ABSTRACT:

Ground improvement is the most imaginative field of geotechnical

engineering. It is a field in which the engineer forces the ground to adopt the project's

requirements, by altering the natural state of the soil, instead of having to alter the design

in response to the ground's natural limitations. The results usually include saving in

construction cost and reduction of implementation time.

There are number of techniques available for improving the mechanical and

engineering properties of the soil. However, each technique has some limitations and suit

abilities to get maximum improvement in the soil conditions with minimum effort. Some

of the important techniques are discussed in this paper.

To improve the strength of the soils, especially in case of granular type of

soils, COMPACTION METHODES are found as best methods among all type of

techniques. Based on the mechanism applied for compacting the soil, it is sub divided

into different methods like dynamic compaction, blasting, vibro techniques ...etc.These

are briefly discussed in this paper.

When there are some limitations encountered for applying the above

technique, grouting techniques, stabilization of soil using different admixtures can be

adopted effectively which can bring variations in the soil conditions. The various types of

above techniques are briefly discussed in this paper.

Finally, recent advancements in ground improving techniques using

GEOTEXTILES, ELECTRIC TREATMENT METHODES are also briefly discussed in

this paper. These techniques are widely used in these days.

2. INTRODUCTION:

Large civil engineering projects are being executed in all over the country in

order to enhance the infrastructure of the country. Infrastructure facilities have to be often

built at sites where the soil conditions are not ideal. The insitu soil characteristics of a

construction site are different from those desired, and almost always far from ideal for a

designed need. With increased urban development, site with favorable foundation

conditions became depleted. At times the civil engineer has been forced to construct

structures at site selected for reasons other than soil conditions. Thus it is increasingly

important for the engineer to know the degree to which soil properties may be improved

or other alterations that can be thought of for construction of an intended structure at

stipulated site.

If unsuitable soil conditions are encountered at the site of a proposed structure,

one of the following four procedures may be adopted to insure satisfactory performance

of the structure.

By pass the unsuitable soil by means of deep foundations extending to a suitable

bearing material.

Redesign the structure and it's foundation for support by the poor soil. This

procedure may not be feasible or economical.

Remove the poor material and either treat it to improve and replace it (or)

substitute for it with a suitable material.

Treat the soil in place to improve its properties.

Rigid foundations such as piling present a solution but these are often expensive.

In such circumstances, ground improvement using different techniques offers a proved

and economic solution. At present a variety of soil improvement techniques are available

for making soil to bear any type of structure on it and also for mitigation of seismic

hazards. The costs of these methods vary widely and the conditions under which they can

be used are influenced by nature and proximity of structures and construction facilities.

3. GROUND IMPROVEMENT TECHNIQUES:

On the basis of mechanism by which they improve the engineering properties of

soil, the most of common of these can be divided into the following major categories.

These are

Densification techniques.

Reinforcement techniques.

Stabilization techniques.

Miscellaneous methods

Apart from the methods listed above, there are some other simple methods like

removal and replacement of soil. In this paper these are discussed first before taking up

above techniques.

3.1. REMOVAL AND REPLACEMENT OF SOIL:

One of the oldest and simplest soil improvement methods is to simply excavate

the unsuitable soil and replace them with compacted fill. This method is often used when

the problem the soil is that it is too loose. In that case, the same soils used to build the fill,

except now it has a higher unit weight (because of compaction) and thus has been better

engineering properties. This is a common way to remediate problems with collapsible

soils.

Removal also may be available option when the excavated soils have other problems,

such as contamination or excessive organics, and need to hauled away. This method can

be expensive because of the hauling costs and the need for imported soils to replace those

that were excavated. It also can be difficult to find a suitable disposal site for the

excavated soils.

Removal and replacement is generally practical only above the ground water

table. Earthwork operations become more difficult when the soil is very wet, even when

the free water pumped out, and thus are generally avoided unless absolutely necessary.

3.2. PRECOMPRESSION OF SOIL:

Another old and simple method of improving soils is to cover

them with a temporary surcharge fill as shown in figure. This method is called

precompression, preloading, or surcharging. It is especially useful in soft clayey and silty

soils because the static weight of the fill causes them to consolidate, thus improving both

settlement and strength properties. Once the desired properties have been obtained, the

surcharge is removed and construction proceeds on improved site.

Advantages of pre compression:

It requires only conventional equipment earthmoving equipment, which is readily

available. No special or proprietary equipment is needed.

