expansive soils and stabilization

Expansive Soils and Stabilization

Abstract

Expansive soils pose a potential threat to the structures. They have the ability to

generate tremendous pressure on the structures such as concrete foundations and

pavements. Experiments show that up to 15,000 pounds per square feet pressure can

be generated by expansive soils that is enough to create problems to foundations. This

document highlights the mechanisms involved in the formation of expansive soils. It

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Table of content

Content Page

Abstract 2

Introduction 3

Description 3

Table 1, Figure 1 & Figure 2 7

Engineering Issues 8

Stabilization and Remedial Measures 9

Case Histories 11

Conclusion 12

References 13


explains about the details of clay minerals such as Smectite family, other silicate

minerals and soils rich with sulfate salts. The behavior of these minerals with the

addition or removal of moisture that causes expansion and shrinkage respectively.

It explains about the complications faced by the engineers and damages caused by this

type of soils throughout the world including heaving, differential settlement,

subsidence, development of fissures in the structures.

It presents the remedial measures used in the industry for stabilization of such soils and

to mitigate the issues that can be created by such soils.

Damage caused by expansive soils in different countries and a case study related to the

settlement of a Breakwater in China which was constructed on soft clay has also been

discussed.

Introduction

Expansive soils could be defined as soils whose volume changes with introduction of

moisture i.e. swell or shrinkage with increase or decrease in moisture content

respectively. Expandable soils, expansive clays, shrink-swell soils and heavable soils are

some of the synonyms used for these soils. The more water expansive soils absorb, the

more their volume increases. Expansion of ten or more percent is not uncommon. This

change in volume can exert enough force on a building or other structure to cause

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damage. This type of soil will also shrink when they dry out. This shrinkage can remove

support from the foundations causing subsidence. It can also develop fissures. These

fissures facilitate the deep penetration of water when moist conditions or runoff occurs.

The spontaneous shrinkage and swelling process can place repetitive stress on

structures.

Description

The expansion potential of any particular expansive soil is determined by the percentage

and type of clay in the soil. Clay particles are very tiny and their shape is determined by

the arrangement of constituent atoms which form clay crystals. Clay is a silicate mineral

and is composed mainly of Silicone, Aluminum and Oxygen. Silicon atoms are positioned

in the center of a pyramid structure called tetrahedron with one Oxygen atom

occupying each of the four corners. Aluminum atoms are situated in the corner of an

octahedron with an Oxygen atom occupying each of the four corners. Because of

electron sharing, the Silicon tetrahedrons link together with one another to form thin

tetrahedral sheets. Aluminum octahedrons also link in same pattern. The actual clay

crystals are a composite of Aluminum and Silicon sheets which are held together by

intra-molecular forces. The presence and abundance of some other elements or ions i.e.

hydrogen, sodium, calcium, magnesium, sulfur, etc can impact the composition and

behavior of the clay minerals.

To understand the concept of swelling or shrinkage, sound knowledge regarding the

formation and constituents of expansive clays is needed. For a group of prominent and

highly expansive clay minerals called Smectites, an octahedral sheet is sandwiched

between two tetrahedral sheets to create the mineral structure. Names and mineralogy

of Smectite family minerals are given in table 1.

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A water molecule consists of two hydrogen atoms sharing electrons with a single oxygen

atom. The water molecule is electrically balanced but within the molecule, the offsetting

charges are not evenly distributed. The two positive charged hydrogen atoms are

grouped together on one side of the larger oxygen atom. As a result water molecule

itself behaves like an electrical “dipole” because it has both positive charge on one side

due to two hydrogen atoms as well as a negative charge on the opposite side due to

bare oxygen atom. The electrical charge on water molecule enables it to interact with

other charged particles. The mechanism by which water molecules become attached to

the microscopic clay crystals is called “adsorption”.

In well drained soils, due to high quantity of oxygen these sheets are combined

together through strong hydrogen bond to form Kaolinite and due to this reason

Kaolinite is not more prone to expansion as it has less affinity for water-dipoles.

Montmorillonate, a member of Smectite family is considered one of the most notorious

mineral to expansion (Wyoming Multi-Hazard Mitigation Plan, 2011) because it is

formed in water deficient areas by isomorphous substitution of magnesium or ferrous

iron for aluminum. Elements in Montmorillonite are bound together through weak

Vander Wall’s forces resulting of having electrical charge on its molecule. Water

molecules called “dipoles” also contain electrical charges so it is easy for

Montmorillonite to adsorb very large amount of water molecules between its crystalline

sheets and therefore has a large shrink-swell potential.

