vi is positive and ψ is

negative and hence -vieψ will be positive. For negatively charged clay surface,

ni,cations > ni0 and ni,anions < ni0.

One dimensional Poisson equation (Eq. 2.4) relates electrical potential ψ, charge

density ρ in C/m3 and distance (x). ε is the static permittivity of the medium (C2J-

1m-1 or Fm-1).

2

2

dx

d (2.4)

ρ = e Σvi ni = e(v+ n+- v- n-)

(2.5)

ni is expressed as ions per unit volume, + and – subscript indicates cation and

anion.

Substituting Eq. 2.3 in 2.5

ρ = e Σvi

kT

evexpn i

0i (2.6)

Hence,

e

dx

d2

2

Σvi

kT

evexpn i

0i (2.7)

Eq. 2.7 represents differential equation for the electrical double layer adjacent to

a planar surface. This equation is valid for constant surface charge. Solution of



this differential equation is useful for computation of electrical potential and ion

concentration as a function of distance from the surface.

Different models representing double layer (Yong 2001)

A) Helmholtz double layer: This model follows the simplest approximation that

surface charge of clays are neutralized by opposite sign counter ions placed at a

distance of “d” away from the surface. The surface charge potential decreases

with distance away from the surface as shown in Fig. 2.10.

Fig. 2.10 Variation of surface charge potential with distance from clay surface (modified from Mitchell and Soga

2005)

In this model, double layer is represented by negatively and positively charged

sheets of equal magnitude (Yong 2001). In this model, positive charges are

considered to be stationery, which is against the reality that cations are mobile. It

is opined that this model is too simple to address the real complexities of double

layer.

B) Gouy Chapman model: Gouy suggested that interfacial potential at the

charged surface can be attributed to the presence of a number of ions of given

sign attached to the surface and to an equal number of opposite charge in the

solution. The counter ions tend to diffuse into the liquid phase, until the counter

potential set up by their departure restricts its diffusion. The kinetic energy of

counter ions affects the thickness of resulting double layer. Gouy and Chapman

proposed theoretical expression for electric potential in double layer by

combining Boltzman equation (2.1) and Poisson equation (2.4), where in Eq. 2.1

relates ion distribution to electric potential and Eq. 2.4 relates electric potential

and distance (Reddi and Inyang, 2000). This combination is given by Eq. 2.7. For

Surface

charge

potential

Distance away from surface



the case of a single cation and anion species of equal valency (i=2) and n0 = n0+

= n0- and v+ = v- = v, then Eq. 2.7 simplifies to Poisson-Boltzmann equation (Eq.

2.8).

kT

vesinh

ven2

dx

d 0

2

2

(2.8)

Solutions of the above are usually given in terms of the dimensionless quantities

as stated below.

y = kT

ve Potential functions (2.9)

z = kT

ve 0

ξ = Kx Distance function (2.10)

where K2 = kT

ven2 22

0

or K =

DkT

nve8 0

22 (2.11)

D is the dielectric constant of the medium. According to Eq. 2.11, K depends on

the characteristics of dissolved salt and fluid phase. However, actual values of

concentration and potential at any distance from the surface would also depend

on surface charge, surface potential, specific surface area and dissolved ion

interaction. This means that the type of clays and pore solution are very

important.

Solution can be obtained for a set of boundary conditions, one at the surface and

other at infinite distance:

y = 0 and d

dy = 0 at ξ = ∞ and y = z =

kT

ve 0at ξ = 0

ψ0 is the potential at the clay surface.

For z << 1, ψ = ψ0e-Kx

(2.12)

For z = ∞, ψ = 2

Kxcothln

e

kT2 (2.13)



For some arbitrary z and ξ >>1, ψ =

e

1e

1e

e

kT42/z

2/z

(2.14)

Eq. 2.12 is commonly referred to as Debye-Huckel equation and 1/K represents

characteristic length or thickness of double layer (Mitchell and Soga 2005).

Knowing electric potential from above equations, it is possible to determine ion

distribution from Eq. 2.3.

For cations: n+ =

kT

evexpn i

0i

(2.15)

For anions: n- =

kT

evexpn i

0i (2.16)

This model is accurate only if the soil behaves like a true parallel particle system.

It does not satisfactorily provide description of ψ immediately adjacent to the

charged particle. This is mainly due to the mechanisms associated with chemical

bonding and complexation. Gouy-Chapman model is ideally suited for qualitative

comparisons. The basic assumption in Botlzmann equation where in the potential

energy is equated to the work done in bringing the ion from bulk solution to some

point, does not consider other interaction energy components.

C) Stern model

According to Stern model total cations required to balance the net negative

charge on clay surface consists of two layers. The first layer is of cations are

adsorbed on to the clay surface and are located within a distance of δ. The clay

surface charges and the adsorbed group of cations are termed as electric double

layer (EDL) or Stern layer. The other group of cations are diffused in a cloud

surrounding the particle and can be described by Boltzmann distribution as

discussed in the previous section. The total surface charge (ζs) is counter

balanced by Stern layer charge ζδ and diffuse layer charge ζdl. The surface

potential (ψs) depends on electrolyte concentration and surface charge (whether

it is constant or pH dependent). It decreases from ψs to ψδ when the distance



increases from surface to the outer boundary of Stern layer. Beyond this

distance, ψ is quantified by using Eq. 2.13.

There are other DDL models like DLVO which deals with complex interactions.

However, these are not discussed in this course. The interested readers can

refer to Yong (2001) for further reading.

Cation exchange capacity

From previous discussion, it is clear that clay surface adsorbs specific

amount and type of cations under a given environmental conditions such as

temperature, pressure, pH and pore water chemistry. The adsorbed cations can

get partly or fully replaced by ions of another type subject to changes in the

environmental condition. Such changes can alter the physico-chemical

characteristics of soil. The most common cations present in the soil are sodium,

potassium, calcium and magnesium. Marine clays and saline soils contain

sodium as the dominant adsorbed cation. Acidic soils contain Al+3 and H+. The

most common anions are sulphate, chloride, phosphate and nitrate.

Cation exchange capacity (CEC) is defined as the sum of exchangeable

cations soil can adsorb per 100 g of dry soil. Its unit is meq./100 g and normally

its value ranges between 1 and 150 meq./100 g. The value represents the

amount of exchangeable cations that can be replaced easily by another incoming

cation. The replaceability of cations depends on valency, relative abundance of

different ion type and ion size. All other factors remaining same, trivalent cations

are held more tightly than divalent and univalent ions. A small ion tends to

replace large ions. It is also possible to replace a high replacing power cation by

one of low replacing power due to the high concentration of latter in the pore

solution. For example, Al+3 can be replaced by Na+ due to its abundance. A

typical replaceability series is given as follows: (Mitchell and Soga 2005)

Na+ < Li+ < K+ < Cs+ < Mg+2 < Ca+2 < Cu+2 < Al+3 < Fe+3

The rate of exchange reaction would essentially depend on clay type, pore

solution concentration, temperature, pH etc. In kaolinite, the reaction takes place

quickly. In illite, a small part of the exchange sites may be between unit layers of



minerals and hence would take more time. In smectite minerals, much longer

time is required because the major part of exchange capacity is located in the

interlayer region.

For a pore solution containing both monovalent and divalent cations, the

ratio of divalent to monovalent cations is much higher in adsorbed layer than in

the equilibrium solution. If M and N represent monovalent cation concentrations,

P the concentration of divalent ions, subscript s and e represent adsorbed ions

on soil and that in equilibrium solution, respectively, then

e

1

sN

Mk

N

M

(2.17)

e

2

12

2

s

2

P

Mk

P

M

(2.18)

where k1 and k2 are selectivity constants, which can be obtained experimentally.

Following, Eq. 2.18 it can be further written as

e

2

1

22s

22

2

MgCa

Nak

MgCa

Na

(2.19)

The concentration of cation is in milliequivalents per litre. The quantity

e

2

122

2

MgCa

Na

is termed as sodium adsorption ratio (SAR) in (meq./litre)1/2. If

the composition of pore fluid and k is known, the relative amounts of single and

divalent cations in the adsorbed cation complex can be determined. The details

of selectivity constants for a wide variety of clays are reported by Bruggenwert

and Kamphorst (1979). Sodium present in the adsorbed layer is normalized with

respect to total exchange capacity as represented by Eq. 2.20 and is termed as

exchangeable sodium percentage (ESP).



ESP = ((Na+)s/(total exchange capacity)) x100

(2.20)

ESP and SAR are considered to be a reliable indicator of clay stability against

breakdown and particle dispersion especially for non-marine clays (Mitchell and

Soga 2005). Clays with ESP > 2 percent is considered as dispersive.

Other quantitative attributes of cation exchange in soils is the property known as

“the percentage base saturation” (Eq. 2.21), which denotes the measure of the

proportion of exchangeable base on the soil exchange complex.

Base saturation (%) = +2 +2 + +Ca +Mg +K +Na

100CEC

(2.21)

Factors influencing CEC of the soil

a. pH of the soil

It is observed that CEC of the soil increases with an increase in pH.

Therefore, it is recommended to maintain a neutral pH (= 7.0) for determining

CEC of the soil.

b. Presence of organic matter

The presence of organic carbon in clays reduces its CEC (Syers et al.,

1970). However, some studies report an increase in CEC with increasing organic

matter contents and this effect was more pronounced in coarser fractions.

c. Temperature

The ion exchange capacity decreases with an increase in temperature.

d. Particle size

It is observed that CEC increase with decreasing particle.

e. Calcium carbonate contents (CaCO3)

Higher amount of CaCO3 in soil leads to higher CEC.

f. Mineralogy

Active clay minerals increase CEC of the soil.



Determination of CEC by Ammonium replacement method

Horneck et al. (1989) have proposed a method for determining the CEC of

soils by using ammonium replacement technique. This method involves

saturation of the cation exchange sites on the soil surface with ammonium,

equilibration, and removal of excess ammonium with ethanol, replacement and

leaching of exchangeable ammonium with protons from HCl acid. It must be

noted that this method is less suited for soils containing carbonates, vermiculite,

gypsum and zeolite minerals. The procedure is discussed as follows:

Take about 10 g of soil, in 125 ml flask, add 50 ml of ammonium acetate solution

and place the flask in reciprocating shaker for 30min. The shaking process is

repeated with blank solution as well.

1 liter vacuum extraction flask is connected to a funnel with Whatman no.5 filter

paper. The soil sample is then transferred to the funnel and leached with 175 ml

of 1 N ammonium acetate. The leached solution is analyzed for extractable K,

Ca, Mg, and Na.

The soil sample in the funnel is further leached with ethanol and the leachate is

discarded.

Transfer the soil to a 500 ml suction flask and leach the soil sample with 225 ml

of 0.1N HCl to replace the exchangeable ammonium. Make up the leachate to a

final volume of 250ml in a standard flask using deionized water.

The concentration of ammonium in the final leachate is measured, and CEC is

calculated using Eq. 2.22.

CEC (meq./100g of soil) = 0.25 100

ammonium concentration14 sample size(g)

(2.22)

IS code (IS 2720, Part 24 1976) and USEPA (EPA SW-846) also provides

alternate methods for determining CEC of the soil. The range of CEC values for

different soil minerals are listed in Table 2.3 (Caroll 1959). It can be noted that

highly active soil minerals such as montmorillonite and vermiculite exhibit high

CEC. Therefore, CEC is important in assessing the chemical properties of the

soil in terms of its reactivity, contaminant retention mechanism etc.



Table 2.3 CEC values of common soil minerals

Mineral CEC (meq./100g) at pH 7

Kaolinite 3-15

Illite 10-40

Montmorillonite 70-100

Vermiculite 100-150

Halloysite 2H2O 5-10

Halloysite 4H2O 40-50

Chlorite 10-40

Allophane 60-70

Quantification of soil water

One of the main attributes that makes soil mechanics different from solid

mechanics is the presence of water in the void spaces. The quality and quantity

of water will significantly influence physical, chemical and engineering properties

of soil such as plasticity, permeability, water retention, mass transport etc. The

water present in the soil voids are quantified as water content, which is also

referred to as capacity factor. Energy status of water is called intensity factor.

Water content is further divided into gravimetric and volumetric water content.

When water content is defined as the ratio of weight of water to the weight of soil

solids (weight basis) it is termed as gravimetric water content, denoted as “w”.

Volumetric water content is expressed as the ratio of volume of water (Vw) to the

total volume of soil (V) and denoted by θ.

θ = Vw/V (2.23)

= Ww/γw 1/V

= Ww/γw (Wd/Wd) 1/V

Wd/V = γd

Ww/ Wd = w

θ = w (γd/ γw) (2.24)

Also, θ = w (G γw/(1+e))/ γw

= w ((Sr e)/ w) 1/(1+e)

= e/(1+e) Sr



= n Sr (2.25)

Where Ww is the weight of water, γw is the unit weight of water, Wd is the weight

of dry soil, γd is the dry unit weight of soil, G is the specific gravity of soil, e is the

void ratio, n is the porosity and Sr is the degree of saturation. Eqs. 2.24 and 2.25

relates θ with w and Sr, respectively. For a fully saturated soil, Sr = 1 and hence θ

becomes equal to n.

There are some dimensionless expressions for water content, which are

important for different modelling application. Some of the important expressions

are given by Eqs. 2.26 and 2.27.

Relative water content, θrel = θ /θsat (2.26)

Reduced or effective water content, Se = (θ-θr) /θsat-θr) (2.27)

Where θsat and θr are saturated volumetric water content and residual water

content. The same expressions are valid in terms of gravimetric water content

also.

Mechanical energy of water

Kinetic energy (KE) of water present in porous media is considered to be

negligible due to the low flow velocity in moderate and low permeable soil.

However, KE is important in granular soils where velocity is significant and also

in the case of preferential flow in soils. Preferential flow is caused in soils due to

the formation of macrocracks which is mostly attributed to the shrinkage cracks in

soil, holes or burrows created by animals, cracks caused by the roots of plants

etc. Water would find an easy path through these cracks and hence known as

preferential path ways.

Potential energy (PE) is the most important energy component of water present

in the porous media. It is the difference in PE between two spatial locations in

soil that determines rate and direction of flow of water. The rate of decrease in

PE is termed as hydraulic gradient (i). PE of water is termed as soil-water

potential. The total soil-water potential (ψt) is the summation of different PE

components as given by Eq. 2.28.

ψt = ψg + ψm + ψp + ψo (2.28)



Where ψg is the gravitational potential, ψm is the matric potential, ψp is the

pressure potential and ψo is the osmotic potential.

ψg is due to the difference in elevation between two reference points and hence it

is also known as elevation head (z). For this a reference datum position is always

defined from which elevation is measured. The point above the datum is

negative, and below is positive. ψm is caused due to the adsorptive and capillary

forces present in the soil. Such a force always retains water towards the soil

surface and hence the potential is always taken as negative. ψp is the pressure

potential below the ground water table and hence the potential is always positive.

It is the head indicated by a piezometer inserted in the soil and hence it is termed

as piezometric head. Such a potential is valid for fully saturated state of the soil.

However, saturation due to capillary rise is not considered since such water is

held under tension. ψo is caused due to salts and contaminants (solutes) present

in the soil pore water. Since the solute present in the water try to retain water

molecules, ψo is negative.

In the absence of solutes, ψm can be expressed as follows (Scott 2000):

ψm = 0e

eln

M

RT (2.29)

Where R is the universal gas constant (8.314 J/K.mol), T is the temperature in

Kelvin, M is the mass of a mole of water in kg (0.018015), ψm is in J/kg, e is the

vapour pressure of soil pore water, e0 is the vapour pressure of pure water at the

same temperature. e is less than e0 due to the attraction pore water on soil

solids. The term e/ e0 is relative vapour pressure.

Problem: Relative vapour pressure at 20 °C is 0.85. Calculate ψm. If relative

vapour pressure becomes 0.989 then what happens to ψm.

ψm = 85.0ln

018.0

293314.8 x

= -21989 J/kg

When relative vapour pressure is 0.989, then ψm = -1496 J/kg.



A higher relative vapour pressure is associated with high water content of the soil

sample. From this example, it can be noted that as water content increases,

matric suction reduces.

Solutes present in soil water results in ψo due to the semi-permeable membrane

effect produced by plant roots, air-water inter phase and clays. As concentration

of solute increases, ψo also increases.

According to Vant-Hoff‟s equation, ψo = RTCs (2.30)

Where ψo is in J/kg, Cs is the concentration in mol/m3, R and T as defined earlier.

According to US Salinity laboratory, ψo = -0.056 TDS (2.31)

Where TDS is the total dissolved solids of soil pore water in mg/L and ψo is in

kPa.

Also, ψo = -36 EC (2.32)

Where EC is the electrical conductivity of soil pore water in dS/m and ψo is in

kPa.

ψo can also be expressed as 0

w

e

eln

M

RTρ

(2.33)

where ρw is the density of water in kg/m3, M is the mass of one mole of water

(kg/mol), R and T as defined earlier, e is the equilibrium vapour pressure of soil

pore water containing solutes, e0 is the vapour pressure of pure water in the

absence of solute, and ψo is in kPa.

Problem: Calculate total potential of a saturated soil at 200C at a point through

which reference datum passes. Saturated volumetric water content is 0.5. 1cm3

of soil at reference datum has 3x10-4 moles of solute. Water table is 1.2 m above

reference datum.

Total potential ψt = ψg + ψp + ψm + ψo

ψg = 0 (at reference datum)

ψm = 0 (soil is saturated)

ψ0 = -RTCs

Cs is moles/m3 in pore water



Therefore we need to find the volume of pore water.

Given, θsat = 0.5 = Vw/V

Vw = 0.5x1x10-6 m3

1cm3 of soil mass will have Cs = 3x10-4/ 0.5x10-6

Thus ψ0 = -[8.31x293x(3x10-4/ 0.5x10-6)]

= -1.46x106 J/kg

1J/kg = 10-6 kPa

ψ0 = 1.46x106 J/kg = -1.46 kPa

ψp = 1.2x9.8 = 11.8 kPa

ψt = ψp + ψo

ψt = 10.34 kPa

Note: It is important to put the sign for each of the potential.

Movement of water: Soil water moves from higher ψt to lower ψt. If we are

concerned only about liquid flow, then the contribution of ψ0 is considered

negligible because the solutes also move along with the flowing water. While

considering flow of water, ψt can be rewritten as ψg + ψp + ψm. This total potential

is termed as hydraulic potential causing flow. Under hydraulic equilibrium, ψt is

same everywhere, spatially.

Problem: A soil has a perched water table above a clay horizon situated at a

depth of 40 cm from ground surface. Height of water ponded above clay layer is

8 cm. Determine the vertical distribution of ψt at 10 cm interval upto 50 cm depth.

Assume conditions of hydraulic equilibrium. Take reference datum at (a) ground

surface (b) at water table. Distance downwards is taken -ve.

The solution to this problem is given in table below. Depth is Z. All potential of

water is expressed in cm. ψo is not considered.

(a) Reference datum at ground surface

Z (cm) ψg ψo ψp ψm ψt



0 0 0 0 -32 -32

10 -10 0 0 -22 -32

20 -20 0 0 -12 -32

30 -30 0 0 -2 -32

40 -40 0 8 0 -32

50 -50 0 18 0 -32

ψg is the distance of the point from the reference datum. Since it is downwards it

is –ve. Since, there is no mention of contamination ψo is taken as zero at all

points. ψp occurs only below water table. Water level is at 8 cm above 40 cm

depth. Therefore, at 40 cm the ψp will be 8 cm. At 30 cm its value will be zero

since it is above water table. At 50 cm, the total height of water is 18 cm. Now the

value of ψm is not known. But we know that below water table its value will be

zero. Therefore, at 40 cm and 50 cm its value is 0. Therefore, the total potential

(ψt) is known at 40 and 50 cm. It is the algebraic sum of all the water potentials.