Any grading contractor can perform the work.

The results can be effectively monitored by using appropriate instrumentation and

ground level surveys.

The method has a long track record of success.

The cost is comparatively low, so long as soil for preloading is readily available.

Disadvantages of pre compression:

The surcharge fill generally must extend horizontally at least 10m beyond the

perimeter of the planned construction. This may not possible for confined sites.

The transport of large quantities of soil onto the sites may not be practical, or may

have unacceptable environmental impacts (i.e., dust, noise, traffic) on the adjacent

areas.

The surcharge must remain in place for months or years, thus delay in

construction.

Figure1: Precompression of soil

3.3 DENSIFICATION TECHNIQUES:

The strength and stiffness of the soil is higher when the particles are packed

in a dense configuration than they are packed loosely. As a result, densification is one of

the most effective and commonly used means of improving soil characteristics. This can

be approaches in following ways.

3.3.1 VIBRO TECHNIQUES:

Vibro techniques use probes that are vibrated through soil deposit in a grid pattern

to densify the soil over the entire area of thickness of the deposit. These are classified in

to the following methods. These are

3.3.1.1VIBRO COMPACTION:

Vibro compaction is a method for compacting deep granular soils by repeatedly

inserting a vibratory probe. It is also known as VIBRO DENSIFICATION.

By inserting depth vibrators, the vibrations are produced by rotating a heavy

eccentric weight with the help of an electrical motor with in the vibrator. The vibratory

energy is used to rearrange the granular particles in a denser state. Penetration of the

vibro probe is typically aided by water jetting at the tip of the probe.

Figure2 : Vibro compaction process

Some of advantages and disadvantages of this method are given below.

It is often an economical alternative to deep foundations, especially when

considering the added liquefaction protection in seismic areas.

It is most effective in granular soils

It cannot be used in cohesive soils

3.3.1.2. VIBRO FLOTATION:

In vibro flotation a torpedo like probe (the vibro float) suspended by a crane is

used to density a soil deposit.Vibro floats usually 12 to 18 inch in diameter and about10

to 16 ft long, contain weights mounted eccentrically on a central shaft driven by electric

or hydraulic power.

The vibro float is initially lowered to the bottom of the deposit by a combination

of vibration and water or air jetting through ports in its pointed nose cone. The vibro float

is then incrementally with drawn in 2 to 3 ft intervals at an over all rate of about 1ft / min

while still vibrating. Water may be jetted through ports in the upper part of the vibro float

to loosen the soil above the vibro float temporarily and aid in its with drawl. The

vibrations produce a localized zone of temporary liquefaction that causes the soil

surrounding the vibro float to densify.

Figure3: Vibro flotation process

The advantages of vibro flotation are;

Vibro flotation is most effective in clear granular soils with the fine contents

less than 20% and clay contents below 3%.

Vibro flotation has been used successfully to density soils to deep [this of up

to 115 ft.]

Table 1: Effectiveness of vibro flotation in different types of soils

Ground Type Relative Effectiveness

Sands Excellent

Silty sands Marginal to good

Silts Poor

Clays Not applicable

Mine spoils Good (if granular)

Dumped fill Depends upon nature of fill

Garbage Not applicable

3.3.2. DYNAMIC COMPACTION:

Dynamic compaction is a ground improvement process for compacting and

strengthening loose or soft soils to support buildings, roadways, and other heavy

construction. The method involves the systematic dropping of heavy weights, 100 to

400kN, from a height of 5 to 30m, in a pattern designed to remedy poor soil conditions at

the proposed building site. In soft ground areas, dynamic compaction has proved to be an

effective and economical alternative to preloading, foundation piling, deep vibratory

compaction, and soil undercutting and replacement.

The depth(D) in meters upto which the method is effective can be determined

from the following relation:

D= c√(MH)

Where,

C= coefficient ranging from 0.5 to 0.75

M= mass in Mg

H= height of drop in m

While using the dynamic method, care shall be taken that harmful vibrations

are not transferred to the adjacent buildings. The radius of influence (R) in meters beyond

which no harmful vibrations are transmitted can be determined from the relation.