The type of soil consisting of Montmorillonite behaves normally under load unless

moisture content increases or decreases in the soil which consequently causes swelling

or shrinkage of the soil. These types of soils are usually found in arid regions due to the

loss of moisture content by evaporation. This prolonged duration of dryness causes an

increase in the area to be affected by the introduction of water. Some other minerals

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like Bentonite, Vermiculite, Attapulgite, Nontronite, Illite and Chlorite also possess

swelling characteristics. There are some sulfate salts that will expand with changes in

temperature. When a soil consisting of large amount of expansive minerals it has

potential of significant expansion and vice versa.

According to Y. Du et al. (1999) dry density, material composition and fabric of the soil

also play an important role the swelling-shrinkage behavior of the soil. Naturally

occurring undisturbed soil develops a certain structure and bond between soil particles

which prevent the occurrence of swelling in the soil in comparison to samples in which

such structural orientation pattern in disturbed due to molding and application of

compressive forces on it. This can be a reason that the fabric of soil does affect the rate

of swelling in the soil.

Soil Engineers or Materials Engineers who identify expansive soils will collect

representative samples at the jobsite and return them to the laboratory for testing. The

tests for calculating percentage of fine particles in a particular sample will be carried

out. If over 50 % of the particles in a sample are able to pass through a number 200

sieve (0.075 mm sieve) is classified as either silt or clay or combination of both. Silt

particles are products of mechanical erosion and could contain mineralogy same as sand

particles. Clay particles are products of chemical weathering and are characterized by

their sheet structure and composition. A significant presence of clay minerals in a

sample can indicate a possible expansive soil problem.

“Atterberg limits” is another test by which engineers can know the potential expansion

property of a soil. Clay soils are tested to determine their “plasticity index”. The

plasticity index is a measure of the range over which the clay sample will retain its

plastic characteristics. A dry clay soil sample after addition of water behaves more like a

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plastic rather than solid or semi-solid. The percentage of moisture at that point is the

plastic limit. As water is added continuously, at some point the clay will cease acting like

a plastic and will start to act like a liquid. That point is called the liquid limit. The plastic

and liquid limits are referred to as the Atterberg limits after the scientist who defined

them. The difference between the liquid limit and the plastic limit is called the plasticity

index. Clay which has plasticity index greater than 50 percent is considered as highly

plastic. These types of clays often called as “fat clays” are expansive clays.

There are some other laboratory tests designed specially to measure the expansion

potential of a particular sample. The American Society of Testing Materials has

published a test method (ASTM D 4829) to quantify the results of Expansion Index. By

adding water to the sample while measuring its deformation, the soil engineer will

compare the result to a scale or Expansion Index.

0 – 2: Very Low; 21 – 50: Low; 51 – 90: Medium; 91 – 130: High; ˃ 130: Very High.

Name of the Mineral Mineralogical FormulaAliettite Ca0.2Mg6((Si,Al)8O20)(OH)4 · 4H2O

Beidellite (Na,Ca0.5)0.3Al2((Si,Al)4O10)(OH)2 · nH2O

Ferrosaponite Ca0.3(Fe2+,Mg,Fe3+)3((Si,Al)4O10)(OH)2 · 4H2O

Hectorite Na0.3(Mg,Li)3(Si4O10)(F,OH)2

Montmorillonite (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O

Nontronite Na0.3Fe2((Si,Al)4O10)(OH)2 · nH2O

Pimelite Ni3Si4O10(OH)2·4H2O

Saliotite (Li,Na)Al3(AlSi3O10)(OH)5Saponite Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2 · nH2O

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Sauconite Na0.3Zn3((Si,Al)4O10)(OH)2 · 4H2O

Stevensite (Ca,Na)xMg3-x(Si4O10)(OH)2Swinefordite Li(Al,Li,Mg)4((Si,Al)4O10)2(OH,F)4 · nH2O

Volkonskoite Ca0.3(Cr,Mg,Fe)2((Si,Al)4O10)(OH)2 · 4H2O

Yakhontovite (Ca,Na)0.5(Cu,Fe,Mg)2(Si4O10)(OH)2 · 3H2O

Zincsilite Zn3(Si4O10)(OH)2 · 4H2O

Table 1 : Names and Mineralogy of Smectite family minerals

Figure 1: Montmorillonite Figure 2: A typical case of differential settlement

shown in a structure, figure reproduced from

Mishra (2012)

Engineering Issues

There are a number of issues related to the structures constructed on expansive soils. It

has been observed that usually light weight structures are more prone to the swell-

shrink behavior of expansive soil due to their inefficiency to resist the variation of soil

strata underneath them (Institution of Civil Engineers, 2012). These include shallow

foundations, pavements and houses. The most common issue faced by geotechnical

engineers around the world is the heaving or differential settlement of the soil.