Therefore, it must be noted here that sign of the potential is very important. ψt at

40 cm and 50 cm is obtained as -32 cm. Since it is under hydraulic equilibrium

(given), ψt at all the points have to be -32 cm. Once ψt at all the points are know,

then ψm at all locations can be determined. For example, at 10 cm depth, ψm =

[ψt-( ψg + ψm + ψp + ψo] will give -32+10 = -22 cm.

(a) Reference datum at water table

Z (cm) ψg ψo ψp ψm ψt

0 32 0 0 -32 0

10 22 0 0 -22 0

20 12 0 0 -12 0

30 2 0 0 -2 0

40 -8 0 8 0 0

50 -18 0 18 0 0



With the change in reference datum, ψg also changes. Due to this change, ψt

also changes. The method of obtaining other potential remains same as in the

previous case.

Problem: Assume water is evaporated from top soil and the matric potential is

given for depth at 10 cm interval upto 50 cm. Water table is at a large depth

greater than 50 cm. Determine total potential and direction of flow. Head is

measured in cm. Distance downwards is taken negative. Reference datum is

taken as ground surface.

Z (cm) ψm ψg ψo ψp ψt

0 -1200 -0 0 0 -1200

10 -250 -10 0 0 -260

20 -165 -20 0 0 -185

30 -80 -30 0 0 -110

40 -50 -40 0 0 -90

50 -40 -50 0 0 -90

Since concentration is not mentioned and water table is at a depth larger than

problem domain, both ψo and ψp will be zero at all points. Only ψg need to be

determined. Between locations at 40 and 50 cm, there will be no flow occurring

due to hydraulic equilibrium. From 40 cm depth, movement of water will occur

upwards because water potential is low at the ground and high at 40 cm depth.

Please note that the magnitude is high at the top (1200) but the potential is

negative. This will draw or attract water towards that location.

Problem: A 10 cm tile drain with water height 2 cm is placed on clay layer at a

depth of 40 cm from ground surface. Find component potential and total potential



at 10 cm interval upto 50 cm depth. Determine the flow direction. Take reference

datum at ground surface. Assume matric potential as one half of the distance to

top of water table.

Z (cm) ψg ψo ψp ψm ψt Flow direction

0 0 0 0 -19 -19 Downward

10 -10 0 0 -14 -24 Downward

20 -20 0 0 -9 -29 Downward

30 -30 0 0 -4 -34 Downward

40 -40 0 2 0 -38 No flow

50 -50 0 12 0 -38 No flow

The above exercise shows that the flow of water takes place towards tile drain

from ground surface. This is based on the values of total potential. Flow takes

place from higher to lower potential. Please note that sign of the potential is very

important.

Hydrologic horizons

For defining water potential, interaction and movement in soil, it is always

convenient to define three hydrologic horizons. These horizons vary in depth and

thickness spatially and temporally. One or more of these horizons may be absent

as well at a particular place. These three horizons are otherwise termed as

zones. These zones are listed as follows and the same is depicted in Fig. 2.11.

As depicted in the figure, the boundaries of these horizons have been shown to

be horizontal for the sake of convenience. In the field these boundaries may be

irregular.

a) Groundwater zone

This zone is otherwise termed as phreatic zone. This zone exists below ground

water table and hence will be fully saturated. Mostly, the saturated soil

mechanics is applicable for this zone. Depending upon the factors such as

season, rainfall, proximity to water bodies etc. the depth of water table varies and



hence the thickness of phreatic zone. All the voids in this zone are filled with

water and water pressure will be always positive.

b) Vadoze zone

The partially saturated zone or unsaturated zone above water table is

termed as vadoze zone. This zone extends from the top of the groundwater table

to the ground surface. The voids are filled with air or water and the relative

percentage is decided by the amount of saturation. The concepts developed for

saturated soil mechanics is not applicable for unsaturated zone. Hence, the

details of this zone are described in detail in the next section. Lowest portion of

this zone can be nearly saturated due to the phenomenon of capillary rise.

However, the water in this zone will be held under tension. The capillary height

(hc) marked in the Fig. 2.11 is expressed as

hc = grρ

2TCosθ

w

(2.34)

T is the surface tension of water, ρw is the density of water, g is the acceleration

due to gravity, r is the soil pore radius, θ is the contact angle made by water-air

interface where it contacts the soil solids.

c) Root zone

This zone corresponds to the top portion of vadoze zone close to ground

surface where the plants and tress grows. Moisture dynamics is more in this

zone due to the fact that roots draw water and nutrients from the soil. This zone

is also subjected to evaporation and evapotranspiration and is in direct

interaction with the atmosphere. During precipitation, infiltration and flow of water

to the subsurface occurs through this zone.



Fig. 2.11 Various hydrologic horizons

Unsaturated soil (vadoze zone)

Unsaturated soil or partially saturated soil and its behaviour come under

the purview of soil-water interaction problem. The classical soil mechanics deals

with a two phase system and the concepts are based on the assumption that soil

is fully saturated (solid particles and water) or fully dry (solid particles and air).

But this assumption may not be valid for some of the real life situations such as

highway and railway embankments, airfields, dams, tunnels, natural slopes,

linings and covers of waste containment facilities, back fill of retaining walls,

stability of vertical excavations, where in soil is generally unsaturated and

becomes a three or multiphase system. Therefore, its behaviour does not comply

with the concepts developed for saturated soil medium.

The interaction of solid, water and air phase present in the soil develops a

complex energy state resulting in negative pore water pressure known as soil

suction. Soil suction is defined as the energy required for extracting unit volume

of water from soil (Fredlund and Rahardjo 1993). The suction present in the

unsaturated soil makes its behaviour highly transient (varies with time) as

compared to the steady state behaviour of saturated soils. Therefore, all constant

parameters such as water potential, permeability, strength etc. of saturated soil

Ground surface

Water table

Groundwater zone or aquifer

Impermeable

layer

Root zone

Vadoze zone

Capillary zone

hc



become a function in unsaturated soil. The study of unsaturated soil behaviour is

dependent on the basic relationship between suction and water content (either,

gravimetric or volumetric) or saturation. Such a graphical plot as shown in Fig.

2.12 is popularly known as soil-water characteristic curve (SWCC) or water

retention characteristic curve (WRCC) in general.

Numerous research works have demonstrated that the WRCC is

mandatory for studying the behaviour of unsaturated soil (Fredlund and Rahardjo

1993). For accurate determination of WRCC, the precise measurement of soil

suction becomes very important. The major components of soil suction include

matric suction (ψm) and osmotic suction (ψo). The sum of these two components

is termed as total soil suction (ψ). Please note that ψm and ψo is same as the soil-

water potential discussed in the previous section. However, ψ is not same as

total water potential since the former constitute only negative water potentials. ψm

is due to the adsorptive and capillary force existing in the soil matrix where as ψo

is the result of salts or contaminants present in the soil pore-water. In the

absence of any contamination, ψm is equal to ψ. The common units for soil

suction are kPa, Atm, pF, centibar. The unit pF is defined as the common

logarithm of height in centimeters of the water column needed to provide the

suction. Table 2.4 summarizes the relationship between different commonly used

units of suction.

Table 2.4 Relationship between different units of soil suction

WRCC obtained by drying and wetting the soil sample is termed as desaturation

(desorption) and saturation (adsorption) curve, respectively. A typical drying and

wetting WRCCs is presented in Fig. 2.12, which indicates a continuous „S‟

shaped hysteretic relationship. Due to hysteresis, drying WRCC has higher

Height in cm of H2O column

pF (log cm of

H2O column)

kg/cm2

kPa Bar Atmosphere (atm)

10 1 0.01 1 0.01 0.01

100 2 0.1 10 0.1 0.1

1000 3 1 100 1 1

10000 4 10 1000 10 10



suction than wetting curve for particular water content. Following are some of the

key points that are relevant for WRCC:

1. The volumetric water content at saturation, θs, describes the water content at

which the soil is completely saturated and typically depicts the initial state for

the evaluation of the drying path.

2. The air-entry value (AEV), ψa, is the suction at which air enter the largest pore

present in the soil sample during a drying process. AEV is less for coarse soil

as compared to fine soils.

3. Residual water content (θr) is the minimum water content below which there is

no appreciable change in θ. Suction corresponding to θr is called residual soil

suction, ψr.

4. The water-entry value, ψw, on the wetting WRCC, is defined as the matric

suction at which the water content of the soil starts to increase significantly

during the wetting process.

A fully saturated soil specimen having a volumetric water content of θs

desaturates in three stages as depicted in Fig. 2.12. In stage 1 termed as

capillary saturation zone extending up to AEV, the soil remains saturated with the

pore-water held under tension due to capillary forces. In the desaturation zone

(stage 2), ranging from AEV to ψr, there is a sharp decrease in water content and

the pores are increasingly occupied by air. The slope of the WRCC in this portion

describes the rate of water lost from the soil. In the third stage known as zone of

residual saturation (>ψr), there is little hydraulic flow. However, there may be

some water vapour movement. Beyond this point, increase in soil suction does

not result in significant changes in water content. The zone of residual saturation

is terminated at oven dry conditions (i.e. water content equal zero),

corresponding to a theoretical soil suction of approximately 106 kPa (Fredlund

and Rahardjo 1993).



Fig. 2.12 Details of idealized WRCC

The slope of WRCC is termed as specific capacity or differential water

capacity and represented by C(θ) =(dθ/dψ). This is an important property

describing water storage and water availability to plants. As C(θ) is more, water

drained out or water availability from that soil is more. For a particular increase in

ψ, the coarse grained soil releases more water than fine grained soil.

A large variety of instruments are available for measuring ψm or ψ in the

field or in the laboratory, either directly or indirectly. A summary of various

instruments used for measuring soil suction is presented in Table 2.5. Each of

these measurement techniques has its own limitations and capabilities, and

active research is ongoing for further improvement.

Table 2.5 Details of different suction measuring instruments

ψw

(ψa, θs)

(ψr, θr)

ψ (kPa)

θ (

%)

Drying

curve

Wetting

curve

3 1 2



Me

tho

d

Instrument Suction

Usage

Range (kPa)

Equilibrium time

Dire

ct

Tensiometer M F 0-80 Hours

Pressure plate apparatus M L/F 0-5000 Hours

Pressure membrane extractor

M L/F 0-1500 Hours

Ridley and Burland‟s apparatus (suction probe)

M L 100-1000 Minutes

NTU Mini suction probe M L 100-1500 Minutes

Suction plate M L 0-90 Hours

Gypsum block M F 60-600 Days

Standpipe lysimeter M L 0-30 Days -Months

Ind

ire

ct

Filter paper-contact M L/F 100-1000 2-5 Days

Filter paper-non contact T L/F 1000-10000

2-14 Days

Transistor psychrometer T L 100-10000 Hours

Thermocouple psychrometer

T/ M L 100-7500 Hours

Thermal conductivity sensor

M L/F 10-1500 Hours-Days

TRL suction probe T F 1000-30000

Weeks

Gypsum block M F 60-600 Days

Centrifuge method M L High Depends on soil

WP4 dewpoint potentiameter

T L 0-40000 Minutes

Pore fluid squeezer O L Entire range

-

Time domain reflectometry

M L Entire range

6-48 Hours

Electrical conductivity sensor

M L/B 50-1500 6-50 Hours

Chilled-mirror psychrometer

T L 500-300000

Minutes

Vacuum dessicator T L 103-105 Months

Porous block M F 30-30000 Weeks

Thermal block M F 0-175 Days

Equitensiometer M F 0-1000 Days

Xeritron sensor T L Entire range

Hours



WRCC models and estimation

It is noted that most of the instruments reported above have limited range

of suction measurement. Therefore, several empirical or semi-empirical

mathematical models have been developed for representing WRCC. The

parameters of these models can be obtained by using the limited range of

measured suction data. Once the parameters are known, the same can be used

for extrapolating or interpolating the results by using the WRCC model. These

WRCC parameters are important input for many of the mathematical models

dealing with unsaturated soils. Two such models which are used widely in the

literature are van Genuchten (1980) model and Fredlund and Xing (1994) model

represented by Eqs. 2.35 and 2.36, respectively.

vgvg

-1m

n

r s r

vg

ψθ ψ = θ + θ -θ 1+

a

(2.35)

ff

-1m

n

r

s 6f

r

ψln 1+

h ψθ ψ = θ 1- ln exp(1) +

a10ln 1+

h

(2.36)

where θ(ψ) is the volumetric water content at any suction, ψ; θr is the residual

volumetric water content; θs is the volumetric water content at saturation; avg and

af are fitting parameters primarily dependent on the air entry value (AEV); nvg and

nf are fitting parameters that are dependent on the rate of extraction of water

from the soil; mvg and mf are fitting parameters which depend on θr; hr is the

suction (in kPa) corresponding to residual state. There are several such

simplified and complex models reported in the literature for defining WRCC.

The experimental procedures adopted for determining SWCC are time

consuming and cost-intensive. Therefore, attempts have been made by

researchers to develop functions (such as pedo transfer functions) for the quick

estimation of WRCC without performing extensive suction measurements. In

F: Field L: Laboratory M: Matric T: Total



such cases, WRCC is based on soil physical parameters that can be quickly

determined in the lab. This indirect approach is less time-consuming, simple, and

more economical. However, such estimations can be soil-specific and case-

specific and would depend mostly on the data used for developing the procedure.

Therefore, estimated WRCC should be used with caution and only in those cases

where suction measurements cannot be performed. For important projects it is

always preferable to obtain measured WRCC for the soil. For more details on

WRCC estimation, readers are requested to go through the wide range of

literature available (Fredlund et al. 1998; Mbagwu and Mbah 1998; Fredlund et

al. 2002; Matula et al. 2007; Nimmo et al. 2007; Soil vision 4.10)

Complexity in modelling the behaviour of unsaturated soil

As discussed above, all the properties of unsaturated soil such as

seepage, strength, and volume change behaviour are dependent upon the

suction existing in the soil. These properties changes when the state of

unsaturation changes and suction changes. The state of unsaturation is defined

by θ, w or Sr. As against the steady state behaviour in saturated soil, an

unsaturated soil therefore exhibit transient behaviour. The complex behavioural

modelling of unsaturated soil is explained with respect to unsaturated hydraulic

conductivity (ku) and flow as an example.

In the case of saturated soil, hydraulic conductivity (ks) remains constant

with time. This is mainly due to the fact that all the pores are filled with water. In

the case of unsaturated soil there is retention forces (suction) acting on water

that would restrict easy movement. Due to this, hydraulic conductivity of

unsaturated soil drastically reduces and is essentially a function of water content

or suction present in the soil. For a particular soil, ku increases as suction

decreases till it approaches ks as shown in Fig. 2.13 (Malaya 2011). This is

mainly due to the fact that suction decreases due to the increase in water content

which results in the increase in ku. Therefore, it is clear that ku is a function and

changes with water content or suction. These functions are highly non-linear and

laborious to determine experimentally. Mostly ku functions are estimated



indirectly from water-retention characteristic curve (or SWCC), which is relatively

easy to determine experimentally.

10-1

100

101

102

103

1x10-13

1x10-11

1x10-9

1x10-7

1x10-5 k

s

ku (

m/s

)

m (kPa)

Fig. 2.13 Variation of unsaturated hydraulic conductivity with suction (Malaya 2011)

The 1-D flow through unsaturated soil can be represented by Darcy‟s law

expressed in the form of Eq. 2.37 where the flux density q is given by

q = k(h)z

H

(2.37)

where q is the flux density or Darcy velocity, H is the total soil water potential, z is

the distance in the direction of flow, k(h) is the hydraulic conductivity which is a

function of matric suction head.

H = z + h

z is the gravitational head and h is the matric suction head or pressure head in

general. In the case of unsaturated state of the soil, pressure head will be matric

suction head or negative pressure head.

z

H

is the hydraulic gradient represented by “i” which can also equal to

Δz

ΔH.



Average i =Δz

ΔH=

)z(z

)z(h)z(h

n1n

nn1n1n

n increases downwards and reference datum is ground surface.

Δz

ΔH=

Δz

Δh1

Δz

ΔzΔh

Problem: For a soil, matric potential head is -75 cm and -50 cm at depth 150 cm

and 200 cm, respectively. Given θ = a h-b h > 45 cm and k = menθ. a= 1000, b

= 2, m = 10-12, n = 45, k is in m/s. Determine soil water flux in m/s and flow

direction. Distance downward is –ve and reference is ground surface.

Average i =

Δz

Δh1

=

200150-

5075-1

=0.5

θ = a (h)-b

= 1000x 62.5-2 h is the average value (75+50)/2

= 0.256

k=10-12xe(45x0.256)

= 1.007x10-7 m/s

Flux q =kΔz

ΔH

= 1.007x10-7 x 0.5

= 5x10-8 m/s

Total potential: at 150 cm = -225 cm and at 200 cm = -250 cm. The flow will take

place in the downward direction.

Soil water diffusivity

While dealing with most of the flow problems in hydrology and

geoenvironmental engineering, the term soil water diffusivity becomes important.



According to Darcy‟s law, flow density is defined by Eq. 2.37 which is re-written

below:

q =- k(h)z

H

In the above, z

H

can be re-written as

θ

H

x

z

θ

θ

H

is the inverse of specific water capacity (C) where in H is considered as the

suction head.

Therefore the above equation becomes q =C

k(h) -

z

θ

In the above representation, C

k(h) is known as soil-water diffusivity (D) and its

unit is m2/s.

q =-Dz

θ

(2.38)

Eq. 2.38 is identical to Fick‟s first law of solute diffusion. Analytical solutions are

proposed by researchers for the above differential equation for simple boundary

conditions. This equation is suitable for highly unsaturated state of the soil and is

not valid of near saturated soil. For nearly saturated soil C approaches zero.

2.2.2 Different soil-water-contaminant interaction mechanisms

The contaminants that can pose serious threat to humans persist in short

or long interval of time. These contaminants can be naturally occurring ones such

as arsenic, fluoride, traces of mercury or anthropogenic substances such as

chlorinated organics, dissolved heavy metals etc. The major role of a

geoenvironmental engineer is to predict the fate of contaminants in the

subsurface and minimize its migration towards groundwater source. Fate

prediction is very essential to understand the presence of contaminants in

groundwater sources or subsurface for long term (50 to 200 years). This would

essentially depend on different interaction mechanisms between contaminant

and soil solids and also between contaminant and dissolved solutes present in



pore water. The knowledge is required to assess the risk or threat posed by

these contaminants to humans and other organisms. Also, the performance and

acceptable criteria of engineered barriers, which minimizes the risk of these

contaminants is assessed based on fate predictions.

Fate of contaminant in geoenvironment is decided by retention and

transport of contaminants. The important mechanisms governing these factors

are as follows (Yong 2001):

(A) Chemical mass transfer and attenuation

(a) Sorption- contaminant partitioning

(b) Dissolution/ precipitation- addition or removal of contaminants

(c) Acid-base reaction- proton transfer

(d) Redox reaction- electron transfer

(e) Hydrolysis/ substitution/ complexation/ speciation- ligand-cation complexes.

(B) Mass transport

(a) Advection- fluid flow

(b) Diffusion- molecular migration

(c) Dispersion- mixing

(C) Other factors

(a) Biological transformations

(b) Radioactive decay

An adequate knowledge of these mechanisms is required to predict the

fate of contaminant. When the contaminated pore fluid passes through the soil

mass, it is bound to undergo weak or strong reactions. Sorption process in which

the contaminants clings on to the soil solids is one of the predominant reactions.