R=130√(MH)

Where,

M= mass in Mg

H= height of drop in m

Dynamic Compaction is normally used under the following circumstances:

To increase in-situ density and in this way improve the bearing capacity and

consolidation characteristics of soils (or waste materials) to allow conventional

foundation and surface bed construction to be carried out. The technique typically

improves the in-situ soils such that allowable bearing pressures of up to 250 Kpa

can be used with foundation settlements of the order of 10 to 20 mm.

To increase in-situ density and in this way improve in-situ permeability and/or

reduce liquefaction potential

What soils are suitable?

Most soil types can be improved, including silts and some clays. The most

commonly treated soils are old fills and granular virgin soils. Soils below the water table

are routinely treated. However, careful control has to be used to allow dissipation of

excess pore pressures created during the weight dropping.

Figure4: Dynamic compaction process

3.3. 3. BLASTING:

Blasting densification involves the detonation of multiple explosive charges

vertically spaced at 10 to 20 ft apart in drilled or jetted bore holes. The bore holes are

usually spaced between 15 to 50 ft apart and back filled prior to detonation. The

efficiency of densification process can be increased by detonating the charges at different

elevations at small time delays. Immediately after detonation, the ground surface rises &

gas & water are expelled from fractures. The ground surface then settles as the excess gas

& water pressure dissipates. Two or three rounds of blasting are often used to achieve the

desired degree of densification.

The depth upto which the blast is effective is limited to about 25m. The uppermost

zone of the soil upto a depth of about 1m gets displaced in a random manner and is,

therefore, not properly densified. This zone should be compacted using the conventional

methods by rollers.

Explosive charges usually consist of about 60% dynamite and 30% special gelatin

dynamite and ammonite. The charges are placed at two-thirds the thickness of the stratum

to be densified. The spacing of the explosive points is kept between 3 to 8m. Three to

five blasts are generally required at each location.

The radius of influence (R) of compaction can be determined using the relation

R=(M/C)1/3

Where,

R=Radius of influence in m

M= mass of charge in kg

C= Constant (= 0.04 for 60% dynamite)

Blasting is most effective in loose sands that contain less than 20% silt and less

than 5% clay.

Although blasting is quite economical, it is limited by several considerations, as it

produces strong vibrations that may damage near by structures or produce

significant ground movements.

3.3.4. COMPACTION GROUTOING:

Compaction grouting uses displacement to improve ground conditions.

A very viscous (low mobility) aggregate is pumped in stages, forming grout bulbs, which

displace and densify the surrounding soils.

A consistency soil cement paste is injected under pressure in to the soil

mass, consolidating, and there by densifying surrounding soils in place. The injected

ground mass occupies void space created by pressure-densification. Pump pressure

transmitted through low mobility grout, produces compaction by displacing soil at depth

until resisted by the weight of over lying soils.

Fine grained soils with sufficient permeability to allow excess water to dissipate

best suits for compaction grouting.

It has also been used successfully in a wide variety of soils and fills.

Figure5: Compaction grouting process

3.4. REINFORCEMENT TECHNIQUES:

In some cases it is possible to improve the strength and stiffness of a existing soil

deposit by installing discrete inclusions that reinforce the soil. These inclusions may

consist of structural materials, such as steel, concrete or timber and geomaterials such as

densified gravel.

3.4.1. STONE COLUMNS:

Soils deposits can be improved by the installation of dense columns of gravel

known as stone columns. They may be used in both fine and coarse grained soils. In fine-

grained soils, stone columns are used to increase the shear strength beneath structures and

embankments by accelerating consolidation (by allowing radial drainage) and introducing

columns of stronger material.

Stone columns can be installed in a variety of ways. (They may be constructed by

introducing gravel during the process of vibroflotation) In the Frankie method, a steel

casing initially closed at the bottom by a gravel plug is driven to the desired depth by an

internal hammer. At that depth part of the plug is driven beyond the bottom of the casing

to form a bulb of gravel. Additional gravel is then added and compacted as the casing is

with drawn. The diameter of the resulting stone column depends on the stiffness and

compressibility of the surrounded soil

Figure5: Stone column proces

3.4.2. COMPACTION PILES:

Granular soils can be improved by the installation of compaction piles.

Compaction piles are displacement piles , usually prestressed concrete or timber, that are

driven into a loose sand or gravel deposit in a grid pattern and left there.

Compaction piles improve the seismic performance of a soil by three different

mechanisms. First the flexural strength of piles themselves provides resistance to soil

movement (reinforcement). Second, the vibrations and displacements produced by their

installation cause densification. Finally, the installation process increases the lateral stress

in the soil surrounding the piles.