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The phenomenon of heaving occurs when water is trapped within the sheets of clay

minerals due to which the overall volume of the soil is increased. This results in lifting

the structure upwards from either its ends or at the centre because of the introduction

of moisture content in the soil or because of the differential settlement of the soil due

to shrinkage after drying (Cutler and Richardson, 1991). This pattern, called “differential

heave” can be observed and measured with a level survey.

Light weight structures may quickly respond to soil movement caused by expansive

soils. Sever cracking and dislocation of these structures can be the result. Once the

cracking begins, more invading water feeds the problem. Pavements, building

foundations, retaining walls and other structures resting on expansive soils can move

laterally along with lifting up, a reflection of the principle that expensive soils exert

pressure in all directions.

Trees should not be planted in the vicinities where these types of soils are present as

trees can play a vital role in failure of the structures causing differential settlement of

the soil as shown in the figure . Roots of trees absorb water from soil causing loss of

moisture content and ultimately shrinkage in soil. It has been mentioned by Cutler &

Richardson (1991) that sometimes the roots of a tree can cause blocking or damaging

the drain resulting in the moisture content of the soil. The details of the effect of

different trees on the swell-shrink behavior of soil could be found in their book (Cutler &

Richardson, 1991).

The shrink-swell properties of expansive soils will often cause a phenomenon called

“slope creep”. As there is always a horizontal movement of such soil on sloping ground,

the period swelling and shrinkage on a slope together with the forces of gravity will

result in an ongoing creep of soil down the face of the slope. Walls and fences in such

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circumstances will rotate down-slope direction. Hillside improvements on creeping soils

must be heavily reinforced and firmly anchored in order to prevent damage and

eventual destruction.

Stabilization and Remedial Measures

Though expansive soils are prohibited to be used in foundations or pavements but if

there is scarcity of suitable soils in a specific locality then immediate attention should be

given to such issues as the cost of introducing a remedial measure is much less than the

cost incurred due to uplift or settlement of a structure.

In new construction where expansive soil is a concern, the engineer may require

controlled pre-wetting of the soil prior to placement of the foundation or pavement.

This will cause pre-expansion of the soil with the idea that further expansion pressure

will be minimized. According to research done in Spain, the issue of swelling could be

handled by eliminating the characteristics of the soil participating in the expansive

behavior along with improving the mechanical properties of the soil (Seco et al., 2011).

Recommendation can be given for removal of several feet of expansive soil layers and

replace those with non-expansive material. Depending upon the severity of expansion

potential, non-expansive soils may be mixed with the expansive soil to lower the

expansion potential to an acceptable level. Expansive nature of the soils can be reduced

by using additives like Fe2O3, CaO, and some polymers (Mirzababaei et al., 2009). The

latest edition of Ground Improvement (Mitchell and Kelly, 2013) provides a brief history

of the remedial measure adopted by people around the world to mitigate the issue of

expansive soils. These include lime, cement, fly ash and other techniques which mainly

involve recycling the waste materials and reusing it is strengthening the soil. The lime

can cause a reaction called “cations exchange” where “ions” or positively charged atoms

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in solution are substituted for other species of ions which are attached to the clay

mineral crystals. Other effects of lime treatment may include “flocculation-

agglomeration” in which small clay particles clump together into larger particles. Lime

also act as a cementing agent increasing the strength of treated soil. High-sulfate soils

do not respond well to lime or other calcium based soil treatment methods. As a result,

highway departments and soil engineering researchers are looking for new and better

option in the form of organic and inorganic polymers.

Piling, thicker foundation slabs, grade beams, concrete piers, screw anchors all with

extra steel reinforcement are another viable options to prevent the transfer of the load

on the expansive part of the soil.

According to Al Rawas and Qamarudin (1998), the extent of damage done to the

structure by expansive soils could be reduced by installing proper drainage. French drain

or subdrain system can be particularly effective where high water tables and subgrade

drainage conditions are bringing high volumes of moisture into the foundation soil. A

French drain is a trench filled with gravel which captures and removes unwanted water.

A perforated pipe in the bottom of trench is also used to accumulate water and forbids

it to saturate the foundation soils.

Concrete pavements could be executed to minimize the absorption of water into the

underlying soil layers. Another interesting method for soil stabilization is by explosive

replacement method. This method was applied during a highway construction in China.

Crushed stones were piled up near the expansive soils and explosives were placed in the

soil. The explosion causes the soil to move out and the same time pushed the stones in

the cavity. This results in the top soil having higher bearing capacity due to the stones

(Yan & Chu, 2004).

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Another way of soil stabilization is by using preloading and prefabricated vertical drains

(PVD). These prefabricated vertical drains increase the consolidation process of the soil

and prevent excessive pore water pressure from developing in the soil and provide a

safe path for water drainage (Yan et al., 2009).