Such a reaction does not ensure permanent removal of contaminants from the

pore fluid, rather attenuation takes place. Attenuation is the reduction in

contaminant concentration during fluid transport due to retardation, retention and

dilution. The extent of interaction between the contaminants and soil fraction

determines reversible or irreversible nature of contaminant partitioning. The term

retention is used for strong sorption of contaminants on the soil particles such

that the concentration of pore fluid decreases with time. The amount of



contaminant concentration reaching a particular target is considerably less than

the source concentration. Chemical mass transfer and irreversible sorption

removes the contaminants from the moving pore fluid. This is a very important

aspect for a contaminant barrier system, where in the contaminants reaching

ground water is minimized. Retardation is mainly governed by reversible sorption

and hence release of contaminant would eventually occur. This will ensure the

delivery of the entire contaminant load to the final target (example ground water),

but with much delay. The process of retention and retardation is depicted in Fig.

2.14. From the figure, it can be noted that for retention process, the area under

the curves (concentration) goes on reducing. For retardation, the area remains

constant (mass conservation), however the concentration of a particular

contaminant reduces. In nature, the effect of contaminated pore fluid is reduced

when it interacts with fresh water (especially during precipitation). This process of

dilution also delays the contaminant migration. However, the process of dilution

is mostly independent of soil interaction.

Fig. 2.14 Attenuation process due to soil-contaminant interaction

For an effective waste management, retention process is more ideal than

retardation. For proper prediction of contaminant fate, it is very essential to know

whether the contaminant is retained or retarded. The important reactions

determining attenuation are discussed as follows:

1) Hydrolysis

Hydrolysis is the reaction of H+ and OH- ions of water with the solutes and

elements present in the pore water. Such a reaction would continue only if the

reaction products are removed from the system. Water is amphiprotic in nature

Distance from source Distance from source

Conta

min

ant

Conce

ntr

atio

n

Conta

min

ant

Conce

ntr

atio

n

Retardation Retention



(Yong 2001), which means it can act as acid or base. According to Bronsted-

Lowry concept an acid is a proton donor and base is a proton acceptor.

According to Lewis, acid is an electron acceptor and base is an electron donor.

As discussed earlier, soil minerals have ionized cations and anions (metal ions)

attached to it that results in a particular pH level in soil-water system. Hydrolysis

reaction of metal ions can be represented as

MX + H2O MOH + H+ + X- (2.39)

The reaction increases with decrease in pH, redox potential and organic content

and increases with temperature. Hydrolysis can be an important reaction in the

process of biodegradation. For example,

(R-X) + H2O (R-OH) + X- + H+ (2.40)

where R is an organic molecule and X is halogen, carbon, nitrogen or

phosphorus and is resistant to biodegradation. The reaction introduces OH in

place of X making organic molecule susceptible to biodegradation.

2) Oxidation-reduction (redox) reaction

Oxidation-reduction (redox) reaction involves transfer of electrons

between the reactants. In general, transfer of electrons is followed by the proton

transfer also. Soil pore water provides medium for oxidation-reduction reaction

which can be biotic and/or abiotic. Microorganisms present in the soil utilize

oxidation-reduction (redox) reactions as a means to derive energy required for its

growth. Hence, these microorganisms act as catalysts for reactions (redox)

involving molecular oxygen, soil organic matter and organic chemicals. For

inorganic solutes, redox reaction results in the decrease or increase in the

oxidation state of the atom. This is important because some ions have multiple

oxidation states and hence would influence the soil-contaminant interaction. It is

found that biotic redox reactions are more significant than abiotic redox reaction.

The redox potential Eh represented by Eq. 2.41 determines the possibility

of oxidation-reduction reaction in the soil-contaminant system.

Eh =

F

2.3RTpE 2.41



E is the electrode potential, pE represent negative logarithm of electron activity e-

, R is the gas constant, T is the absolute temperature, and F is the Faraday

constant. At a temperature of 250C, Eh = 0.0591pE. Factors affecting Eh include

pH, oxygen content or activity, and soil water content.

3) Complexation

Complexation is the reaction between metallic cations and anions called

ligands. The inorganic ligands such as Cl-, B-, F-, SO4-2, PO4

-3, CO3-2 and organic

ligands such as amino acids take part in complexation reaction. For example,

Mn+2 + Cl- MnCl+

Complexation can also occur in series, such that complex formed from one

reaction can react with another ligand as shown below (Reddi and Inyang 2000).

Cr+3 + OH- Cr(OH)+2

Cr(OH)+2 + OH- Cr(OH)2+

Cr(OH)2+ + OH- Cr(OH)3

0

This indicates that the concentration of metals in the form of complexes also

needs to be taken into account in addition to the free metal ion concentration.

Else, the concentration of the metal transported downstream would always be

more than the predicted concentration of metal ion.

4) Precipitation and dissolution

The process of precipitation and dissolution is an important mass transfer

mechanism in the subsurface, where in dissolution increases and precipitation

decreases the concentration of contaminants in pore water. Water is a good

solvent for a variety of solids, liquids and gases. Dissolution is the process of

complete solubility of an element in groundwater. Some natural minerals also

undergo dissolution. For example,

SiO2 + 2H2O Si(OH)4 (dissolution of quartz)

Kaolinite + 5H2O 2Al(OH)3 + 2H4SiO4 (dissolution of kaolinite)

Precipitation is reverse process of dissolution where in dissolved element comes

out of the solution due to the reaction with dissolved species. Due to

precipitation, the concentration of the element reduces in pore water. For



example, Lead gets precipitated from pore water due to its reaction with sulfides,

carbonates or chlorides. Iron, zinc and copper precipitates due to hydrolysis

reaction, and chromium, arsenic precipitates due to redox reaction. In some

cases, both dissolution and precipitation occurs one after the other as the pore

water advances.

pH is important factor governing dissolution and precipitation. An element

has a solubility limit in water. Beyond the solubility limit the solution becomes

supersaturated and starts precipitating. pH governs the solubility limit and hence

when pH changes, there is a possibility of precipitation reaction. It is found that

solubility reduces with pH, reaches a minimum value and then again increases.

This indicates that there exists an optimal pH where precipitation will occur. Metal

hydroxides are amphoteric (increasingly soluble at both low and high pH) and the

pH for minimum solubility (optimum precipitation) is different for different metal.

For example, cadmium-pH 11, copper-pH 8.1, chromium-pH 7.5, zinc-pH 10.1,

nickel-pH 10.8. A small change in pH would therefore result in considerable

changes in precipitation reaction.

5) Exsolution and volatilization

This process involves mass transfer between gaseous and liquid or solid

phase. Similar to precipitation this process removes mass from pore fluid to

gaseous phase. This process is mostly governed by the vapour pressure

(pressure of gaseous phase) with respect to liquid or solid at a particular

temperature. There are a lot of volatile contaminants disposed into subsurface

that finds its way to atmosphere. A thorough knowledge on the exsolution and

volatilization is required to understand the mass transfer mechanism of these

organic contaminants.

6) Radioactive decay

In this process, unstable isotopes decay to form new ones with release of

heat and particles from element nucleus. The process is known as α or β decay

depending on whether the element looses α particle (helium) or a β particle

(electron). The process of decay is irreversible and daughter isotope increases in



quantity. The disposal of radioactive waste generated from nuclear installations,

mining etc. to subsurface will considerably increase the heat. Moreover, the

radioactive isotope such as uranium, plutonium, cesium etc gets transported to

far field and would pollute the groundwater. Preventing such harmful pollution

and reducing the ill effect of overheating of subsurface is a challenging

geoenvironmental problem.

7) Sorption and partitioning

When contaminant laden pore water flow past the soil surface, mass

transfer of these contaminants takes place on to the solids. The process is

referred to as sorption or partitioning. The amount of partitioning depends on the

soil surface (sorbent) and the reactivity of contaminant (sorbate). This is one of

the predominant mechanisms governing the fate of contaminant once it is

released into the geoenvironment. The term sorption refers to the adsorption of

dissolved ions, molecules or compounds on to the soil surface. The mechanism

of sorption includes physical and chemical sorption as well as precipitation

reaction. These reactions are governed by surface properties of soil, chemistry of

contaminant and pore water, redox potential and pH of the environment. Physical

adsorption refers to the attraction of contaminant on to the soil surface mainly

due to the surface charge (electrostatic force of attraction). Physical sorption is

weak bonding and can be reversed easily by washing with extracting solution.

Chemical sorption is strong force of attraction due to the formation of bonds such

as covalent bond. High adsorption energy is associated with chemical sorption

and it can be either exothermic or endothermic reaction. The details of sorption

reaction and mass transport mechanisms will be discussed in detail in module 3

on how to use these information for predicting the fate of contaminants in

geoenvironment.

8) Biological transformation

Biological transformation is the degradation or assimilation of

contaminants (mostly organic) by microorganisms present in the soil.

Transformations from biotic processes occur under aerobic or anaerobic

conditions. The transformation products obtained from each will be different. The



biotic transformation processes under aerobic conditions are oxidation reaction.

The various processes include hydroxylation, epoxidation, and substitution of OH

groups on molecules (Yong 2001). Anaerobic biotic transformation processes are

mostly reduction reaction, which include hydrogenolysis, H+ substitution for Cl–

on molecules, and dihaloelimination (Yong 2001). The major application of

biological transformation is in organic contaminant remediation which is

discussed in module 4.



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Model Questions

The knowledge of clay minerals is important in geoenvironmental engineering.

Explain?

Summarize important properties of clay minerals.

Bring out the major difference between the three clay minerals: Kaolinite, Illite

and montmorillonite.

What are the important mechanisms of soil-water interaction?

Explain the formation of diffused double layer.

What are the important assumptions for formulation DDL theory?

Discuss in brief, Gouy Chapman DDL model.

Explain the significance of cation exchange capacity and method of its

determination.

How does CEC and SSA influence reactivity of soil?

Define volumetric water content? How does the volumetric water content

influence the flow properties of a soil medium?

Derive relationship between volumetric and gravimetric water content.

What are the different components of soil-water potential?

Compare saturated and unsaturated state of soil?

Explain important features of water retention curve?

Explain the complexity in modeling unsaturated behavior as compared to that of

saturated soil.

Discuss soil-water diffusivity.

Explain in detail the various contaminant retention and transport mechanisms in

soil.

What is the difference between retardation and retention of contaminants?

Discuss in detail, the important contaminant attenuation reaction in soil.

Explain the significance of soil sorption behavior in waste management?

Fig. Q2.1 represents the equilibrium condition (after time t) of water separated

from NaCl by using a semi-permeable membrane at a room temperature of 270C.

There is a rise in solution level by 5 cm.



Illustrate using a figure, the condition at time t=0 and explain what has happened

thereafter till time t and why?

Calculate the molar concentration of the NaCl solution. The value of R is

0.0820Litre.atm/Kelvin.mole. The density of NaCl is 1.2 g/cc. (1 atm=0.1 MPa).

5 cm rise

NaCl water Fig. Q2.1



Waste minimization (Reduction)

Reuse

Value addition of waste (Recycle)

Incineration

Land disposal with

energy recovery

Land

disposal

without

energy

recovery

Module 3

WASTE CONTAINMENT SYSTEM

3.1 Evolution of waste containment facilities and

disposal practices

Increased events of environmental pollution and its realization have led to

the evolution of planned and engineered waste management facilities. The waste

management essentially comprises of collection, transport, disposal and/or

incineration of wastes. A sustainable waste management is founded on 3 R’s,

namely Reduce, Reuse and Recycle so that the quantity of waste to be disposed

on land is considerably reduced. For better clarity, the waste management

hierarchy is presented in Fig. 3.1. The major focus is to reduce the quantity of

waste production by efficient process control, try to reuse the by-products or

waste products from a process, and try to recycle the left out waste products by

value added transformation.

Fig. 3.1 Waste management hierarchy (Modified from Munier 2004)



Some of the major challenges faced in the implementation of an efficient waste

management scheme are the non-awareness of public and the need for

systematic functioning of various divisions like collection, transportation, disposal

and site management.

The concept of waste management started in 1800 century. However, the

need for an integrated solid waste management program (ISWMP) has been

realized in late 1980s. The main aim of ISWMP is to optimize all aspects of solid

waste management to achieve maximum environmental benefits cost-effectively.

It essentially consists of

1) Waste source identification and characterization.

2) Efficient waste collection

3) Reduction of volume and toxicity of waste to be discarded.

4) Land disposal and/or incineration.

5) Optimization of first four steps to reduce cost and environmental impact.

The wastes which are produced include non-hazardous municipal solid

waste, construction and demolition waste, partially hazardous medical wastes,

agricultural waste, highly hazardous industrial and nuclear waste. The handling

and management of hazardous and non-hazardous waste varies a lot.

When the wastes are disposed on to the land, the percolating rainwater

interacts with it and produces liquid known as leachate (contaminated liquid that

comes out of the waste matrix). In the due course of time, the leachate

percolates through the soil and reaches the groundwater and moves along with it

as shown in Fig. 3.2.



Rain water

Waste Primary containment/ landfill

Ground surface

Leachate

flowing out

of landfill

Unsaturated

Natural soil

(Secondary

containment)

Top of aquifer

Groundwater

flow

Impervious layer

Fig. 3.2 A conceptual waste disposal facility on a global scale

In the past, it was presumed that leachate generated from waste dumped directly

on natural soil is completely attenuated (purified) by the subsurface before it

reaches or interacts with groundwater. In the figure, subsurface is the

unsaturated natural soil which provides an indirect containment of harmful

contaminants leaching out. In view of the above, all forms of non-engineered land

disposal such as gravel pits were acceptable. Since, 1950 onwards there were

considerable increase in the ground water pollution. The cause for such pollution

was traced back to such indiscriminate casual waste disposals. This gave way to

the development of engineered waste disposal facilities known as landfills. The

properties of soils used for the construction of landfills and the natural soil

beneath the landfill become very important. In this course, major emphasis is laid

on understanding the concepts of landfill and the role of soil in minimizing the

harmful pollution of geoenvironment and ground water.

3.2 Landfills

There are two types of landfills namely natural attenuation landfill and

containment landfill as depicted in Fig. 3.3. Natural attenuation landfill is similar



to what has been discussed in the previous paragraph where there is no

provision below the wastes to minimize the migration of harmful contaminants.

The unsaturated subsurface below the wastes naturally attenuate harmful

contaminants before it reaches ground water. It is presumed that the

contaminants reaching ground water will be well within the permissible limit, even

though in most of the cases it would not be. For the same reason, these types of

landfills are not preferred in spite of its simplicity.

In the containment landfill, there is an engineered layer of soil known as

liner on which the waste is disposed or dumped. Liners are tailor made soil layer

with some desirable properties meeting the regulations set by the pollution

control board. The design of these liners is done in such a way that the

contaminants leaching out seeps at a very low pace and gets attenuated. The

concentration of contaminants reaching the ground water within the prescribed

design life is expected to be well within the permissible limit. This type of landfill

is mandatory for containing hazardous wastes such as industrial and nuclear

wastes. All the working elements of such landfills are properly designed. This

module essentially deals with the role of geoenvironmental engineers in deciding

and designing engineered containment landfills.

Fig. 3.3 Conceptual depiction of types of landfill (a) Natural attenuation (b) Containment

Cover

Waste

Groundwater

Aquifer

Rock

Cover

Waste

Groundwater

Aquifer

Rock

Containment liner

(a) (b)



3.2.1 Engineered landfills

The first and foremost task in the planning of engineered landfills is its site

selection. There are several socio-economic concerns which need to be satisfied

before a site can be decided for waste disposal. The major concern is social

since nobody likes wastes to be dumped in their neighbourhood. This would

necessitate mass education and awareness program on the pros and cons of the

waste management project. Apart from public acceptance the other factors which

are important in site selection are locational, geotechnical and hydrogeological

criteria. Another important aspect in landfill site selection is establishing search

radius, which is the maximum distance of waste hauling (transport). Waste

hauling is one of the costliest items in landfill operations.

Three important steps of landfill site selection are

a) Data collection

b) Locational criterion

c) Obtaining public reaction and acceptance

a) Data collection: The data pertained to landfill site selection are summarized as

follows:

i) Topographic maps: This include information on contour, natural surface, water

drainage, location of streams, wetlands etc. Ideally landfills should be avoided on

land contributing to groundwater recharge. The surface flow should be in such a

way that water flow away from the landfill site. In case the flow is towards the

landfill then adequate measure has to be taken to prevent excessive water

seeping into the landfill.

ii) Soil maps: Gives information on the type of soil available at a particular place.

This information is important before going for an in depth subsurface

investigation. A high permeable soil strata is normally avoided for landfills.

iii) Land use maps: These maps are very important as it gives the land value and

its importance. There will be some zoning restriction for some lands laid down by

the government, which can be assessed based on land use maps. For example,

landfills should be located away from the flood plain.



iv) Transportation: The data on transportation would include the present network

and the futuristic development. It is very essential that the landfill site is easily

accessible and waste hauling is optimal. At the same time, the site should be

away from important facilities like airport. It is essential to refer road and rail

network details before site-selection.

v) Waste type and volume: The primary question is whether the waste is

hazardous or not. The philosophy of waste containment changes depending on

whether it is municipal or industrial waste. Stringent specifications need to be

followed for industrial waste and in no case the waste can be dumped in open

pits. Around 50% of the total waste comes from domestic municipal sources. A

waste generation rate of 0.9-1.8 kg/person/day is a reasonable estimate for

determining municipal waste volume. The population and its growth during the

active life of landfill need to be computed.

Waste volume per year = population per year x waste generation rate

The landfill volume is the sum of daily, intermittent and final cover volume and

waste volume. Waste: daily cover ratio of 4:1 is needed if soil is used as the

cover.

b) Locational criterion: Following are some of the important points to be followed

while deciding location for waste containment.

Lake or pond: Away by 300m. The distance can be reduced for engineered

waste containment. Surface water need to be monitored continuously for

pollution in future.

River: Away by 100 m.

Flood plain: Not to construct municipal waste containment within 100 year flood

plain. For hazardous waste containment this requirement is 500 year flood plain.

Highway and public park: Away by 300 m.

Airport: Away by 3 km to avoid bird menace.

Water supply well: Away by 400 m.

Crowded habitat, wetland, unstable area to be avoided.



The geology of the place should be suitable with no faults and folds. Maximum

horizontal acceleration for the site caused by earthquake should not exceed 0.1g

in 250 years.

c) Preliminary assessment of public reaction: Public education on the short term

and long term advantages of the facility should be carrier out extensively. Not in

my backyard (NIMBY) sentiment can prevent the execution of landfill. Some of

the major concerns are noise, odour, increase in traffic volume, reduction in

property value, fear of groundwater contamination etc. The public needs to be

assured that the above mentioned concerns would be tackled efficiently. This is

one of the challenging issues for geoenvironmental engineers and municipal

authorities in the planning and execution of such projects.

3.2.2 Methods for landfill site selection

There are different qualitative and quantitative methods available for

landfill site selection by assessing the extent of environmental impact caused by

the project. Essentially the decision on landfill siting is done based on subsurface

and burrow source investigation. The subsurface investigation includes the

assessment of hydrogeology of a place to understand permeability, strength,

compressibility, contaminant interaction, presence of faults and folds, seismic

hazard investigation etc. Borrow source investigation reveals the quality of

material available near to the probable landfill site and its utility in landfill

construction. If soil near by is suitable, it would considerably reduce the cost of

the project by minimizing transportation and material expenditure.