Compaction piles generally densify the soil with in a distance of 7 to 12 pile

diameters and consequently are usually installed in a grid pattern. Between compaction

piles a relative density of up to 75% to 80% are usually achieved. Improvement can be

obtained with reasonable economy to depth of about 60ft.

3.4.3 DRILLED INCLUSIONS:

Structural reinforcing elements can also be installed in the ground by drilling or

auguring. Drilled shafts, some times with very large diameters, have been used to

stabilize many slopes.

Soil nails, tie backs, micro piles have been used for this purpose. The installation

of such drilled inclusions can be quite difficult. However in the loose granular soils that

contribute to increase the strength of the soil in a every effective manner.

3.5 GROUTING AND MIXING TECHNIQUES:

Grouting techniques involve of cementitious materials into voids of the soil or

into fractures in the soil so that the particle structure of the majority of the soil remains

intact.

Mixing techniques introduce cementitious materials by physically mixing them with

the soil, completely disturbing the particle structure of the soil. Grouting and mixing

techniques tend to be expensive but can often be accomplished with minimal settlement

or vibration.

3.5.1 PERMEATION GROUTING:

Permeation grouting involves the injection of low viscosity liquid grout into the

voids of the soil without disturbing the soil structure. Particulate grouts (i.e., aqueous

suspensions of cement, fly ash, bentonite, micro fine cement or some combination there

of) or chemical grouts (e.g., silica & lignin gels, or phenolic & acrylic resins) may be

used.

Grout pipes are typically installed in a grid pattern at spacing of 4 to 8 feet. The

grout may be injected in different ways. In ‘stage grouting’, a boring is advanced a short

distance before grout is injected through the end of the drill rod. After the grout sets up,

the boring is advanced another short distance and grouted again. This process continues

until grout has been placed to the desired depth.

Permeation grouting produces soil improvement by two mechanisms. First the

grout tends to strengthen the contacts between individual soil grains, there by producing a

soil skeleton that is stronger and stiffer than that of the un grouted soil. Second, the grout

takes up space in the voids between soil particles, reducing the tendency for

densification.

Stopping leaks in below-grade structures

Stopping leaks in below-grade utilities

Excavating support of non-corrosive soils

Strengthening of soil mass to accept new loads.

Figure6: permeation grouting process

3.5.2. JET GROUTING:

In Jet grouting the soil is mixed with cement grout injected horizontally under

high pressure in a previously drilled bore hole.

Jet grouting uses a special pipe equipped with horizontal jets that inject grout into

the soil at high pressure. The pipes are first inserted to the desired depth, then they are

raised and rotated while the injection is in progress, thus forming a column of treated soil.

Because of high pressure, this method is usable on a wide range of soil types.

Figure7: Jet grouting process

3.6. STABILIZATON USING ADMIXTURES:

SOIL STABILIZATION:

It is the process of improving the engineering properties of soil

by mixing some binding agents thus binding the soil particles .In a broader sense it also

includes compaction, pre consolidation and many more such process. Soil stabilization is

classified as

Mechanical stabilization

Chemical stabilization

3.6.1MECHANICAL STABILIZATION:

Mechanical stabilization is the process of improving the properties

of soil by changing its gradation. Two (or) more types of natural soils mixed to obtain

composite which is suspension to any of its components. To achieve the desired grading,

sometimes the soil with coarse aggregate are added or the soils with fine particles are

removed. Mechanical stabilization is also known as granular stabilization.

3.6.2. CHEMICAL STABILIZATION:

Chemical stabilization is the form of lime, cement, fly ash and the

combination of the above is widely used in soil stabilization to

Reduce the permeability of the soil.

Improve shear strength.

Increase bearing strength.

Decrease settlement.

Soil and chemicals are mixed either mechanically in place or by bath process .the

optimum benefit of using these agents in stabilization must be determined by laboratory

testing. The general principle of these admixtures as stabilizers is discussed below.

3.6.2.1. LIME STABILIZATION:

This is done by adding lime to soil. It is useful for stabilization of

clayed soils. When lime reacts with soil, there is exchange of cations in the adsorbed

water layer and a decrease in plasticity of soil occurs .The resulting material is more

friable than the original clay and is therefore more suitable as sub grade.

