article 17 vol iii issue iii 2012

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.III/ Issue III/July-Sept, 2012/79-84

Research Paper

PROSPECT OF USING GRANULAR PILES FOR

IMPROVEMENT OF EXPANSIVE SOIL Kumar Rakesh.

1 and Jain P.K.

2

Address for Correspondence 1 Asst. Professor

2Professor, Department of Civil Engineering, M.A.N.I.T. Bhopal 462 051, India

ABSTRACT An expansive soil is treacherous from civil engineering construction point of view as it is volumetrically unstable due to

seasonal moisture variation. Its strength decreases and compressibility increases tremendously on wetting. Different ground

improvement techniques have been proposed in the literature to counteract the ill effects of soil instability. The construction

of granular piles/stone columns has been proved successful in improving soft marine clays, which are very poor from

strength and compressibility criteria. The expansive soil may also be considered as soft soil under wet condition. The

technique of granular pile may be applied in expansive soil too to have its improved behaviour. The paper discusses the

outcome of one such attempt made at MANIT, Bhopal wherein Rathod (2012) has carried a model study on granular pile

made in expansive black cotton soil. The load settlement behavior of the soil was determined for different size of the

granular pile. Geo-grid encased granular piles were also installed in the soil. It is observed that significant improvement in

load carrying capacity is obtained with installation of granular pile without and with geo-grid encasement. For a given value

of applied load, the settlement of the soil with granular pile is found to reduce significantly in comparison to that without the

pile.

KEYWORDS: Granular Pile/Stone column, Bearing Capacity, Settlement, Soft Soil (Expansive Soil)

1. INTRODUCTION Expansive soils are found in many parts of the world.

Argentina, Australia, Burma, Canada, Cuba,

Ethiopia, Ghana, India, Iran, Mexico, Morocco,

Rhodesia, South Africa, Spain, Turkey, U.S.A.,

Venezuela and Israel are reported to have significant

area having expansive soil regions Donaldson,

(1969).

In India about one -fifth of the area is covered by

expansive soil. The soil is popularly known as black

cotton soil as it is black in color and is good for

growing the cotton. The soil behaves like a soft soil

under wet/saturated condition. There are a number of

methods available to stabilize expansive soil such as

soil replacement, sand cushion, cohesive non-

swelling layer, mechanical, chemical and thermal

stabilization etc. Where the construction on a large

area is to be carried, such as the construction of

highways over the expansive soil belts, these

techniques are not very much effective or become

costly. The concept of granular pile, used to improve

weak marine clays, have prospect of being utilized in

improving the behaviour of expansive soils in such

regions.

Granular piles/stone columns are constructed in soft

soils by making circular holes and filling them with

granular materials such as natural stone, sand or stone

chips. When about 10 to 35% weak soil is removed

and replaced with granular material in the form of

piles, the load carrying capacity of the ground

increases and settlement decreases significantly and

the ground becomes useable to support the structure.

With this in view, an attempt has been made at

MANIT Bhopal to study the behaviour of saturated

expansive black cotton soil installed with granular

piles. A review of literature on granular piles

installed in soft clays suggest that the failure of

granular piles takes place by bulging of the granular

pile material near the upper portion of the pile, hence

geo-grid encased granular piles have also been

installed in the saturated expansive soils and the load

settlement behaviour of the soil was studied.

The paper reviews the various ground improvement

techniques commonly employed in expansive soils,

discusses their merits and demerits and briefly

describes the concept of granular pile in soft soils.

The process of installation of granular piles in soft

soils and failure modes of the granular piles are also

discussed. The model study carried by Rathode

(2012) is then presented.

2. COMMON METHODS OF EXPANSIVE SOIL

IMPROVEMENT

2.1. Soil Replacement

In this method the poor soil is excavated up to certain

depth and is replaced by good soil which is not

expansive. This is possible only where the non

problematic soil is easily and cheaply available

nearby. Removal and replacement is generally

practical only above ground water table. Earthwork

operation is difficult when the soil is wet or

submerged.

2.2. Sand Cushion Method

When the entire depth of the expansive soil stratum

or a part there of is removed and replaced with the

sand; compacted to the desired density and thickness,

the ill effects of poor soil are minimized

Satyanarayana, (1969). The basic advantage of the

sand cushion method is its ability to adapt itself to

volume changes in the soil. However, the sand

cushion method has several limitations particularly

when it is adopted in deep strata. The high

permeability of sand creates conditions conducive to

easy ingress and accumulation of water from surface

runoff.

2.3. Cohesive Non Swelling Layer

Replacement by soils with relatively impervious

material may, to a great extent offset the

disadvantages of sand cushion method. The method

proposed by Katti (1979) uses cohesive non-swelling

(CNS) layer to reduce the effects of swelling.

The heave of expansive soil underlying a CNS layer

reduces exponentially with increase in thickness of

the CNS layer and attains a value of no heave around

a depth of 1m.The shear strength of the underlying

expansive soil at the interface and below increases

with the thickness of CNS layer. The method is

recommended for construction of canals in black

cotton soil area.