Some of the qualitative and quantitative methods for landfill site selection

are briefly discussed below. Qualitative methods for landfill site selection are

only used for preliminary evaluation as discussed below:

(a) Check list: It is a simple list consisting of different criteria that are important

for knowing potential impact due to a project on the environment. It includes

factors related to environment, social and ecosystem considering its

beneficial or adverse impact. For instance:

1. Population likely to be affected by project.



2. Soil, air, water.

3. Flora and fauna

4. Land use etc.

A descriptive check list gives list of impacts during the various stages of

project which can be used as criteria for understanding environmental impact.

A weight scale check list is used to recognize the relative importance of

different factors or environment.

(b) Network analysis: In this method, cause and effect relationship is detected by

analyzing different areas likely to be affected by the project. A block diagram

shown in Fig. 3.4 is used to show the connectivity between action and

consequence. The connectivity is shown by solid arrow for direct

consequence and broken for indirect consequence. It provides an effective

and visual way to illustrate positive or adverse impacts of a project.

Fig 3.4 Network analysis block diagram (Munier 2004)

(c) Overlays

In this method, thematic transparent maps are developed for flora, fauna,

geology, population, rivers, slopes, roads, agricultural land etc. These maps are

placed on a glass table, one on top of the other, forming layers of information

about the zone .When an intense electric lamp is placed beneath the glass table,

Consequence 3

Consequence 2

Consequence 1

Action 1 Action 3

Action 2



light reaching the top layer indicate the area that is feasible for a project under

study. The physical limitation in the application of this method is that no more

than 10 overlays can be used. These days GIS (geographic information system)

is an effective quantitative method to combine the overlays.

Some of the quantitative methods for landfill site selection include the

following:

(a) Matrix method

This method relates activities of a project and its impact on the

environment. An example problem of site selection for landfill is presented in

Table 3.1. The table corresponds to the assessment of one of the alternative

(Site 1). As listed in the table, an importance value is assigned to different

environmental parameters. Further, the impact of different activities (denoted as

A, B, C, D in table) on these environmental parameters is defined by assigning

magnitude of impact, which can be negative or positive. A, B, C, D corresponds

to activities like disposal of solid waste, reclamation, transportation etc.

Table 3.1 Details of matrix method

SITE-1

Environmental Parameters Importance Value A B C D

Air Quality 100

Water Quality 95

Health 90 3

Land Use 85

Human Settlement 80 -5

A matrix is formed by assigning importance values to the environmental

parameters selected for the problem. Further a value is assigned in the matrix

which shows the magnitude of impact (positive or negative depending upon the

sign of magnitude) due to the activity on the environment. Importance values are

multiplied with magnitude of impact and summation is done for rows as well as

for columns. Best site is then decided based on the maximum summation value

of row, column or both.



(b) Multi-criteria analysis

In this method, best possible optimal criteria are selected for evaluation of

sites. A total score of 1000 is apportioned among the assessment criteria based

on their importance .There is no hard and fast rule for total score. A site

sensitivity index (SSI) is developed for different attribute qualities on a scale of 0

to 1. Based on SSI, score for each parameter of various sites is computed.

Ranking is done for the individual site alternatives based on summation of the

score.

(c) Hatzichristos and Giaoutzi (2006) demonstrated the use of fuzzy set approach

integrated with geographical information system (GIS) for landfill siting. The fuzzy

set is considered effective to take decisions on those criteria that are not discrete

and which overlap with one another. It is opined that the fuzzy set approach

integrated with GIS platform is most relevant for applications where the decision

criteria are not discrete and the boundaries between regions are fuzzy or

overlapping.

(d) Chang et al. (2008) have presented a fuzzy multi-criteria decision analysis

along with a geospatial analysis for the selection of landfill sites. The study

developed a spatial decision support system (SDSS) for landfill site selection in a

fast-growing urban region. Thematic maps in Geographical information system

(GIS) are used in conjunction with environmental, biophysical, ecological, and

socioeconomic variables leading to support the fuzzy multi-criteria decision-

making (FMCDM). It differs from the conventional methods of integrating GIS

with multi-criteria decision making for landfill site selection because the approach

follows two sequential steps rather than a full-integrated scheme. The purpose of

GIS was to perform an initial screening process to eliminate unsuitable land

followed by utilization of FMCDM method to identify the most suitable site using

the information provided by the regional experts with reference to different

chosen criteria.



3.3 Subsurface investigation for waste management

Subsurface investigation for waste management is required for deciding

the site for landfills and also for delineating the extent of contamination. Several

hydrogeological parameters required for landfill site selection are obtained from

subsurface investigation conducted for different potential sites. The methodology

for subsurface investigation remains similar to any other geotechnical

investigation (for example, open pit, bore holes). In addition, several geophysical

methods such as electrical resistivity imaging, seismic refraction, ground

penetration radar, etc. are used for defining the zone of contamination,

establishing the depth of aquifer, and also to reduce the number of bore holes.

3.4 Design of landfills

An engineered landfill essentially consists of a barrier layer or liner which

is a low permeable zone to prevent the leaching of waste from the landfill. Above

the liner, a drainage layer is placed which collects the leachate from the waste for

treatment. Such a layer also minimizes the head causing flow in liner due to the

timely removal of leachate from the landfill. The third important layer is the cover

to the landfill, which is a multi-layered system to cut off the harmful effect of

waste on the atmosphere. The various aspects required for planning and design

of landfill are as follows:

1. Waste Characterization

2. Assessment of leachate and gas generation

3. Landfill elements to be provided

4. Liner and cover materials

5. Landfill design approach



3.4.1 Waste Characterization:

Waste characterization is important to understand the following:

1. Physical and chemical tests are preformed to evaluate whether waste is

hazardous or non-hazardous.

2. Whether waste can be landfilled directly or necessitate processing (reduction,

recycling etc.) before disposal.

3. Approximate rate of waste volume generated.

4. Assessment of leachate quantity.

5. Assessment of leachate quality for judging liner compatibility, treatment plant

design, ground water monitoring program design.

6. Safety precautions to be followed during landfill operations.

7. Identify waste reduction alternatives.

3.4.2 Assessment of leachate and gas generation

Leachates are produced when water or other liquids percolates and

interacts with waste. The information on quality and quantity of leachate and gas

generated during active life and after closure are important for realistic and

efficient design of a landfill. Leachate contains a lot of dissolved and suspended

materials. Gases produced include CH4, CO2, NH3 and H2S due to anaerobic

decomposition of waste. These gases either escape to atmosphere or dissolve in

water leading to further reactions. Contaminated liquids of high concentration are

formed due to chemical reaction taking place within the waste. The percolating

water increases the quantity of leachate but would help to dilute the

concentration.

Factors influencing leachate quality

a) Refuse composition

b) Elapsed time: Leachate quality (concentration) increases and reaches peak

during the working period of landfill and then start decreasing with time as shown

in Fig. 3.5. All the contaminants present in the leachate do not exhibit peak at the

same time and may not be of same shape.



Fig. 3.5 Variation of leachate quality (concentration) with elapsed time

c) Temperature: Temperature affects bacterial growth and chemical reactions,

there by affecting leachate quality.

d) Available moisture influences biodegradable and subsequent leaching of

wastes.

e) Available oxygen influences leachate quality due to the fact that chemicals

released due to aerobic decomposition is different from anaerobic

decomposition. Anaerobic condition would arise due to landfill cover or covering

due to fresh waste.

Factors influencing leachate quantity

a) Amount of precipitation received.

b) Ground water interaction when the landfill base is below groundwater table.

c) Moisture content of waste increases biodegradation and increases leachate

production. Such a scenario is mostly applicable in the case of municipal solid

waste and due to sludge that are disposed.

d) Final cover reduces leachate quantity due to low percolation through

compacted covers. Also vegetation in the top soil of final cover reduces

infiltration by increased evapotranspiration.

Conce

ntr

atio

n

Time



Estimation of leachate quantity

The quantity of leachate is directly dependent on precipitation received.

Pre-closure and post-closure leachate generation from a landfill vary significantly.

Pre-closure leachate generation rate is required for designing leachate collection

pipes in the landfill, fixing the size of leachate collection tank and treatment plant.

Post-closure leachate generation rate is required to plan the management of

leachate and cost incurred for it. Leachate quantity considerably reduces after

closure and construction of covers.

Leachate volume (Lv) is given by Eq. 3.1.

L v = P + S - E - AW 3.1

Where P is the precipitation volume, S is the volume of pore liquid squeezed

from the waste, E is the volume lost by evaporation and AW is the volume of

liquid lost through absorption in waste.

Pore squeeze leachate volume (S)

When sludge in disposed, liquid within the pores gets squeezed due to

self- weight of sludge and weight of waste dump and cover soil. Such an action is

similar to the consolidation process occurring in a saturated soil. Primary

consolidation of waste accounts for the majority of pore squeeze leachate. The

primary consolidation properties of sludge are used to predict leachate

generation rate.

Loss due to evaporation depends on ambient temperature, wind velocity,

difference in vapour pressure etc. Leachate absorbed in waste (AW) is depended

on field capacity (FC) of waste. FC is the maximum moisture content that waste

can retain against gravitational force without producing down ward flow. When

the moisture content is within FC, the waste has the capacity to retain water

without causing downward flow.



Post closure leachate generation rate

Only water that can infiltrate through the final cover of the landfill

percolates through the waste and generates post closure leachate. Water

balance method expressed by Eq. 3.2 is a popular method for estimating post

closure leachate generation.

L’V = P - ET - R - S 3.2

Where L’V is the volume of post closure leachate, P is the volume of precipitation,

ET is the volume lost though evapotranspiration, R is the volume of run off and S

is the volume of moisture stored in soil and waste. Potential ET is obtained based

on appropriate empirical equation.

R = Cr I A 3.3

Where Cr is the run off coefficient, I is the rainfall intensity and A is the area of

landfill surface.

Soil moisture storage (S): A portion of infiltrating water is stored by soil and only

a part of this is used for vegetation. Soil moisture storage capacity is the

difference between field capacity and wilting point. Wilting point is the moisture

content at which plants cannot draw moisture and starts wilting. Normally,

moisture content corresponding to 1500 kPa matric suction is taken as wilting

point.

Water balance method if not done properly results in large errors

especially when used for long term leachate generations rate. The disadvantages

of water balance method are: (i) it does not account permeability of cover layer

(ii) evapotranspiration is sometimes wrongly calculated due to over prediction of

root length in vegetation layer. In reality root would not have penetrated entire

thickness of vegetation layer. Some of the freely downloadable software such as

hydrologic evaluation of landfill performance (HELP) model by US Environmental

Protection Agency (USEPA) is a handy tool for performing water balance studies.



Gas generation rate

Gas generation rate is mostly valid for municipal solid waste (MSW)

landfill where organic matter decomposition results in the production of gases.

Gas production in MSW landfill occurs due to anaerobic degradation resulting

from hydrolysis and fermentation (attributed to bacterial activities), acetogenesis

and dehydrogenation, and methanogenesis. Hydrogen gas is produced due to

the oxidation of soluble products to organic acids. Some of the other gases

produced from MSW are methane, carbondioxide, hydrogen sulphide and

nitrogen. Gas production reaches a stable rate and then decreases as biological

activity in MSW landfill start decreasing. The assessment of time dependent

percentage production of methane from a MSW landfill is important for

recovering methane as an energy source, and there by reducing greenhouse gas

effect.

3.4.3 Engineered containment landfills

The engineered landfill includes designed man made barrier layers for

minimizing the migration of harmful contaminants from the place of disposal to

the groundwater. The provisions in engineered landfill depends upon the type of

waste is receives. For example, comparison of a typical MSW landfill and

hazardous landfill is shown in Fig. 3.6.

Fig. 3.6 A typical engineered landfill provision

Municipal solid

waste

Soil filter or

geotextile

Leachate collection

layer

Clay barrier

Permeability

less than 10-7

cm/s

15 cm

30 cm

≥60 cm

Hazardous waste

Soil filter or

geotextile

Leachate collection

layer

Clay barrier

Permeability

less than 10-7

cm/s

15 cm

60 cm

≥100 cm

Water table

Minimum 3 m



Major role of soil in engineered landfill

As indicated in Fig. 3.7, the major role of soil in an engineered landfill can

be summarized as follows:

Fig. 3.7 Role of soil in an engineered landfill

1) Compacted liner or barrier which minimize the migration of contaminant to

groundwater and hence it is the most integral and important part of a

landfill. The reduction in migration is due to low permeability and

contaminant retention capacity of the clayey soil used in liners.

2) Leachate collection system provided below the waste to collect the

leachate and effectively drain to a collection source for further treatment.

3) After the service life of the landfill, an integrated multi layer cover system

is provided on top of the waste to isolate it from the environment and

minimize the generation of post closure leachate.

4) Natural soil is used as daily cover material for waste during the operational

phase of landfill.

5) The unsaturated natural soil below the liner act as an additional buffer

layer in reducing the migration of contaminants to groundwater.

6) In addition, suitable geosythetics, geotextiles, geomembrane, geonets etc.

are used individually or in combination with soil to act as liner, drainage

layer, filtration layer or separation layer. The use of geosynthetic helps to

reduce the thickness of liner layer.

Multi layer cover on waste

Waste

Leachate collection layer

Daily cover (natural soil)

Liner or barrier

Natural soil



3.4.3.1 Compacted liner

Soil used for compacted liners include natural clays, glacial till, residual

soil, shale, mud, bentonite etc. Natural or locally available soils with high clay

content are preferred to commercial soil like bentonite due to cost effectiveness.

In the absence of suitable natural soil, swelling clays like bentonite is mixed with

locally available soil, fly ash, sand etc. to achieve the desired performance of

liners. In recent years, geosynthetic materials have been used along with clays to

enhance the performance of liners due to its low permeability. A typical eg; is

geosynthetic clay liner popularly known as GCL. These are factory manufactured

hydraulic and gas barriers typically consisting of bentonite clay or other low

permeability clay materials sandwiched between synthetic materials such as

geomembranes or geotextiles or both, which are held together by needling or

chemical adhesives. The thickness of GCLs is much less than that of compacted

clay liners. The main advantage in using geosynthetic materials are their ready

availability, small volume consumption, better performance, durability, low cost

and homogeneity as compared to soils. The simplest compacted liner is that of

compacted clay liner (CCL), which is widely used as hydraulic barriers for water

and waste containment. Other configurations of liners include single, multiple and

composite layers and are used depending on the importance of the project and

vulnerability of waste. The thickness of liner varies from 60 cm for an ordinary

solid waste facility to approximately 300 cm for highly hazardous waste. It is

reported that even for a homogenous liner, a thickness of less than 60 cm would

result in a sharp increase of leakage through the liner (Kmet et. al., 1981). As the

liner thickness is increased, the flow through the liner is significantly decreased.

The trend of decreasing flow is observed until a thickness of 1.2 m to 1.8 m is

reached. Beyond this, the decrease in flow with further increase in thickness is

minimal (Benson et. al., 1999). As such, it is recommended that a minimum liner

thickness of 1.2 m to 1.8 m be used to provide an effective flow barrier. This

factor of safety is required to account construction errors and to compensate the

difficulty of ensuring quality control for such a large aerial extent of liner.



It is very important to assess the suitability of geomaterial for compacted

liner construction. One of the universally accepted criteria to be satisfied by

compacted liner is that the permeability (k) should be ≤ 10-9 m/sec. Therefore,

this requirement becomes the primary criterion for deciding the suitability of

geomaterial as compacted liner. There are different other criteria available in the

literature for assessing the suitability of material for liner construction based on

soil properties such as unconfined compressive strength (UCS), index properties

etc (Younus and Sreedeep 2012 a, b). UCS of not less than 200 kPa is desirable

for liner material to bear the overburden placed above. In some cases plasticity

characteristics are used for initial assessment of geomaterials. Clay with liquid

limit less than 90%, plasticity index (PI) between 6% and 65% and clay content

greater than 10% is found suitable for liners. However, these guidelines are

qualitative and need to be ascertained with the permeability characteristics of

compacted liner material. Daniel and Benson (1990) recommend that the soil

liner materials should contain at least 30% of fines, where as other state

regulatory agencies recommend at least 50% fines.

Compaction is one of the most important factors that govern permeability

of liners. Most of the liners are compacted with footed rollers, which are fully or

partially penetrating the soil layer. The dry unit weight of compaction in the field

should be 96-98 % of maximum dry unit weight established in the lab. Water

content of the soil is normally 0 to 4% of OMC on wet of optimum. A broader

range of compaction water content resulting in target permeability is desirable

from workability point of view. Sufficient care is required to guard against

desiccation of the compacted liner due to the loss of water content. Desiccation

results in cracks and preferential pathways for liquid leading to enhanced

permeability. The problem of desiccation can be alleviated by covering the liner

by natural soil, using clayey sand with low shrinkage, specify the range of

compaction water content and dry unit weight that ensures both low permeability

and low shrinkage.



3.4.3.2 Design philosophy of compacted liner

For the design of compacted liner it is important to understand the

governing mechanism of contaminant transport through soil. Knowing the

governing mechanism, the appropriate governing differential equation is

formulated. The solution of governing equation is used to predict the

concentration of contaminant with respect to space and time. Such predictions

are used to evaluate whether the thickness of compacted liner (with a specific set

of properties) would be able to protect the groundwater aquifer from pollution for

the period of design life (which may be as high as 100 years). If not, then the

thickness or the material of liner is modified to meet the requirements. To start

with, the governing mechanisms of contaminant transport are discussed below:

1) Advection: It is the movement of contaminant along with the flowing water.

Seepage velocity (vs) become important. Movement of contaminant with velocity

equal to ground water is termed as plug flow.

Mass flux of contaminant transported by advection is f = n. vs. C = v. C (3.4)

Mass flux is defined as the amount of mass transported across a given cross

section in unit time. n is the porosity and C is the concentration.

Total mass flux due to advection ma = A * t

df0

. (3.5)

= A * t

s dCvn0

...

ma is the mass of contaminant transported from landfill by advection. A is the

cross section area through which contaminant passes. For non-reactive

contaminant, contaminant moves with a velocity equal to flow velocity. If velocity

is negligible, contaminant movement by advection is minimal.

2) Diffusion: It is the process of solute transport from a region of higher

concentration to a region of lower concentration. The process is termed as

molecular diffusion, Dm, when the solute migrates in pure water. However,

diffusion in the porous media is restricted only to pore space and can be



expressed by Fick’s first and second laws (Rowe et al. 1988), which corresponds

to steady (Eq. 3.6) and transient diffusion (Eq. 3.7), respectively.

z

CDF ed n (3.6)

where, me DD and 2

eL

L

2z

C2

eDt

C

n (3.7)

where Fd is the mass flux due to diffusion of solute per unit area per unit time, De

is the effective diffusion coefficient, Dm is the molecular diffusion coefficient, is

the tortuosity coefficient, z

C

is termed as concentration gradient, L is the straight

line distance of the flow path, Le is the actual distance traveled by the solute

through the pore space and z is the distance of solute travel.

Total mass flux due to steady state diffusion md = A *

t

e dz

CDn

0)...( (3.8)

Advective-dispersive transport:

Mechanical dispersion (Dmd) occurs when the flow velocity is high or when there

is sudden variation in flow velocity or due to non-homogeneity in porous media.

Dispersion and diffusion process are normally lumped together and known as

hydrodynamic dispersion coefficient (D).

D= (De+Dmd) (3.9)

For low permeable soils like clays, De dominates and for high permeable soils

like sands Dmd dominates. Dmd is represented as a linear function of velocity as

represented by Eq. 3.10.

Dmd = α.v (3.10)

α is known as dispersivity (in m). It is scale dependent and changes with the

extent of problem domain.