This method is not effective for sandy soils. However these soils can be stabilized

in combination with clay, fly ash or other pozzolanic materials, which serve hydraulically

reactive in gradients.

3.6.2.2. CEMENT STABILIZATION:

Cement stabilization is done by mixing pulverized soil and

Portland cement with water and compacting the mix to attain a strong material .The

material obtain by mixing soil and cement is known as soil cement .The mix becomes

hard and durable structural material as the cement hydrates and develops strength.

The soil cement is quite weather resistant and strong. It is commonly used for

stabilizing sandy and other low plasticity soils. Cement interacts with the silt and clay

fractions and reduced their affinity for water .It reduces the swelling characteristics of the

soil.

3.6.2.3. FLY ASH STABILIZATION:

Fly ash is a by product of the pulverized coal combustion

process. Fly ash has silica, alumina and various oxides and alkalis as its constituents .It is

fine grained and pozzolanic in nature. Fly ash reacts actively with hydrated lime and

hence is used in combination with lime as a stabilizer. A mixture of about 10 to 35 % of

fly ash and 2 to 10 % of lime forms as effective stabilizer for stabilization of highway

bases and sub bases .Soil-lime-fly ash mixes are compacted under controlled condition

with adequate quantity of water.

3.7. GEOTEXTILES:

Soil conditions can be improved in an excellent manner by using geo

textiles. Geotextiles are porous fabrics manufactured products and others such as

polyester ,polyethylene, polypropylene and polyvinylchloride, nylon, fiber glass and

various mixtures of these. These are having permeabilities comparable in range from

coarse gravel to fine sand.

Geotextiles have been used in a variety of civil engineering works. Thus in the

selection of a proper geotextile, due importance has to be given to the major function that

the geotextile is intended to perform. These are majorly used as follows.

1. They acts as separators between two layers of soils having a large difference in

particle size to prevent migration of small size particles into the voids of large size

particles

2. They act as filter. When the silt laden turbid water passes through the

geotextile, the silt particles are prevented from movement by the geotextile.

3. Geotextiles themselves function as a drain because they have a high water

transporting capacity than that of the surrounding material.

4. They serve as REINFOREMENT in soil since they are a good in tensile

strength.

Figure8: Geotextiles

3.7. ELECTRO OSMASIS AND ELCTRO CHEMICAL HARDENING

METHOD:

The electroosmasis process can be used to increase the shear strength and reduce

the compressibility of soft clayey and silty soils beneath foundation. By introducing an

electrolyte such as calcium chloride at the anode, the base exchange reaction between the

iron anode and surrounding soil is increased, resulting in the formation of ferric

hydroxides which bind the soil particles together. However because cost of electric power

and wastage of electrodes, electroosmasis with or without electrochemical hardening can

be considered only for special situations where the alternative of piling cannot be

adopted.

4. CONCLUSION:

1. Unfavorable soil conditions can frequently be improved using soil improvement

techniques. A variety of soil improvement techniques have been developed.

However a suitable technique has to be adopt according to necessity of the

structure and economy.

2. Mainly soil improvement techniques can be divided in to four broad categories;

Densification technique, Reinforcement technique, grouting or mixing technique

and stabilization technique.

3. Densification is probably the most commonly used soil improvement technique.

Most densification techniques relay on tendency of granular soils to densify when

subjected to vibrations. However there is a possibility of damaging adjacent

structures and pipelines due to application of this technique.

4. Reinforcement techniques introduce discrete inclusions that stiffen and strengthen a

soil deposit. The high stiffness and strength of the inclusions also tend to reduce

the stresses imposed on the weaker material between the inclusions.

5. Grouting techniques involve the injection of cementitious materials into the voids of

the soil or into fractures of the soil, so that the particle structure of the majority of

soil remains inject. In permeation grouting, very low viscosity grouts are injected

intothe voids of the soil with out disturbing the soil structure. In intrusion

grouting, thicker and more viscous grouts are injected under pressure to cause

controlled fracturing of the soil.

6. Now a days, geotextiles are extensively used for improving the soil conditions. These

have multiple applications as they act as filters, reinforcement, separations...etc.

5. REFERENCES:

1. “Geotechnical Engineering Principles & Practices” by Donald P.Coduto

2. “Foundation Design & Cinstruction “by M.J.Tomlinson.

3. “Geotechnical Engineering” by Purshotham raj

4. “Geotechnical Earthquake Engineering” by Steven L.Kramar.

.