Case Histories

Expansive soils are present throughout the world and are known in every US state.

American Society of Civil Engineers estimates that ¼ of all homes in the United States

have some damage caused by expansive soils. In a typical year in the United States they

cause a greater financial loss to property owners than earthquakes, floods, hurricanes

and tornadoes combined. Every year they cause billions of dollars in damage throughout

the world. It has been mentioned that the annual loss to structures due to expansive

soils is about 400 million pounds a year in the United Kingdom (Driscoll and Crilly, 2000).

According to the hazard mitigation plan, United States of America faces a total loss of

3.2 billion dollars per annum due to expansive nature of soils (Wyoming Office of

Homeland Security, 2011). Whereas China experiences 1 billion dollars loss annually

(Yanjun Du, 1999).

Yan et al. 2009 have discussed a case of failure of the settlement of an offshore

Breakwater constructed on soft clay in China. A Breakwater was constructed near

Shanghai Port which experienced settlement and lateral movement of the structure due

to floods. Analysis of the structure showed that the strength factor was not taken into

consideration in the original design by the design engineers as floods were not very

common in that place which consequently resulted in the soil having insufficient bearing

capacity. That issue was resolved by installation of prefabricated vertical drains (PVDs).

Those PVDs increased the undrained shear strength of the soil which ultimately

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increased the factor of safety from 0.83 to 2.51 making it suitable for construction. This

method proved to be successful as the breakwater sustained more floods after its

reconstruction and has shown no sign of settlement till then.

Conclusion

Expansive soils pose a threat to the safety and economic conditions of people. A

detailed explanation of the mechanisms involved in the soil which causes it to swell or

shrink has been discussed in this document. It highlights the issues faced by engineers

around the world regarding expensive soils and the extent of damage caused by them. It

has been given that the main cause of swelling or shrinkage of the clay soil is due to the

introduction or removal of moisture which depends upon number of factors. They

include:

● Improper drainage system of a structure

● Trees planted near foundation of a structure

● Damaged drains along the pavements

For remedial actions, treatment of soil with additives such as fly ash, iron oxide, calcium

oxide, polymers, lime and cement are prescribed. If the expansive soil is distributed in

relative small area then it is more economical to remove and replace it with non

expansive soil rather than adding additives. Design of proper drainage system is also

recommended in the literature. It has also been observed that the main focus of

research by the engineers is either on introducing new ways to treat expansive soils or

on using an existing method in a more efficient manner. More work is needed to be

done on defining and classification procedure of expansive soils which is a potential

research area.

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References

http://www.academia.edu/7599700/Expansive_Soils

http://geology.com/articles/expansive-soil.shtml

http://www.foundation-repair-guide.com/expansive-soil.html

Al-Rawas, A. A., & Qamaruddin, M. (1998).Construction problems of engineering structures founded on expansive soils and rocks in northern Oman. Building and Environmental, Vol. 33, 159-171.

Cutler, D. F., & Richardson, I. B. (1991). Tree roots and buildings. Essex: Longman Scientific& Technical, Longman Group UK Limited.

Driscoll, R., & Crilly, M. (2000). Subsidence damage to domestic buildings. Lessons learned and questions asked. London: Building Research Establishment

Institution of Civil Engineers, ICE. (2012). ICE Manual of Geotechnical Engineering. London: Institution of Civil Engineers

Mirzababaei, M., Yasrobi, S., & Al-Rawas, A. (2009). Effect of polymers on swelling potential of expansive soils. Ground Improvement - Institute of Civil Engineering, 111-119.

Mishra, G. (2012). Allowable settlement for different structures. Retrieved 11 28, 2013, from the Constructor: http://theconstructor.org/geotechnical/allowable-settlement-for-different-structures/6899/

Mitchell, J. K., & Kelly, R. (2013). Addressing some current challenges in ground improvement. Ground Improvement - Institution of Civil Engineers, 127-137.

Seco, A., Ramirez, F., Miqueleiz, L., & Garcia, B. (2011). Stabilization of Expansive Soils for use in construction. Applied Clay Science 51, 348-352.

Wyoming Office of Homeland Security. (2011). Wyoming Multi-Hazard Mitigation Plan. Wyoming: Wyoming Office of Homeland Security.

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http://www.foundation-repair-guide.com/expansive-soil.html

http://geology.com/articles/expansive-soil.shtml

http://www.academia.edu/7599700/Expansive_Soils


Yanjun Du, S. L. (1999). Swelling-shrinkage properties and soil improvement of compacted expansive soils, Nign-Liang Highway, China. Engineering Geology, 351-358

Grim, R.E., Kulbicki, G. (1961) Montmorillonite: high-temperature reactions and classification. American Mineralogist: 46: 1329-1369

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expansive soils and stabilization

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