Total mass flux due to advective-dispersive transport is then given by



md = A *

t

dz

CDn

0s )...C . vn. ( (3.11)

Sorption

Sorption process, as discussed in chapter 2, is an important contaminant

retention mechanism that slow down or remove the contaminant from flowing

pore water there by delaying its presence in groundwater. Therefore, for reactive

contaminants, sorption plays an important role in deciding its fate (presence of

contaminant with respect to space and time). Sorption is governed by physico–

chemical properties of both solute and soil. Many soils can preferentially adsorb

some type of contaminants to others.

When water containing dissolved contaminants (reactive) comes in contact with

soil, the total mass of the contaminant will partition between solution and the soil.

Concentration of contaminant sorbed on to the soil solids is given by

Cs = (Ci – Ce).(V/Ms) (3.12)

Where Ci is the initial concentration of contaminant in pore water, Ce is the

concentration of contaminant in pore water at equilibrium sorption reaction, Cs is

the concentration of contaminant sorbed on soil mass, V is the volume of pore

water which has interacted with Ms mass of soil. V/Ms is known as liquid to solid

ratio.

For water flowing at a sufficiently low pace, the sorption reaction reaches

equilibrium. The equilibrium sorption reaction is mathematically defined by using

sorption isotherms. These isotherms define the equilibrium relationship between

sorbed concentration on soil and equilibrium concentration present in solution.

Cs = f(Ce) (3.13)

The simplest case of sorption can be modelled using linear isotherm represented

by Eq. 3.14.

Cs = Kd. Ce (3.14)

Kd is the partition coefficient representing the amount of sorption on soil. Such

linear isotherms are good approximations for low concentration range. For higher

range of concentration, sorption is non-linear. Two commonly used non-linear



isotherms are Langmuir isotherm and Freundlich isotherm as represented by

Eqs. 3.15 and 3.16, respectively.

1

m es

e

S bCC

bC

(3.15)

n

s f eC K C (3.16)

Where mS is the maximum capacity of sorption at all available sorption site (mono

layer), b is a constant representing rate of sorption, Kf and n are empirical

constants. Once the sorption isotherm are defined for a particular contaminant-

soil system, then the solute sorbed on soil for any concentration of solution can

be determined.

Sorption characteristics of contaminant-soil system are determined by

batch test procedure (ASTM D 4646). The liquid to solid ratio and required pH for

the batch sorption test is decided. Based on the expected range in the field, the

range of concentration of solution is finalized. The soil is mixed with solution in

the chosen liquid to solid ratio and shaken for 16 hrs using a mechanical shaker.

The solution is then filtered and analyzed for equilibrium concentration (Ce).

Knowing the initial concentration, the sorbed mass (Cs) can be determined based

on Eq. 3.12. Plot the results of Cs vs. Ce and use appropriate sorption isotherm to

define the trend mathematically.

Governing differential equation for contaminant transport

By considering conservation of mass within small soil volume and

summing up the process explained above, the governing differential equation for

contaminant transport (Fetter 1992) can be expressed as

n Cnt

S

z

f

t

C

(3.17)

f is the mass flux due to advective-dispersive transport = z

CDn

..C . vn. s , S is

the sorbed concentration of contaminant and equal to Cs (Eq. 3.13), n is the

porosity, C is the concentration of pore water at time t and distance z, D is the



hydrodynamic dispersion coefficient, ρ is the dry density of soil, λ represent first

order decay reaction such as radioactive decay.

Substituting for f, assuming the simplest linear sorption isotherm (t

S

=Kd

t

C

)

and neglecting first order decay Eq. 3.17 can be represented as

nt

CK

z

Cnv

z

CnD

t

C

ds

2

2

(3.18)

z

Cv

z

CD

t

C

n

Ks

d

2

2

1

(3.19)

n

K d1 is termed as retardation coefficient “R” when linear sorption is

assumed for contaminant-soil interaction. This assumption is valid for low

concentration range of contaminant.

When the contaminant is reactive with the soil, the velocity of its travel may be

less than the seepage velocity due to the retention process. To take this into

account, relative ionic velocity (vs/vion ) is represented as

vs/vion =

n

K d1

vion is the average velocity of reactive (non-conservative) contaminant species.

For a non-reactive (conservative) contaminant, Kd will be negligible and hence vs

is equal to vion. Eq. 3.19 is valid only for saturated soil where porosity is equal to

volumetric water content (θ). For unsaturated soil n is replaced by θ.

Determination of hydrodynamic dispersion and retardation

coefficient

A simple soil column test set up can be used to determine hydrodynamic

dispersion and retardation coefficient simultaneously in the laboratory. A detailed

description of these test procedures are discussed in the literature (Rowe et al.

1988). The soil sample is packed in the soil column with the compaction state

expected in the field. The soil sample is saturated with water and the required

contaminant solution of particular concentration is transferred on top of the



compacted soil. Depending upon the test facilities, the flow of contaminant

solution can be under constant head or under constant flow rate. Constant flow

rate is possible only for high permeable soil. The contaminant solution after

flowing through the soil is collected as effluent from the bottom of the column.

The effluent is collected at regular intervals of time, filtered and analyzed for

concentration. This measured concentration is designated as Ct (concentration at

time t). Concentration variation of effluent can be related to time or pore volume.

Once the test is over, the soil is sliced and the concentration sorbed on soil mass

is determined. This will give the concentration variation with depth. Therefore,

measured Ct can be obtained as a function of time, pore volume or depth. The

solution to the governing differential equation (Eq. 3.19) can be best fitted to the

experimental data to obtain the values of R and D. Analytical solution for Eq. 3.19

for simple boundary conditions given below is represented by Eqs. 3.20 and 3.21

for non-reactive and reactive contaminants, respectively. The solution is

applicable for barrier which is assumed to be infinitely deep and subject to a

constant source concentration.

Initial condition C (z, 0) = 0 z >0 Boundary conditions C (0, t) = Co (initial concentration) t ≥ 0

C (∞, t) = 0 t ≥ 0

Dt

tvzerfc

D

zv

Dt

tvzerfc

C

C ssst

2exp

22

1

0

(3.20)

RDt

tvzerfc

D

zv

RDt

tvzerfc

C

C ssst

/2exp

/22

1

0

(3.21)

For a given liner, it is essential to check whether the provided thickness is

sufficient or not. For this purpose, the parameters governing contaminant

transport such as vs, D and R is obtained as discussed above for the liner

material and model contaminant used. vs is obtained by determining discharge

velocity and knowing the compaction state. Numerical or analytical modelling is

performed to determine the fate of model contaminant (position of contaminant

with respect to space and time). For 1-D modelling as discussed above, space

refers to depth. Based on the numerical modelling, it is checked whether the liner



of given thickness and properties will be able to contain the contaminant for the

given design life. It is expected that the concentration of contaminant reaching

groundwater aquifer should not exceed the safe drinking water standards for the

specified design or operational life. In case, it exceeds then the thickness or the

material need to be reconsidered till it becomes safe. In certain cases,

groundwater table is assumed at the bottom of the liner as worst case scenario.

This means that the role of natural soil below liner is not considered. In the above

modelling, the deterioration of liner material with aging is not considered. The

modelling is done with a gross assumption that the material properties remain

same with age.

Determination of diffusion coefficient

For determining diffusion coefficient of the contaminant the half-cell

assembly, depicted in Fig. 3.8 (Sreedeep 2006), can be employed. This is mostly

applicable for unsaturated soil where the flow component (advective component)

is negligible. The contaminated soil half (source) is packed along with the

uncontaminated soil half (receiver) as shown in the figure. With time, the

contaminant migrates only by diffusion from source to receiver. After the test

duration, the soil mass is sliced and analyzed to obtain concentration variation

with depth. The analytical or numerical solution for differential equation for

diffusion (Eq. 3.22) (Shackelford 1991) is fitted to the experimental results to

obtain De and R parameters.

2

t

2

et C

R

D

t

C

z

(3.22)

where Ct is the concentration at any time t, De is the effective diffusion coefficient,

R is the retardation coefficient and z is the distance.



Fig. 3.8 Details of the half-cell

Solution of Eq. 3.22 depends on the boundary conditions, as presented below:

(i) When the concentration profile does not reach at the ends of half-cell, the soil

medium can be considered to be infinite and the origin for x-axis is taken at

the interface of the half-cell, as depicted in Fig. 3.8 (a). The initial and

boundary conditions for this case can be stated as follows:

Initial conditions: Ct (z, t) = C0 (for z≤0, t=0); Ct (z, t) = 0 (for z>0, t=0)

Boundary conditions: Ct (z, t) = C0 (for z = -∞, t>0); Ct (z, t) = 0 (for z = ∞,

t>0)

The solution for Eq. 3.22 corresponding to case (i) can be represented by

Eq. 3.23 (Crank 1975):

RtD

zerfc

C

C

e

t

/22

1

0

(3.23)

(ii) When the concentration profile reaches at the ends of half-cell, the soil

medium can be considered to be finite and the origin for x-axis is taken at the

end of the source half-cell, as depicted in Fig. 3.5(b). The initial and boundary

conditions for this case can be stated as follows:

Initial conditions: Ct (z, t) = C0 (for z≤0, t=0); Ct (z, t) = 0 (for z>0,

t=0)

Boundary conditions: 0

z

Ct (for z=0, t>0); 0

z

Ct (for z=Lc, t>0)

where Lc is the total length of the cell.

The solution for Eq. 3.22 corresponding to case (ii) can be represented as

follows (Carslaw and Jaeger 1959):

Ct/C0

Source Receiver

Lc

0 +z -z

Direction of diffusion

(a) Infinite cell

Source Receiver

Lc

z0 z 0

Direction of diffusion (b) Finite cell

Ct/C0



ccm

cde

c

t

L

mz

L

mz

m

LRtmD

L

z

C

C 0

1

222

0

0

sincos.)/exp(2

(3.24)

where z0 is the interface between source and receiver.



References

1. ASTM D 4646 (2004) “Standard test method for 24-h batch-type

measurement of contaminant sorption by soils and sediments”, Annual Book

of ASTM Standards, Vol. 04.11, ASTM International, West Conshohocken,

PA, USA.

2. Benson, C. H., Daniel, D. E. and Boutwell, G. P. (1999) “Field performance of

compacted clay liners”, Journal of Geotechnical and Geoenvironmental

Engineering, ASCE, Vol. 125, No. 5, pp. 390-403.

3. Carslaw, H. S. and Jaeger, J. C. (1959) “Conduction of heat in solids”, Oxford

University Press, New York.

4. Chang, N. Parvathinathanb, G. and Breedenc, J. (2008) “Combining GIS with

fuzzy multicriteria decision making for landfill siting in a fast-growing urban

region”, Journal of Environmental Management, Vol. 87, No. 1, pp. 139-153.

5. Crank, J. (1975) “The mathematics of diffusion”, Oxford University Press,

New York.

6. Daniel, D. E. and Benson, C. H. (1990) “Influence of clods on hydraulic

conductivity of compacted clay” Journal of Geotechnical Engineering, ASCE,

Vol. 116, No. 8, pp. 1231-1248.

7. Fetter, C. W. (1992) “Contaminant hydrogeology”, Macmillan publishing

Company, New York.

8. Hatzichristos, T. and Giaoutzi, M. (2006) “Landfill siting using GIS, fuzzy logic

and the Delphi method”, Journal of Environmental Technology and

Management, Vol. 6, No.2, pp.218–231.

9. Kmet, P., Quinn, K. J. and Slavic, C. (1981) “Analysis of design parameters

affecting the collection efficiency of clay-lined landfills”, Proc., Fourth Annual

Madison Conf. of Appl. Res. and Practice on Municipal and Industrial Waste,

Univ. of Wisconsin, Madison, Wis., Sept., 250–265.

10. Munier, N. (2004) “Multicriteria environmental assessment”, Kluwer Academic

Publishers, Netherlands.



11. Rowe, R. K., Caers, C. J. and Barone, F. (1988) “Laboratory determination of

diffusion and distribution coefficients of contaminants using undisturbed

clayey soil”, Canadian Geotechnical Journal, Vol. 25, pp. 108-118.

12. Shackelford, C. D. (1991) “Laboratory diffusion testing for waste disposal- a

review”, Journal of Contaminant Hydrology, Vol. 7, pp. 177-217.

13. Sreedeep, S. (2006) “Modeling Contaminant Transport in Unsaturated Soils”,

Ph. D. Thesis submitted to the Department of Civil Engineering, Indian

Institute of Technology Bombay, India.

14. Younus, M. M. and Sreedeep, S. (2012a) “Evaluation of bentonite-fly ash mix

for its application in landfill liners”, Journal of Testing and Evaluation, ASTM,

in print.

15. Younus, M. M. and Sreedeep, S. (2012b) “Re-evaluation and modification of

plasticity based criterion for assessing the suitability of material as compacted

landfill liners”, Journal of Materials, ASCE, in print.



Model Questions

1. Explain the concept of 3Rs and waste management hierarchy? 2. What is the aim of integrated solid waste management program? 3. Bring out the difference between a natural attenuation landfill and an

engineered landfill. 4. Explain the important details required for deciding landfill site. 5. Discuss in detail the multicriteria method for landfill site selection. 6. What is the importance of waste characterization? 7. What are the factors influencing leachate quality and quantity? 8. How to estimate leachate and gas generation rate? 9. With a neat figure, explain a conceptual liner and cover in landfill. 10. What is the major role of soil in a waste containment facility? 11. What are the requirements of compacted liner? 12. Explain in steps the design philosophy of waste containment liner system. 13. Starting from the basics, derive the differential equation for defining

contaminant transport for reactive contaminant. Every phenomena governing differential equation need to be discussed in detail.

14. With neat figures, explain laboratory method for establishing a) hydrodynamic dispersion coefficient, b) retardation coefficient, c) diffusion coefficient of unsaturated soil with low water content d) partition coefficient.

15. What are the major differences between physisorption and chemisorption? 16. Explain the batch method for establishing sorption characteristics of the

soil-contaminant system. 17. Explain the physical significance of sorption characteristics and its

importance in contaminant transport modeling. 18. What are the different isotherms used for establishing sorption

characteristics? 19. What are the different contaminant transport phenomena? 20. What is diffusion and when it is expected to dominate contaminant

transport phenomena? 21. What is retardation coefficient and how it is helpful in determining ionic

velocity? 22. A column test was conducted to determine dispersion coefficient. The soil

used was a silty clay with specific gravity 2.7. The diameter and height of the saturated soil column is 5 cm and 7cm, respectively with a water content of 35%. Calculate the pore volume of the soil column. An advective flux equal to 0.003 kg/day/m2 of 1000 mg/l SrCl2 has flown through the soil column for 5 hrs. Determine the total pore volume and number of pore volume for 5 hrs. The longitudinal hydrodynamic dispersion coefficient is 1.267 x 10-9 m2/s with a tortuosity coefficient of 0.7. The molecular diffusion coefficient of Sr+2 is 7.9 x 10-10 m2/s. Determine the longitudinal dispersivity for the soil-contaminant system.



23. A batch test was conducted for 3 soil samples A, B, C with an initial concentration of 100 mg/l of SrCl2. 5 g of each of the soil sample is mixed with 50 ml, 100 ml, and 250 ml of SrCl2 and the values of Ce for A are 10, 8 and 6 mg/l, for B it is 12, 10 and 8 mg/l and for C it is 4, 3, 2 mg/l respectively. Compare the reactivity of the soil-contaminant system of the three soils and comment on the role of liquid to solid ratio on the sorption capacity of the three soil. Make any suitable assumptions.

24. Specific discharge in the field is given as 1.68x10-8 m/s. Bulk density of fully saturated porous medium is 1.6 g/cc with volumetric water content of 0.4. Partition coefficient of lead obtained by linear isotherm is 10 ml/g. Determine average velocity of lead. What will be the velocity of lead if it is assumed as non-reactive with porous medium?

25. A drainage pipe became blocked during a storm event by a plug of sand and silty clay as shown in figure Q3.1. When the storm ceased, water level above ground is 1 m. Permeability of sand is 2 times that of silty clay. a) Obtain variation of head components and total head for the length of drainage pipe b) Calculate pore water pressure at centre of sand and silty clay c) Find average hydraulic hydraulic gradient in both soil plugs.

26. Determine the quantity of flow and seepage velocity for constant head set ups given below (Fig. Q3.2) in SI units.

Total height of air tube is 10 cm

in which 2 cm is below water

20 cm

10 cm

5 cm

5 cm

10 cm

ksat= 3*10-5

cm/s

saturated volumetric water content = 0.5

Total height of air tube is 10 cm

in which 2 cm is below water

20 cm

10 cm

5 cm

5 cm

10 cm

ksat= 3*10-5

cm/s

Specific gravity = 2.65; w=25 %

Datum and water exit

A C B

3.3 m

1.5 m 0.5 m

Sand Silty

clay

Water level

Fig. Q3.1

Fig. Q3.2



Module 4

CONTAMINATED SITE REMEDIATION

Soil contamination by organic or inorganic pollutants is caused by a number of

industries such as chemical, pharmaceuticals, plastics, automobile, nuclear

industries, biomedical wastes, mining industries, municipal solid waste. At times

it becomes essential to decontaminate soil. Broadly the soil decontamination is

done in two ways: (a) pump and treat in which the pollutant is pumped out using

external energy source, treated using methods such as incineration, radiation,

oxidation etc (b) removal of contaminated soil, treat it and then returning back to

its original place. This module is meant to briefly introduce various soil/ water

decontamination processes. The scientific basis and the reactions involved in

these processes are acid-base chemistry, solubility-precipitation, ion exchange,

redox, complexation, sorption, etc. which are discussed in module 2.

4.1 Contaminated site characterization/ assessment

Broadly, site characterization or contaminated site assessment (CSA) is

important for:

a) Determining concentration and spatial distribution of harmful pollutants under

consideration.

b) Determining the extent of site remediation (zonation) based on which the

suitable remediation technique is selected.

c) For assessing environmental and human health risk due to contamination.

More specifically, CSA is required to answer the following questions:

a) What is the source of contaminants?

b) What is the type and physical form of contaminants?

c) Spatial and depth wise extent of contamination

d) Whether the contaminants are stationery or movable?

e) If they are movable, then identify the significant pathways.

f) Identify the potential receptors of contaminants.



4.2 Selection and planning of remediation methods

Fig. 4.1 (USEPA 1991) presents a flowchart on various processes

involved in the planning of site remediation.

Fig. 4.1 Processes involved in deciding contaminated site remediation

It can be noted from Fig. 4.1 that the most important step for making a decision

on site remediation is collection of data. Table 4.1 summarizes the essential data

to be collected as part of site reconnaissance and site characterization.

Table 4.1 Summary of data required for planning contaminated site remediation

Data Details Method of acquisition

1) Site history and land use pattern

a) Population density within 3 km from the contaminated site b) Proximity to important geographical features like airport, railways, river etc. c) Ownership of the land d) Extent of contamination

Field

2) Geologic and hydrologic

a) Topography b) Soil profile up to bed rock c) Information on aquifer d) Groundwater depth and flow direction

Field

3) Geotechnical a) Soil sampling and classification b) Permeability of soil c) Chemical characteristics of soil

Field Field Lab

Site reconnaissance: Assessment of distribution, reaction and migration potential

Site characterization and sampling

Mass balance analysis using predictive mathematical modeling

Sufficient information to decide remediation Select, evaluate and apply remediation

Lab/ field studies to understand distribution, reaction and migration

Sensitivity analysis to understand the effect of various design

parameters on remediation performance

Field verification of remediation effectiveness

Yes

Information sufficient to demonstrate

remediation optimization

No

No



d) Soil strength Lab

4) Waste a) Water quality b) Identifying the type of contamination c) Concentration of contaminants d) Spatial extent of contamination e) Depth of contamination f) Contaminant retention characteristics g) Contaminant transport characteristics h) Hazard assessment and zonation

Field/ Lab Field/ Lab

Lab Field/ Lab Field/ Lab

Lab Lab Lab

4.3 Risk assessment of contaminated site

Risk assessment or hazard assessment is required to decide the extent of

contaminant remediation required for a particular site. The factors influencing risk

assessment are:

Toxicity

A material is deemed toxic when it produces detrimental effects on

biological tissues or associated process when organisms are exposed to

concentration above some prescribed level. Acute toxicity is the effect that

occurs immediately after exposure where as chronic toxicity deals with long term

effects. It is expressed as mass unit of toxicant dose per unit mass of receiving

organism. It must be noted that concentration is an important factor while

deciding toxicity. Only when a contaminant crosses a particular concentration, it

becomes toxic. If the concentration is within the prescribed limit then no

remediation need to be performed. Only those site which have toxic level of

contaminant concentration needs remediation. For example, toxic contamination

level leading to cancer becomes the basis for some of the site clean-up

programs.

Test protocols such as toxicity characteristics leaching procedure (TCLP)

(Method 1311, EPA) have been developed for extraction of chemicals from

wastes to verify whether the concentration is within the prescribed toxicity limit.

TCLP is designed to determine the mobility of both organic and inorganic

analytes present in liquid, solid, and multiphase wastes. Several regulatory



agencies such as central pollution control board (CPCB), India, United States

Environmental Protection Agency (USEPA) have prescribed toxic concentration

levels for various chemicals that get leached from the waste samples by

conducting TCLP. In some cases, multiple extractions from the wastes become

necessary. For performing TCLP appropriate extraction fluid need to be used.

Glacial acetic acid mixed with water is used as the extraction fluid. In some cases

sodium hydroxide is also added. For detailed procedure, readers are advised to

refer to Method 1311, EPA.

Reactivity

It is the tendency to interact chemically with other substances. These

interactions become hazardous when it results in explosive reaction with water

and/or other substances and generate toxic gases.

Corrosivity

Corrosive contaminants degrade materials such as cells and tissues and

remove matter. It is defined as the ability of contaminant to deteriorate the

biological matter. Strong acids, bases, oxidants, dehydrating agents are

corrosive. pH < 2 or pH > 12.5 is considered as highly corrosive. Substances that

corrode steel at a rate of 6.35 mm/year is also considered hazardous.

Ignitability

It is the ease with which substance can burn. The temperature at which

the mixture of chemicals, vapour and air ignite is called the flash point of

chemical substances. Contaminants are classified as hazardous if it is easily

ingnitable or its flash point is low.

Based on the above four factors the risk associated with a particular site is

determined by specifying maximum acceptable risk using risk estimation

equations (Reddi and Inyang 2000). Risk assessment provides a numerical

quantification of the probability of harm from hazardous or toxic contamination.

Risk management uses this input of risk assessment in deciding how much

regulation and corrective measure need to be taken. The corrective action is

mostly the practice of remediation of the contaminated site. The maximum

possible concentration that could lead to the maximum acceptable risk is back



calculated. If the level of concentration at a particular site is greater than the

maximum possible concentration, then it requires remediation. This approach

would clearly indicate the extent of remediation required for the contaminated

site. Appropriate remediation scheme is then selected to bring the concentration

level much less than the maximum possible concentration. Since risk

assessment and risk management is a very broad topic, it is difficult to discuss

the mathematical formulation in this course. Interested readers are requested to

go through additional literature (USEPA 1989; Asante-Duah 1996; Mohamed and

Antia 1998).

4.4 Remediation methods for soil and groundwater

Based on the toxic level of contaminants and the risk it pose to the

environment, a suitable remediation method is selected. It must be noted that the

remediation does not aim for entire decontamination. The major focus is to bring

the contamination level well below the regulatory toxic limit. This is done by

removing the toxic contaminants and/or immobilizing the contaminant that

prevents its movement through subsurface geoenvironment. The remediation

methods are broadly classified as physico-chemical, biological, electrical, thermal

and combination of these methods.

4.4.1 Physico-chemical methods

Removal and treatment of contaminated soil

One of the simplest physical methods for remediation is by removing the

contaminated soil and replacing it with clean soil. Essentially it is a dig, dump

and replace procedure. Such a method is practically possible only if the spatial

extent and depth of the contaminated region is small. The dug out contaminated

soil can be either disposed off in an engineered landfill or subjected to simple

washing as shown in Fig. 4.2.



Fig. 4.2 Soil washing for granular soils contaminated with inorganic pollutant

However, washing procedure is mostly suitable for granular soils with less clay

content and contaminated with inorganic pollutants. For clay dominated soils, a

chemical dispersion agent need to be added to deflocculate and then chemical

washing is employed to break the retention of contaminants with the clay surface.

Incineration is suggested for soils contaminated with organic pollutants. In case,

it is necessary to remove organic pollutants then certain solvents or surfactants

are used as washing agents.

The method is directly applied in situ where solvent, surfactant solution or

water mixed with additives is used to wash the contaminants from the saturated

zone by injection and recovery system. The additives are used to enhance

contaminant release and mobility resulting in increased recovery and hence

decreased soil contamination.

Vacuum extraction

This method is one of the most widely used in situ treatment technologies.

The method is cost-effective but time consuming and ineffective in water

saturated soil. The technique, as depicted in Fig. 4.3, is useful for extracting

contaminated groundwater and soil vapour from a limited subsurface depth. The

contaminated water is then subjected to standard chemical and biological

treatment techniques. Vacuum technique is also useful when soil-water is

contaminated with volatile organic compound (VOC). The method is then termed

Contaminated soil Grinding/ dispersing

and slurry preparation

Filtering (liquid solid

separation)

Polluted water for

treatment

Clean solid for reuse

To disposal Reuse of clean water



as “air sparging”. Sometimes biodegradation is clubbed with air sparging for

enhanced removal of VOC. Such a technique is then termed as biosparging.

Fig. 4.3 A schematic diagram for vacuum extraction procedure (Reddi and Inyang, 2000)

The vacuum extraction probe is always placed in the vadoze zone. The success

of the method depends on the volatilization of VOC from water into air present in

voids. An injecting medium is used to extract soil-water and/ or soil-air. When

oxygen is used instead of nitrogen as the injecting medium, it enhances aerobic

biodegradation.

Soil structure influences a lot on the passage of extracted water and

vapour and hence on the success of vacuum extraction technique. It is not only

important that the injecting medium is delivered efficiently but also the extracted

product reaches the exit with less hindrance. Granular soils provide better

passage where as the presence of clay and organic matter impedes the

transmission of both fluid and vapour. Organic matter provides high retention

leading to less volatilization. High density and water content also minimize

transmissivity. Apart from soil, the VOC properties such as solubility, sorption,

vapour pressure, concentration etc. also influence the extraction process.

Solidification and stabilization

This is the process of immobilizing toxic contaminants so that it does not

have any effect temporally and spatially. Stabilization-solidification (SS) is

Vadoze zone

Saturated zone

Nitrogen/

oxygen Nitrogen/

oxygen

Aeration Aeration

Contaminated

water

extraction

Water trap

Vapour

extraction Vacuum

Contaminated air for

treatment



performed in single step or in two steps. In single step, the polluted soil is mixed

with a special binder so that polluted soil is fixed and rendered insoluble. In two

step process, the polluted soil is first made insoluble and non-reactive and in the

second step it is solidified. SS process is mostly justified for highly toxic

pollutants. In-situ SS process is mostly influenced by the transmissivity

characteristics of the soil, viscosity and setting time of the binder. Well

compacted soil, high clay and organic content do not favour in-situ SS.

In ex-situ methods, polluted soil is first grinded, dispersed, and then

mixed with binder material. The resultant SS material need to be disposed in a

well contained landfill. It is essential that the resultant SS product does not

undergo leaching. The common binders used in practice include cement, lime, fly

ash, clays, zeolites, pozzolonic products etc. Organic binders include bitumen,

polyethylene, epoxy and resins. These organic binders are used for soil

contaminated with organic pollutants.

Chemical decontamination

This method is mostly applicable for those soils which have high sorbed

concentration of inorganic heavy metals (IHM). The first process in this method is

to understand the nature of bonding between the pollutant and the soil surface. A

suitable extractant need to be selected for selective sequential extraction (SSE)

of IHM from the soil mass. The extractants include electrolytes, weak acids,

complexing agents, oxidizing and reducing agents, strong acids etc. The use of

these extractants in single or in combination will depend upon the concentration

of IHM and nature of the soil mass.

In-situ application (as depicted in Fig. 4.4) of extractants would remove

IHM from the soil surface and enter into the pore water. The pore water is

pumped and treated (pump and treat method) on the ground. While treating the

pumped water, both extractants and IHM are removed.



Fig. 4.4 A schematic diagram for in-situ chemical decontamination

Another method is to allow the contaminated pore water to flow through a

permeable reactive barrier (PRB). Hence the placement of the barrier is

determined by the direction of flow of ground water. The material packed in the

barrier will retain IHM by exchange (sorption), complexation or precipitation

reaction. The transmission and the reaction time determine the thickness of the

reactive barrier to be provided. The material to be provided in the barrier is

influenced by the knowledge of IHM to be removed. This is mainly due to the fact

that the above mentioned reaction occurs differently when IHM is present as

single or as multiple species.

The successful use of PRB or treatment wall (TW) depends upon its

location such that majority of the contaminated groundwater flows through it. It is

essential to have a good knowledge on the hydrogeological conditions where

such barriers need to be placed. In some cases, sheet pile walls are used to

confine the flow towards the permeable barrier. Some of the materials used in

PRBs are exchange resins, activated carbon, zeolites, various biota, ferric

oxides, ferrous hydroxide etc. Hydraulic conductivity of the PRB should be

greater than or equal to the surrounding soil for proper permeation to occur. The

Ground surface

Ground water

flow direction

Row of injection wells

Extraction Permeable

reactive barrier

Contaminated

zone with

extractants and

IHM



knowledge on reaction kinetics and permeability of the barrier would determine

the thickness of the wall to be provided such that enough residence time is

achieved for the removal reaction to occur.

4.4.2 Biological methods

Remediation by biological treatment is mostly applicable for soil

contaminated with organic pollutants and the process is termed as

bioremediation. In this method, certain soil microorganisms are used to

metabolize organic chemical compounds. In the process these microorganisms

degrade the contaminant. If naturally occurring microorganisms such as bacteria,

virus or fungi is not capable of producing enzymes required for bioremediation,

then genetically engineered microorganisms would be required. At the same

time, it should be ensured that such microorganisms do not produce any

undesirable effect on the geoenvironment (such as toxins). The process of

bioremediation is dependent on reactions such as microbial degradation,

hydrolysis, aerobic and anaerobic transformation, redox reaction, volatalization

etc. An example of bioremediation is discussed in the next section where in the

process is used for the remediation of oil spill land.

4.4.3 Electro-kinetic methods

Electro-kinetic methods are popular field method for decontaminating a

particular site by using electrical principles. The procedure is more effective for

granular type of soils. Two metal electrodes are inserted into the soil mass which

acts as anode and cathode. An electric field is established across these

electrodes that produces electronic conduction as well as charge transfer

between electrodes and solids in the soil-water system. This is achieved by

applying a low intensity direct current across electrode pairs which are positioned

on each side of the contaminated soil. The electric current results in

electrosmosis and ion migration resulting in the movement of contaminants from

one electrode to the other. Contaminants in the soil water or those which are

desorbed from the soil surface are transported to the electrodes depending upon



their charges. Contaminants are then collected by a recovery system or

deposited at the electrodes. Sometimes, surfactants and complexing agents are

used to facilitate the process of contaminant movement. This method is

commercially used for the removal of heavy metals such as uranium, mercury etc

from the soil.

4.4.4 Thermal methods

Thermal methods include both high temperature (>5000C) and low

temperature (<5000C) methods and are mostly useful for contaminants with high

volatilization potential (Evangelou 1998). High temperature processes include

incineration, electric pyrolysis, and in-situ vitrification. Low temperature

treatments include low temperature incineration, thermal aeration, infrared

furnace treatment, thermal stripping. High temperature treatment involves

complete destruction of contaminants through oxidation. Low temperature

treatment increases the rate of phase transfer of contaminants from liquid to

gaseous phase there by causing contaminant separation from the soil. Radio

frequency (RF) heating is used for in situ thermal decontamination of soil having

volatile and semi-volatile organic contaminants. Steam stripping or thermal

stripping is another process useful for soils contaminated with volatile and semi-

volatile organic contaminants. It is an in situ process in which hot air, water or

steam is injected into the ground resulting in increased volatilization of

contaminants. Sometimes vacuum is applied to extract air or steam back to the

surface for further treatment. The effectiveness of this method is increased by the

use of chemical agents that are capable of increasing the volatility of the

contaminants. High cost and its ineffectiveness with some contaminants (with low

volatilization potential) make thermal method less attractive. Also, in some cases

incineration process produces more toxic gases.

4.5 Some examples of in-situ remediation

Harbottle et al. (2006) have compared the technical and environmental

impacts of taking no remedial action with those of two remediation technologies.



The main objective of this study is to verify the sustainability of remediation

technologies. The two remediation technologies evaluated in this study are

solidification/ stabilization (S/S) and landfilling. In both these methods

contaminants are contained rather than destroyed. Therefore, it is extremely

important to analyze the long-term effect to avoid any potential problems in

future. In this study, sustainable remediation project is defined as the one that

satisfies the five criteria listed as follows:

Criterion 1: Future benefits outweigh the cost of remediation.

Criterion 2: Overall environmental impact of the remediation method is less than

the impact of leaving the land untreated.

Criterion 3: Environmental impact of remediation process is minimal and

measurable.

Criterion 4: The time-scale over which the environmental consequences occur is

part of the decision-making process.

Criterion 5: The decision making process

The site selected in this study was an industrial location polluted by BTEX

(benzene, toluene, ethylbenzene and xylene) and TPH (total petroleum

hydrocarbon). About 4400 m3 of contaminated soil has been remediated. The

stabilization mix used was cement:bentonite of 2.5:1 and water:dry grout of 3.8:1.

It was found that due to S/S, groundwater pollution reduced by 98 percent and

the leachate from S/S sample was well within the limit. S/S process resulted in

the increase in strength, reduction in permeability and increase in pH of the soil.

The same quantity of contaminated soil has been landfilled at a distance of 96

km from the source. In long term, S/S has been found to perform better than

landfill and no action taken for remediation. Other advantages of S/S are low

material usage, low off-site waste disposal, potential ground improvement for

immediate re-use, and lesser impact on the local community. However, the

contaminants remain on the site which increases the level of uncertainty in long

term. In the case of landfilling, long term impacts are less due to the fact that

contaminated soil is removed from the site. The resources that need to be

mobilized for landfill are more than S/S.



Ludwig et al. (2011) have explained the use of permeable reactive barrier

(PRB) for the treatment of Cr6 in groundwater. PRB in the form of trench and fill

system, chemical redox curtain or organic carbon based biotic treatment zone

induce reduction condition for converting Cr6 to relatively immobile and non-toxic

Cr3. The most efficient trench and fill application is granular zero valent Iron (ZVI)

fillings, which rapidly converts Cr6 to Cr3. Alternatively, organic mulch and

compost has been used to initiate microbially active Cr6 reduction. However, the

use of organic matter as well as organic carbon does not have the longevity of

ZVI. The study quotes an example of ZVI based PRB installed at North Carolina

in 1996. This PRB is of 10 m depth, 0.6 m wide and 46 m long. This PRB is

found to treat groundwater containing Cr6 (approximately 15 mg/l concentration)

for more than 15 years. This study also quotes the use of chemical reducing

agent such as sodium dithionite at US department of energy, Hanford, site for

treating large Cr6 containing groundwater plume.

Asquith and Geary (2011) have compared bioremediation of petroleum

contaminated soil by three methods, namely, biostimulation, bioaugmentation

and surfactant addition. Bioremediation process depends on microbial activity for

biodegrading petroleum hydrocarbons. Since it is a natural process, it is a slow

reaction. The above mentioned three methods are used for increasing the rate of

bioremediation reaction. Biostimulation enhances the growth and activity of

microorganisms by the addition of nutrients and/or additives. Bioaugmentation is

the addition of hydrocarbon degrading microbial cultures. Surfactant addition

would enhance solubility, emulsify and disperse hydrophobic contaminants to

overcome the problem of low contaminant bioavailability. Sandy loam soil with

total petroleum hydrocarbon (TPH) > 30000mg/kg has been used to evaluate the

three methods. It was noted from this study that biostimulation with nutrients

enhanced bioremediation process. Organic amendments provided a better

bioremediation than inorganic amendments. Surfactant addition was found to

increase bioavailability of hydrocarbon and hence enhance bioremediation.

Ascenco (2009) has discussed about contaminated site characterization

and clean up based on two case studies. The first case study pertains to the



excavation and washing of soil in an industrial estate site of 0.12 km2.

Preliminary investigation of the site revealed contamination upto a depth of 6m

with TPH, volatile aromatics such as toluene, ethylbenzene and xylene. Soil was

found to be free of heavy metals. A quantitative risk assessment indicated the

need for remediation. 40000 tonnes of soil was excavated from the affected site

and subjected to soil washing. Washing has been performed in a unit with a

capacity of 70 tonnes/ hour. Washed soil has been declared safe after adequate

laboratory testing and the clean soil reused in the site. The soil has been first

homogenized and sieved. The required surfactant and extracting agents were

mixed with water and used for soil washing. The waste water which comes out

after washing has been treated and reused. Contaminated sludge and fines after

waste water treatment and oversized soil mass rejected during sieving was

transferred to landfills.

The second case study is another industrial area of 3 km2 near Lisbon.

The industrial site comprised mainly of organic and inorganic chemistry industries

producing pesticides, acid, copper, lead, zinc, iron pyrites etc. The site consists

of 52000 tonnes of hazardous sludge from zinc metallurgy and iron pyrite ashes.

The site required investigation and remediation due to the placement of an

airport in the vicinity of this site. The groundwater exhibited high levels of arsenic,

lead, mercury, cadmium, copper, zinc, cobalt. In some areas the pH was as low

as 1, which increased metal mobility. The investigations were mainly focused on

developing a conceptual site model and environmental risk analysis for defining

remediation options. The efforts are still on for this particular site.



References

1. Asante-Duah, D. K. (1996) “Management of Contaminated Site Problems”,

Lewis Publ., CRC Press Inc., Boca Raton, Florida.

2. Ascenco, C. (2009) “Contaminated site characterization and clean up-two

case studies”, NASA/c3p- 2009 International workshop on environment and

alternative energy: Global Collaboration in Environmental and Alternative

Energy Strategies” http://www.wspgroup.com/en/WSP-

Group/Sustainability/Case-Studies-3/Land- remediation (Website visited on 7-

11-2011)

3. Asquith, E. A. and Geary, P. (2011) “Comparative bioremediation of

petroleum hydrocarbon-contaminated soil by biostimulation, bioaugmentation

and surfactant addition”, 4th International Contaminated Site Remediation

Conference, Clean up 2011, Adelaide, South Australia, pp. 261-262.

4. Evangelou, V. P. (1998) “Environmental soil and water chemistry: principles

and applications”, Wiley-Inderscience, New York.

5. Harbottle, M. J., Al-Tabbaa, A. and Evans, C. W. (2006) “Assessing the true

technical/ environmental impacts of contaminated land remediation – a case

study of containment, disposal and no action”, Land, Contamination and

Reclamation, Vol. 14 (1), pp. 85-99.

6. Ludwig, R. D., Wilkin, R. T. and Su, C. (2011) “Treatment of Cr6 in

groundwater using prb systems”, 4th International Contaminated Site

Remediation Conference, Clean up 2011, Adelaide, South Australia, pp. 7-8.

http://www.cleanupconference.com/program.html (website visited on 7-11-

2011)

7. Method 1311, EPA “Toxicity characteristic leaching procedure”,

www.epa.gov/osw/hazard/testmethods/sw846/pdfs/1311.pdf (website

accessed on 7-10-2011).

8. Mohamed, A. M. O. and Antia, H. E. (1998) “Geoenvironmental engineering”

Elsevier, Amsterdam, Netherlands.



9. Reddi, L. N. and Inyang, H. I. (2000) “Goenvironmental engineering:

principles and applications”, Marcel Dekker Inc., New York.

10. US EPA (1989) “Risk assessment guidance for superfund Vol. 1. Human

health evaluation manual (Part A)”, United States Environmental Protection

Agency, Cincinnati, OH, EPA/540/1-89/002.

11. US EPA (1995) “Guidance for scoping the remedial design”, United States

Environmental Protection Agency, Cincinnati, OH, EPA/540/R-95/025.

http://www.epa.gov/superfund/cleanup/rdra.htm (website visited on 7-11-

2011).

12. US EPA. (1991) “Site characterization for subsurface remediation”, Seminar

Publication, EPA/625/4-91/026, Office of Research and Development, United

States. Environmental Protection Agency, Washington, DC.



Model Questions

1) What are the important points to be kept in mind for contamination assessment?

2) What are the processes involved in the planning of contaminated site remediation?

3) What are the important data required for planning contaminated site remediation?

4) Discuss the important physico-chemical methods for performing contaminated soil remediation.

5) Prepare a scheme for the design of permeable reactive barrier. 6) Based on the literatue, explain how to plan and design electro-kinetic

remediation. 7) Discuss case histories related to contaminated site remediation and identify

the most popular method.



Module 5

ADVANCED SOIL CHARACTERIZATION

Many a times solution to geoenvironmental problems necessitates advanced

characterization of soil. These characterization results serve as inputs for

mathematical modelling, parameterization of certain soil related functions,

verification or validation of some phenomenon, field investigation, physical

modelling of soil behaviour, indirect estimation of properties etc. While the list of

such advanced soil characterization is exhaustive due to the recent

developments in electronics and instrumentation, only some of the important and

common advanced characterizations for geoenvironmental problem are

discussed in the following.

5.1 Soil contaminant analysis

A wide variety of instruments are available for analyzing the concentration

of organic and inorganic contaminants present in the soil. In most of these

methods, the contaminant present in the soil need to be first brought into solution

form by using suitable methods. The contaminated soil is washed using water or

suitable extractants in single, multiple or sequential steps (ASTM D 3974; Reddy

and Chintamreddy 2001; Dean 2003; Maturi et al. 2008). Another process for

extracting soil contaminants into solution form is by acid digestion method

(Method 3050B, EPA). The contaminant in solution form is then analyzed using

the appropriate method for contaminant analysis such as atomic absorption

spectrometer (AAS), inductively coupled plasma mass spectrometer (ICP MS),

ion chromatograph, gas chromatograph, flame photometer, UV visible

spectrophotometer. The choice of contaminant analysis methodology would

depend upon the type of contaminant and whether single or multiple

contaminants need to be analysed. The accuracy of all these methods would

depend upon the precise calibration performed by the user. In the process of

calibration, instrument parameter is correlated to the contaminant concentration



using standard contaminant solution of known concentration. Further, for a

solution of unknown concentration, instrument parameter is measured and the

concentration determined using the calibration equation.

5.2 Electrical property of soil

The knowledge of soil electrical property of soil system (solid, liquid and

gaseous phase) is required for several applications in engineering and

geosciences. Electrical properties of soil system have multiple phases due to the

following reason (Fang and Daniels 2006): (a) Soil and water has inherent

electrical characteristics, (b) electrical energy is related to thermal and magnetic

properties and difficult to separate (c) electro-chemical interaction in soil-water

system is sensitive to surrounding environment. The important factors influencing

soil electrical properties are particle size distribution, compaction, water content,

mineral structure, mineral surface condition, characteristics of pore fluid and ion

exchange reaction. The direction of electric current is the direction of flow of ions.

The zone of electric field depends on the magnitude of electric charge and soil-

water system. The electrical property of soil is defined in terms of electrical

resistivity, conductivity, capacitance and dielectric property. Resistivity and

conductivity quantifies the flow of electric current through a medium. Electrical

resistivity is the most common method for defining electrical property of soil-

water system. There are a lot of literature that describe the use of resistivity or

conductivity for indirectly assessing water content, extent of soil contamination or

salinity, unit weight, porosity, frost depth, buried objects etc. (Fang and Daniels

2006). Capacitance is the charge storage capacity of a material. Dielectric

property defined in terms of dielectric constant (κ) implies the ability of a material

to perform as an insulator. This property is not measured but computed by Eq.

5.1.

κ = C x (d/A) (5.1)



C is the capacitance in Farad, d is the length of specimen and A is the cross

sectional area of specimen. κ is an important property that has been used

extensively for indirect correlation with different soil properties.

When the soil is fully dry the electrical resistivity is very high because

there is little interaction between the electrical charge (or energy) and ions

present in the soil. When the soil is wet, resistivity decreases and electrical

conductivity increases due to the formation of water film around soil surface.

Such a film act as a bridge between electrical charge and ions present in the soil.

Flow of electricity through soil can be due to direct current (DC) or due to

alternating current (AC) of particular frequency. The effect produced by both on

soil is different. To assess the effect of flow of alternating current in soils, it is

necessary to determine κ and electrical conductivity (σec) of the soil

corresponding to the frequency of the current (Smith-Ross 1933). This is

because these characteristics are dependent on the frequency of AC. The

density, water content of soil and frequency of AC are the important parameters

affecting electrical properties of soil under AC. The κ value for dry soil and

minerals varies between 2.8 to 2.6 for a frequency variation from 100 to 10000

kHz. As moisture content increases, the κ variation with frequency increases

considerably. For pure water, κ value is close to 80. Such a wide variation in κ

values is used for indirectly determining volumetric water content of soils.

5.2.1 Uses of electrical properties of soil

Electrical properties of subsurface are used extensively for oil and mineral

exploration, subsurface exploration, to delineate contaminated land etc. Soil

electrical properties are used for in situ soil mapping and monitoring when the

studied soil property is dependent on the mobile electrical charges in the soil. It is

used for characterizing soil morphology, develop accurate soil maps for

agricultural purposes, identify the extent of soil pollution, forensic and

environmental applications (Anatoly and Larisa 2002). The important soil

properties studied are soil salinity, texture, stone content, groundwater depth,

and horizon sequence in soil profiles (Larisa 1999). Some of the geophysical



methods measure soil electrical properties such as electrical conductivity,

resistivity and electrical potential from soil surface to a particular depth without

soil disturbance. These electrical properties are then correlated to the

appropriate soil parameters such as salinity, water content, density, porosity,

degree of saturation, permeability, swelling potential, liquefaction potential etc. by

using some empirical equation (Shah and Singh 2004, 2005; Sreedeep et al.

2004). However, the success of such methods depends upon detailed knowledge

of subsurface electrical properties and systematic procedure for data

interpretation, which is still an open area of research.

5.2.2 Measurement of electrical properties of soil

There are different types of probe and box arrangement for measuring

electrical property of compacted soil in the lab or in situ soil. Rhoades and

Schilfgaarde (1976) have used an electrical conductivity probe for determining

soil salinity based on the principle of Wenner four electrode method (Halvorson

et al., 1977). Arulanandan (1991), Rao et al. (2007) have used an impedance

analyzer to measure dielectric constant k of various soils. Fam and Santamarina

(1997) have measured dielectric permittivity of soils with a coaxial terminator

probe integrated with a network analyzer. Lee et al. (2002) have measured

capacitance of the saturated contaminated sands using impedance analyzer in

the frequency range of 75 kHz to 12 MHz. A descriptive methodology for

electrical resistivity box and probe reported by Sreedeep et al. (2004) is

discussed below.

Electrical resistivity box (ERB) consists of a perspex cubical box, 100 mm

in dimension and 10 mm thick, as depicted in Fig. 5.1, which works on the

principle of two-electrode method (Abu-Hassanein 1994). ERB can be used for

measuring electrical resistivity of disturbed and undisturbed soil samples in all

the three dimensions and can also be used for layered soil deposits. Each face of

the ERB is provided with three brass screw electrodes of length 12.5 mm and

diameter 2.5 mm, which can be screwed into the compacted soil sample. This

arrangement insures proper contact of the electrode with the soil. A known AC



voltage V is applied between the two electrodes mounted on the opposite faces

of the box and the current I passing through the medium is measured using a

digital multimeter. Hence, the resistance RERB and electrical resistivity ERB

offered by the medium can be determined by Eqs. 5.2 and 5.3, respectively.

RERB=V/I (5.2)

ERB = a.RERB (5.3)

a is a constant that depends on the geometry of the box, which can be

determined by measuring resistance of the standard KCl and NaCl solutions of

known electrical resistivity.

Electrical resistivity probe (ERP) is more appropriate for measuring the

soil electrical resistivity in situ. As depicted in Fig. 5.2, four annular copper rings,

which act as electrodes are mounted on an ebonite rod of 16 mm outer diameter,

at a center-to-center spacing of 25 mm. The two outer electrodes are the current

electrodes while the inner electrodes are used for measuring the voltage. For

sufficient insertion and ensuring perfect contact of the ERP with the soil mass, a

100 mm long and 15 mm diameter hole is created in soil with the help of a

dummy rod. AC of intensity I is applied to the outer electrodes and the potential

drop V across the two inner electrodes is measured. Soil resistance (RERP) can

be obtained, which can be correlated to the resistivity ERP using an appropriate

parameter b that depends on the geometry of the probe, as discussed above for

ERB.



12 c

m

3cm

3

cm

3cm

12 c

m

12 cm

3cm

Electrodes

Figure not to scale

Fig. 5.1 A conceptual electrical resistivity box (Sreedeep et al. 2004)

Fig. 5.2 A conceptual electrical resistivity probe (Sreedeep et al. 2004)

Ebonite rod

Stainless steel cone

C: Current electrode (1,4)

V: Voltage electrode (2,3)

C

V

C

V

1 2

4 3

C

V

V

C



5.3 Thermal property of soil

Thermal property of soil are of great importance in several engineering

projects where heat transfer takes place through the soil. These projects include

underground power cables, high level nuclear waste repository, hot water or gas

pipes and cold gas pipelines in unfrozen ground, agriculture, meteorology and

geology. The thermal properties of soil include thermal conductivity (K= 1/ρ), ρ is

the thermal resistivity, thermal diffusivity (D), and heat capacity (C). K is defined

as the amount of heat passing in unit time through a unit cross- sectional area of

the soil under a unit temperature gradient applied in the direction of heat flow.

Considering a prismatic element of soil having a cross-sectional area A at right

angles to the heat flow q, then K is defined as

2 1

qK=

A(T -T )/l (5.4)

Where, l is the length of the element, T1 and T2 are temperature where T2>T1.

The heat capacity C per unit volume of soil is the heat energy required to raise

the temperature of unit volume of soil by 1°C. It is the product of the mass spe-

cific heat c (cal/g °C) and the density ρ (g/cc). Thermal diffusivity is the ratio of

thermal conductivity to specific heat. It indicates how materials or soil adjust their

temperature with respect to the surroundings. A high value of the thermal

diffusivity implies capability for rapid and considerable changes in temperature.

5.3.1 Factors influencing soil thermal resistivity

Fine grained or cohesive soil and peaty soils exhibit high ρ than granular

soil. Sand with quartz as the principal constituent has low ρ. The type of clay

minerals present in soil also influences ρ. Expansive clay minerals such as

montmorillonite would cause the soil particles to be forced apart during swelling

action when it comes in contact with water, thereby increasing ρ. Well-graded

soils conduct heat better than poorly graded soils because the smaller grain can

fit in the interstitial positions between the larger grains thus increasing the density



and the mineral-to-mineral contact. The shape of the soil particles determines the

surface contact area between particles which affects the ability of the soil to

conduct heat. ρ increases with decreasing particle size due to reduced surface

contact between adjacent particles.

The density of soil has an important influence on ρ. The presence of air

with its high ρ decreases the overall ρ of the soil as compared to that of its solid

components. Therefore, a well compacted soil will have low ρ due to low total

void volume and better contact between the solid grains. When water is added to

the soil, it tends to distribute itself in a thin film around solid grain of the soil. This

water film provides a path for the heat and hence bridges the air gap between the

solid particles. Additional water, over and above that required for film formation,

serves to fill voids which were initially occupied with air. Since ρ of air is much

higher than water, inclusion of water in soil would considerably decrease ρ of

soil. The moisture content also has an indirect influence on ρ since higher density

can be achieved by adding water to the soil. The ρ of soil is also influenced by

temperature, because each of the constituents has temperature dependent ther-

mal properties. The ρ of all crystalline minerals increase with increasing

temperature, however, the ρ of water and gases exhibit the inverse effect.

5.3.2 Measurement of soil thermal resistivity (ρ)

Thermal resistivity (ρ) measurement of soil could be categorized as

steady state and transient state methods. For steady-state method, a known

thermal gradient is established in soil specimen with definite shape and length

and ρ can be determined based on recording the heat flow through the soil. In

transient-state method, known time-rate of energy is applied into soil

specimen and the corresponding temperature change with time is recorded

and analyzed to determine ρ. The thermal gradient across the soil sample

being tested may induce appreciable moisture migration in unsaturated soils

there by changing the properties it is attempting to measure. Therefore,

selection of appropriate method of ρ measurement should be based on the



condition of the materials. Some of the methods employing steady state and

transient measuring principle are discussed below.

5.3.2.1 Steady state method

In this method, the soil sample being tested should be in steady state

when the measurements are made. Attainment of such a state is time consuming

after the initial temperature difference has been applied. Also, there is possibility

of moisture changes by the time the steady state is reached. The methods based

on steady state are described below:

Guarded hot plate method

The most important steady state method for measuring the ρ of soils is the

guarded hot plate (GHP) test as depicted in Fig. 5.3 (ASTM C 177). As shown in

figure, two identical specimens are placed above and below a flat-plate main

heater unit which is surrounded by an outer guard heater. The guard eliminates

horizontal heat losses and causes heat from the main heater to flow vertically up

or down through the test specimen. Liquid-cooled heat sinks are placed adjacent

to the outer surfaces of the specimens. A certain temperature drop is obtained

across each specimen of certain thickness. K of the specimen material is

calculated from Eq. 5.5.

1/ ρ =Q L

K=A ΔT

(5.5)

Where, Q is the heat flow through soil, A is the area of soil specimen, L is the

length of heat flow, and T is the temperature drop. The GHP test is time

consuming and only suitable for laboratory use.



Fig. 5.3 Schematic diagram of the guarded hotplate method for determining thermal conductivity (ASTM C 177)

Heat flux meter

The ρ of soil can be determined by measuring temperatures at two

points and the heat flows between these points with the help of a heat flux meter.

The heat flux meter is a thin plate of suitable material with known ρ, and installed

with thermal couples on both side. The temperature difference (gradient) between

both sides multiplied by the ρ of the plate gives the heat flux per unit area across

the plate. This method is described in detail in ASTM C 518. The heat flux meter

also requires long measuring time. The contact between the plate and the

specimen need to be perfect to eliminate the influence of contact thermal

resistance. Therefore, a contact pressure needs to be applied, which may alter

the soil state (density or volumetric water content).

5.3.2.2 Transient state method

In transient method, temperature of the soil varies with time. Such

methods are less time intensive and can be easily performed than the steady

state methods. Thermal probe and point-source method based on transient state

method are discussed below.

Thermal probe method

The thermal probe or needle is a rapid and convenient method for

measuring ρ of soils in situ or in the laboratory. The theory of the probe method is

based on the theory of the line heat source placed in a semi-infinite,

Cold plate

Heater

Cold plate

Soil specimen

Soil specimen

Thermocouple



homogeneous and isotropic medium. This method is described in detail in ASTM

D 5334. The heat flowing from a line heat source through a medium of thermal

diffusivity must conform to the following equation:

2

2

x

TD

t

T

(5.6)

T is the temperature at time t in x direction. For cylindrical coordinates Eq. 5.6

becomes:

r

T

r

1

r

TD

t

T2

2

(5.7)

Where, r is the radial distance from the line source. Assuming heat is produced

from t=0 at a constant rate q per unit length of probe, the solution of Eq. 5.7 is

given by Eq. 5.8.

∆T=

4Dt

rEi

K

1

4π

q 2

(5.8)

Where, Ei (-x) is an exponential integral and K or (1/ρ) is thermal conductivity.

The apparatus for thermal probe method shall consist of the following:

1. Thermal needle probe: A device that creates a linear source and incorporates a

temperature measurement element (thermocouple or thermostat) to measure

variation of temperature at a point along the line.

2. Constant current source: A device to produce a constant current.

3. Thermal read out unit: A device to produce a digital read out of temperature in

0C.

4. Voltage-Ohm-Meter (VOM) - A device to read voltage and current to the

nearest 0.01 V and ampere.

5. Stopwatch measuring time to the nearest 0.1 s for a minimum of 15 min.

6. Equipment capable of drilling a straight vertical hole having a diameter as close

as possible to that of the probe and to depth at least equal to the length of the

probe.



This method can be utilized on both undisturbed and remolded sample.

For undisturbed sample, thermal probe shall be pushed into the pre-drilled hole

on dense specimens or directly inserted into soft ones. The length of the soil

sample should be large enough to accommodate the probe length. During the

measurement, a steady current is applied while the temperature is recorded as a

function of time. Temperature is then plotted as a function of time on semi-log

graph. A straight line is drawn through points that exhibit linear trend (pseudo

steady state portion). K can be expressed in terms of the slope of this line:

qT= ln

4t c

K (Jackson and Taylor, 1965) (5.9)

slope= q

4 K

Where q=heat flow rate (q= i2.r’), t is the time, T is the temperature, K is the

thermal conductivity of soil, I is the current applied, r’ is the resistance per unit

length of probe.

Point-source method

This method eliminate the disadvantages of thermal probe due to large-

sized samples in which controlling water content becomes difficult, thermal

resistance produced between the soil sample and the probe inserted, and

movement of water occurring due to high temperature. This method is comprised

of recording the voltage variations of the thermistor and variable resistor in the

measuring circuit over a period of time. The variations in temperature and heat

production with time for the thermistor are calculated from the measured voltage

values. Then, the thermal diffusivity of sample is determined by inverse analysis

based on the Eqs. 5.6 and 5.10 (Chu 2009).

KD=

γc (5.10)

Where, K is Thermal conductivity, D is thermal diffusivity, c is Specific heat, T is

temperature and γ is density of soil.



5.4 Water content and permeability measurements

5.4.1 Volumetric water content sensors

Determination of gravimetric water content, w, is simple and employs

direct methods such as oven drying, sand bath method, alcohol method, infrared

lamp method and calcium carbide method (IS 2720 part II: 1973). However,

gravimetric water content does not provide instant measurement of water content

and cannot be monitored continuously. Such requirements are common in

geoenvironmental projects where water content has to be monitored

continuously. This can be done by measuring volumetric water content (), which

is defined as the ratio of volume of water to the total volume of soil. is one of

the vital parameter correlated to different soil properties such as compaction

state, permeability, seepage, soil suction, volume change etc. Its determination is

mainly based on indirect techniques such as electrical resistivity, capacitance

and dielectric property of the soil mass (Topp et al., 1980). The fundamental

approach of measurement is that electrical properties such as capacitance,

dielectric constant, resistivity is strongly related to the soil water content. A

calibration equation is developed between any of the electrical property and

known volumetric water content of the soil. The same calibration equation can be

used to monitor the variation of by measuring electrical properties.

There are different resistivity, capacitance, dielectric, probes available in

the market such as time domain reflectometry (TDR), frequency domain

reflectometry (FDR), theta probes for insitu measurement of . As an example,

two low cost probes EC-5 and EC-TE (Decagon Devices, Inc., USA) as shown in

Figs. 5.4 and 5.5 are explained below. The value of κ for water is close to 80, dry

soil minerals is around 4 and for air it is 1 (Topp et al., 1980). Therefore, κ of soil

medium is highly sensitive to changes in water content. κ is dependent on the

capacitance property or the charge storage property of the soil mass. The probe

measures the capacitance property which is converted to κ. is determined

based on κ value.



Fig 5.4 Two prong EC-5 probe details

Fig. 5.5 Details of EC-TE probe

The probe comprises of an oscillator working at a particular frequency, which

generates an electromagnetic (EM) field. The EM field charges the soil around

the probe. This stored charge is measured using copper traces provided on the

C: Connection cable

P: Prongs

B: Body of the probe

C

P

B

P

C: Connection cable

P: Prong

P1: Gold coated Prongs

B: Body of the probe

C

P

B

P1 P1



prongs and is proportional to κ and . It must be noted that the electromagnetic

field thus produced by the probe decreases with distance from the probe surface

and has little or no sensitivity at the extreme edges of the probe. The stored

charge thus measured would confine to a zone of influence of 5 cm measured

from the edge of the prong.

5.4.2 Guelph permeameter

This is a handy instrument for measuring insitu permeability of natural and

compacted soil for hydrogeological investigations at shallow depth. As depicted

in Fig. 5.6, Guelph permeameter consist of a reservoir which stores and releases

water into a hole (termed as well) under constant head. The constant head is

maintained with the help of Marriot bubble principle. There are two reservoirs,

one outer tube and smaller inner tube. For high permeable soil, bigger outer

reservoir is used and for low permeable soil smaller inner reservoir is used. The

scale attached to the inner reservoir is used to measure rate of fall of water in the

reservoir. When air tip is raised, water flows out of the reservoir into the bore hole

(or well). Water height in the well is established based on the height of air inlet

tube tip. This height (constant head causing flow) can be set and read using well

height indicator connected with the head scale. The determination of permeability

is done by either single head or double head method by the procedure discussed

in Ref. 29.



Fig. 5.6 Guelph permeameter (Ref. 31)

5.4.3 Tension Infiltrometer (TI)

Tension Infiltrometer (TI) as depicted in Fig. 5.7 is a handy instrument for

measuring infiltration characteristics and permeability of nearly saturated soil. It

consists of three major components namely, reservoir assembly, infiltrometer foot

assembly and Marriot bubbler assembly. In tension infiltrometer, water is allowed

to infiltrate the under lying soil at a slower rate than the infiltration rate that would

have been established when water is ponded on the soil surface. This is

accomplished by maintaining a small negative pressure (maximum tension of 20

cm) maintained with the help of Marriot bubbler on the water as it moves out of

the infiltrometer disc into the soil. Water can only flow out of the infiltrometer disc

at the base and infiltrate into the soil. The amount of infiltration is measured

based on the fall of water level in the reservoir. Saturated permeability is

determined indirectly based on the infiltration characteristics (Zhang 1997).



.

Fig. 5.7 Tension Infiltrometer (Ref. 31)

5.4.4 Minidisk infiltrometer

Mini disc infiltrometer as shown in Fig. 5.8 is similar to the working of

tension infiltrometer but with a lower range of suction applied to the infiltrometer

disc (Ref. 30). Since the infiltrometer is small in dimension (total length of the

infiltrometer is 32.7 cm), it can be used for measuring infiltration and near

saturation permeability in lab and field. The upper and lower chambers of the

infiltrometer are both filled with water. The top chamber controls the suction

head. The lower chamber contains the volume of water that infiltrates into the

soil. The minidisc infiltrometer is tension infiltrometer and it can measure the

hydraulic conductivity in the unsaturated medium (close to near saturation) for

adjustable suction ranging from 0.5 cm to 7 cm. At time zero, the infiltrometer is

placed on the soil surface. The volume of water that infiltrate into the ground has

been recorded as a function of time, based on which infiltration and permeability

characteristics is determined.



Fig. 5.8 Minidisk Infiltrometer (Ref. 32)

5.5 Ground Penetrating Radar for site evaluation

Ground penetrating radar (GPR) is a non destructive and non intrusive

geophysical method to measure electrical properties at various depth of

subsurface. It works by generation, transmission, propagation, reflection and

reception of discrete pulses of high frequency (1 MHz to 1 GHz) electromagnetic

energy. The depth of imaging would depend on the frequency of electromagnetic

wave. A lower frequency is essential for imaging larger depth where as shallow

imaging requires higher frequency. The fundamental issue with its application is

the efficiency in processing the electrical data to interpret subsurface information

accurately. As the electromagnetic wave propagates downwards it experiences

materials of differing electrical properties, which alter its velocity. If velocity

changes are abrupt with respect to the dominant radar wavelength, some energy

is reflected back to the surface. The reflected signal is detected by the receiving

antenna. In systems with a single antenna, it switches rapidly from transmission

to reception. The time between transmission, reflection and reception is referred



to as two-way travel time (TWT) and is measured in nanoseconds. Reflector

TWT is a function of its depth, the antenna spacing (in systems with two

antennae), and the average radar-wave velocity in the overlying material. GPR is

used to detect the underground buried objects such as pipes, beams, tunnels,

buried walls, salinity, water content, ground contamination, depth of ground water

table, and properties of ground water. GPR applicability in certain type of soils

such as clay is a subject of debate due to the high attenuation of electromagnetic

waves. A lot of research is still required for exploring the full utility of GPR for

efficient subsurface investigation.

5.6 Introduction to geotechnical centrifuge modelling

A geotechnical centrifuge is used to conduct physical modeling of

geotechnical problems for which gravity is the primary driving force. These

studies include determination of settlement of embankments, stability of slopes

and tunnels, flow and contaminant migration characteristics of soil (Cooke and

Mitchell 1991; Singh and Gupta 1999). The basic principle of centrifuge

modelling is that when a soil sample model of (N times smaller than its prototype)

is subjected to N times the acceleration due to Earth’s gravity (Ng) by

centrifugation, it results in identical self-weight stresses at homologous points in

the model and the prototype as depicted in Fig. 5.9 (a) (Taylor 1995). In the

figure, ρ is the mass density of soil, g is the acceleration due to gravity, ω is the

angular velocity of rotation in rad/sec, re is the effective radius represented by Eq.

5.11, where rt is the distance from axis of rotation to the top of the soil sample. It

can be clearly seen that the stress in prototype and N-g model is identical where

as the geostatic stress scale down by N in a 1-g model.



Fig. 5.9 Basic principle of the centrifuge modelling

3

Lrr m

te (5.11)

5.6.1 Similitude in centrifuge modeling

The results of centrifuge model, which is used to understand a mechanism

or process, can be extrapolated to corresponding prototype condition using

suitable scaling laws. To formulate these scaling laws, three types of similitude

conditions have to be considered, as discussed in the following.

Geometrical similarity

This can be achieved if there is a constant ratio of length, L, between the

homologous points in the model and the prototype.

Lm/Lp = = 1/N (5.12)

where subscripts m and p correspond to the model and its prototype,

respectively, and is the scale factor.

(a)

Lm Lp=N.Lm

Prototype (1-g)

Model (N-g)

Lm

re

Axis of

rotation

N.g = re.

2

(b)

Lp

Stress=ρgLp

1-g model N-g model

Stress=ρg(Lp/N) Stress=ρNg(Lp/N)

= ρgLp



Kinematic similarity

The model and the prototype are said to be kinematically similar if their

ratio of velocity, v, and acceleration, a, are constant. Hence:

vm/vp = (5.13)

am/ap = n (5.14)

where and n are constants.

Dynamic similarity

This similarity can be ensured if there is a constant ratio between the

forces in the model and its prototype.

Fm/Fp = (5.15)

where F is the force and is a constant.

5.6.2 Modeling of mass in Ng model

Mass M = ρ.V (5.16)

ρ is the density and V is the volume of soil mass.

pp

mm

p

m

Vρ

Vρ

M

M (5.17)

Subscripts m and p stands for model and prototype, respectively.

If the material used in model and prototype are same, then the mass density will

be same (ρm = ρp).

3

p

m

p

m

p

m

N

1

L

L

V

V

M

M

(5.18)

Unit weight γ = ρ.g (5.19)

pp

mm

p

m

gρ

gρ

γ

γ (5.20)

For Ng model, ρm = ρp and gm = Ngp



Therefore, γm = N γp (5.21)

5.6.3 Scale factor for body forces or geostatic forces

F = Mg (5.22)

3

pp

mm

p

m

N

1

gM

gM

F

F for 1-g model where gm = gp (5.23)

2

pp

mm

p

m

N

1

gM

gM

F

F for N-g model where gm = Ngp (5.24)

5.6.4 Potential of geotechnical centrifuge for geoenvironmental

project

Geotechnical centrifuge has potential application in geoenvironmental

problems such as fluid and contaminant transport that is mostly governed by

seepage forces. The permeability of high compacted liners is very low.

Therefore, determination of permeability and contaminant transport parameters

(advective-dispersive) is extremely time consuming with normal 1-g modelling.

For establishing advective-dispersive contaminant transport parameters, it is

essential that the contaminant solution flows through the soil column as

discussed in module 3. This is time intensive even for a small soil column. Using

geotechnical centrifuge for simulating seepage can considerably reduce the time

required for experimentation as discussed below.

Seepage force (SF) = i. γw.V (5.25)

= (v/k).W

V is the volume of soil mass, i is the hydraulic gradient, v is the discharge

velocity, k is the hydraulic conductivity or permeability, γw is the unit weight of

water and W is the weight of seepage water.

p

m

m

p

p

m

p

m

W

W.

k

k.

v

v

SF

SF (5.26)

m

p

m

p

p

m

p

m

t

t.

N

1

t

t.

L

L

v

v (5.27)



t is the time.

k can be represented by Eq. 5.28.

μ

.gK.ρ=k w

(5.28)

where w is the fluid density, is the dynamic viscosity of the fluid, and K is the

intrinsic permeability. If the same pore fluid and the soil are used in the model

and prototype, then Eq. 5.28 can be written as:

Ng

g

μ

g.ρ.K

μ

g.ρ.K

k

k

p

m

p

m

p

m

(5.29)

km=N.kp (5.30)

2

p

m

p

m

p

m

N

1

g

g.

M

M

W

W (5.31)

Substituting Eqs. 5.27, 5.30 and 5.31 in Eq. 5.26, and considering seepage force

as a body force with scale factor represented by Eq. 5.24, we get

2

m

p

2

p

m

N

1.

N

1.

t

t

N

1

N

1

SF

SF (5.32)

2

p

m

N

1

t

t (5.33)

The above derivation clearly indicates that the seepage phenomenon is

accelerated at N-g due to increase in velocity of flow. The time for seepage in

model is reduced by 1/N2. Therefore, permeability of compacted liner can be

determined in short interval of time with the help of geotechnical centrifuge model

and the prototype permeability can be obtained by using the scale factor derived

above. The advective-dispersive transport parameters can also be established in

relatively short duration due to accelerated seepage in geotechnical centrifuge.



References

1. Abu-Hassanein, Z. S. (1994) “Use of electrical resistivity measurement as a

quality control tool for compacted clay liners”, M. S. Thesis, University of

Wisconsin, Madison.

2. Anatoly, P. and Larisa, P. (2002) “Electrical field and soil properties”, 17th

WCSS, August, Thailand, pp. 1558-1-1558-11,

www.landviser.com/.../17WorldCongress%20of%20Soil%20Science.pdf,

(website accessed on 18-11-2011).

3. Arulanandan, K. (1991) "Dielectric method for prediction of porosity of

saturated soil, Journal of Geotechnical Engineering, ASCE, Vol. 117, No. 2,

pp. 319-330.

4. ASTM C 177 (2010) “Standard test method for steady-state heat flux

measurements and thermal transmission properties by means of the guarded-

hot-plate apparatus”, Annual Book of ASTM Standard, ASTM International,

West Conshohocken, USA.

5. ASTM C 518 (2010). “Standard test method for steady-state thermal

transmission properties by means of the heat flow meter apparatus”, Annual

Book of ASTM Standard, ASTM International, West Conshohocken, USA.

6. ASTM D 3974-81 (1994) “Standard practices for extraction of trace elements

from sediments”, Annual Book of ASTM Standards, 04.08, ASTM

International, West Conshohocken, PA, USA.

7. ASTM D 5334 (2000) “Determination of thermal conductivity of soil and soft

rock by thermal needle probe procedure”, Annual Book of ASTM Standard,

ASTM International, West Conshohocken, USA.

8. Chu, C-A. (2009) “Measurement and modeling for thermal conductivity of

geomaterials”, Ph. D. Thesis submitted to Department of Civil Engineering,

National Central University, Taiwan. URL: thesis.lib.ncu.edu.tw/ETD-db/ETD-

search-c/getfile?URN...pdf (website visited on 6-4-2012).



9. Cooke, B. and Mitchell, R. J. (1991) “Physical modelling of a dissolved

contaminant in an unsaturated sand”, Canadian Geotechnical Journal, Vol.

28, pp. 829-833.

10. Dean, J. R. (2003) “Methods for environmental trace analysis”, John Wiley

and Sons Ltd., England.

11. Fam, M. A., and Santamarina, J. C. (1997) “A study of consolidation using

mechanical and electromagnetic waves”, Geotechnique, Vol. 47, No.2, pp.

203-219.

12. Fang, H-Y. and Daniels, J. L. (2006) “Introductory geotechnical engineering:

An environmental perspective”, Routledge, Taylor and Francis, New York,

USA.

13. Halvorson, A. D., Rhoades, J. D. and Reule, C. A. (1977) “Soil-four electrode

conductivity relationships for soils of the northern great plains”, Soil Science

Society of America Journal, Vol. 41, pp. 966-971.

14. IS 2720, Part II, (1973) “Methods of Test for Soils: Determination of water

content”, Indian Standards Institute, New Delhi, India.

15. Jackson, R. D. and Taylor, S. A. (1965) "Heat transfer. in methods of soil

analysis. Part 1. Agronomy, Vol. 9 (Black C A ed.), pp. 349-356.

16. Larisa P. (1999) “Electrical properties of soils”, Ph. D. thesis submitted to the

department of renewable resources, University of Wyoming, Laramie, WY.

17. Lee, J., Oh, M., Park, J., Ahn, K. H. and Kim, H. (2002) “Application of

dielectric constant for estimating moisture content and contamination in

sand”, Second Japan-Korea Joint Seminar on Geoenvironmental

Engineering, pp. 75-82.

18. Maturi, K., Khodadoust, A. P. and Reddy, K. R. (2008) “Comparison of

extractants for removal of lead, zinc and phenanthrene from a manufactured

gas plant field soil”, Practice Periodical of Hazardous, Toxic, and Radioactive

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19. Method 3050B, EPA “Acid digestion of sediments, sludges and soils”,

http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/3050b.pdf (website

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20. Rao, B. H., Bhat, A. M., and Singh, D. N. (2007) “Application of impedance

spectroscopy for modeling flow of ac in soils”, Geomechanics and

Geoengineering: An International Journal, Vol. 2, No. 3, pp. 197-206.

21. Reddy, K. R. and Chintamreddy, S. (2001) “Assessment of electrokinetic

removal of heavy metals from soils by sequential extraction analysis”, Journal

of Hazardous Material, Vol. 84, No. 2-3, pp. 279-296.

22. Rhoades, J. D. and Schilfgaarde, J. V. (1976) “An electrical conductivity

probe for determining soil salinity”, Soil Science Society of America Journal,

Vol. 40, pp. 647-650.

23. Shah, P. H. and Singh, D. N. (2004) “A simple methodology for determining

electrical conductivity of soils”, Journal of ASTM International, Vol. 1, No. 5,

published on line, 11 Pages.

24. Shah, P. H. and Singh, D. N. (2005) “Generalized Archie’s law for estimation

of soil electrical conductivity”, Journal of ASTM International, Vol. 2, No. 5,

published on line, 20 Pages.

25. Singh, D. N. and Gupta, A. K. (2000) “Permeability modeling in a small

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26. Smith-Ross, R. L. (1933) “Electrical properties of soils for alternating currents

at radio frequencies” Proceedings of the Royal Society of London. Series A,

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electrical resistivity using a resistivity box and a resistivity probe” Journal of

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Geotechnical Centrifuge Technology, Taylor (ed.), Blackie Academic and

Professional, Glasgow, pp. 19-33.

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determination of soil water content: Measurements in coaxial transmission

lines”, Water Resour. Res. Vol. 16, pp. 574–582.



30. Zhang, R. (1997) “Determination of soil sorptivity and hydraulic conductivity

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31. http://www.soilmoisture.com/operating.html (website visited on 4-1-2012).

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tension-infiltrometer/ (website visited on 4-1-2012).



Model Questions

1. Prepare a review on different methods of soil contaminant analysis and clearly list its limitations.

2. The concentration of contaminant sorbed on the soil need to be determined. What are the different single and sequential procedures for extraction of contaminants from soil?

3. Based on the available information in literature, try to device a scheme for measuring electrical and thermal property of soil.

4. What are the uses of measuring electrical property of soil? 5. What is the difference between calibration and validation procedure? 6. Discuss about the dielectric and electrical properties of soil-water-

contaminant system and its important features. 7. Explain steady state and transient methods for measuring thermal properties

of soil. 8. What is application of thermal property of soil? 9. What are the factors influencing thermal and electrical property of soil? 10. What are the various methods used for measuring volumetric water content of

soil? 11. From the available literature, prepare the procedure for measuring

permeability using Guelph permeameter, tension and minidisk infiltrometer. 12. What are the different modeling approaches in geotechnical and

geoenvironmental engineering? Discuss the relative merits and demerits of each method.

13. What are the different geophysical methods for subsurface investigation/ 14. Explain the principle and working of ground penetrating radar for delineating

subsurface contamination. 15. Explain the philosophy of accelerated physical modeling and how the stress

similitude is achieved. 16. With respect to permeability of soil, demonstrate mathematically how

accelerated physical modeling is useful in studying any seepage induced phenomenon.

17. Suggest and justify a less time consuming procedure in the lab for obtaining advective-dispersive contaminant transport parameters for a compacted bentonite soil layer

18. A falling head permeability test is conducted in centrifuge. The details of falling head test is as follows: Area of stand pipe is 0.28 cm2. Area of soil column is 80 cm2. Length of soil column is 10 cm. There is a change in head from 90 cm to 84 cm for a time of 15 minutes. The centrifuge is rotated at 700 RPM. Effective radius is 50 cm. Determine prototype permeability, prototype length, model velocity and prototype velocity, prototype seepage velocity. (report all results in SI and time in seconds). Weight of wet soil sample is 1500 g and after oven drying the weight reduced to 1200 g. Specific gravity is 2.45. What will be the time taken in days if the same test is conducted at 1g.

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