2.4 Mechanical Stabilization Mechanical stabilization is the process of improving

the properties of soil by changing its gradation. Two

or more type of natural soils is mixed to obtain

composite material which has better strength.

Generally coarse grained materials such as sand,

crusher dust, moorum (a soil predominantly coarse

grained, red in color having fine silt and clay) etc are

mixed with fine grained soil.

2.5 Chemical Stabilization

Mixing of lime, cement, fly ash and combination of

these in small quantities changes the physico-

chemical characteristics around and inside of clay

particles and the soil gives improved behaviour. A

brief description of these methods is as follows.

2.5.1 Stabilization with Lime

Lime stabilization has been used successfully on

major projects to minimize swelling of the expansive

soil. In general all lime treated fine grained soil

exhibit decreased plasticity, improved workability

and reduced volume change characteristics.

Generally, 3 to 8% by weight of hydrated lime is

added to the top several inches of the soil. Lime

continues to be widely used additive for modification

of expansive clays in view of its cost-effectiveness

although limited success in many instances. Lime

diffusion into soil either from lime piles or lime

slurry pressure injection is also used. It is reported

that hardly 38 to 50mm diffusion of lime in to soil

takes place in 1 to 4 years unless extensive fissure

and crack system is present.

Venkataswamy et.al.(2003) studied the improvement

of expansive clay by deep in-situ technique. They

concluded that the pozzolonic reaction due to

presence of lime has shown marked increase in

unconfined compressive strength and reduction in

swelling pressure as well as plasticity index. They

made a hole of 150mm diameter by pushing a steel

pipe in to the soft ground up to a depth of 5.5 m and

poured a mixture of sand and calcium in1:1

proportion. All the area covered for 21 days by gunny

bags then undisturbed samples were collected at

different radial distances. The improvement in

properties was found up to radial influence zone is of

750 mm at 3.5m, 4.5m and 5.5m depths.

2.5.2 Stabilization with Cement

Portland cement can be used either to modify or

improve the quality of the soil or to transform the soil

in to a cemented mass with increased strength and

durability. The amount of cement used depends upon

whether the soil is to be modified or stabilized

Kowasliki, et. al.( 2007) .The hydration of Portland

cement is a complex pozzolanic reaction that

produces a variety of different compounds and gels.

The results of mixing cement with clay soil are

similar to that of lime. It reduces liquid limit, the

plastic index and the potential of volume change. It

increases the shrinkage limit and shear strength.

Addition of 2 to 6% cement by weight of soil can

produce a soil that acts as a semi rigid slab.

Some investigators have tried and succeeded in

minimizing the swelling of expansive soil using

chemicals like calcium chloride (CaCl2), calcium

sulphate (CaSo4), potassium chloride (KCL),

aluminium chloride etc.

2.5.3 Stabilization with Fly ash

As fly ash is freely available, for projects in the

vicinity of thermal power plants, it can be used for

stabilization of expansive soil.

Phanikumar and Sharma (2004) studied the effect of

mixing fly ash (content 5, 10, 15 and 20% by dry

weight of soil) on engineering properties of

expansive soil through an experimental investigation.

Free swell index was found to reduce by 50% on

addition of 20% fly ash. The hydraulic conductivity

of expansive soil decreases with increase in fly ash

content. The undrained shear strength increases with

increase in the ash content.

White et al. 2005 reported that addition of fly ash

changes soil compaction characteristics, compressive

strength, wet/dry durability, freeze/thaw durability,

hydration characteristics, and rate of strength gain

and plasticity characteristics.

Ramarao et al., (2005) studied the developments of

cohesive bonds in a lime-stabilized fly ash cushion.

The combination of lime and fly ash is expected to

produce an environment similar to the one obtained

in CNS material following saturation and

consequently arrest heave. The results of the study

showed a new solution to the problem of heave of

expansive soil could be in the form of Fly ash

cushion method. It also solves the problem of fly

ash utilization and disposal to some extent. If at a site

containing black cotton soil, the depth of the active

zone is 3m, it would be sufficient if 1.5m of

expansive clay is removed and replaced with fly ash

cushion to reduce heave significantly. With the

superstructure load causing further reduction of

heave, the amount of sub-excavation and replacement

with lime stabilized fly ash cushion can be further

reduced.

2.6 Stabilization of Expansive Soil using

Reinforcement

Reinforcing the soil is usually accomplished by one

of the following methods: Soil nailing, Soil

anchoring, Micro piles, Stone columns and Fiber

reinforcement. Using fibres like jute fabrics, coir

ropes, rubber tire chips, waste plastics, synthetic

fibres etc one can stabilize the expansive soils. The

soil and its reinforcing elements act in combination

and increase the shear strength of the soil mass,

reduce its settlement under the load, and improve its

resistance to liquefaction.

The work reported by Al-Omari and Hamodi (1991)

showed the feasibility of using tensile geo grid for the

purpose of controlling the swell of plastic soils.

Swelling tests using an enlarged odometer revealed

promising results. The reinforcements were

cylindrical geo grid of varying stiffness values

embedded in clays of different plasticity indices. The

reduction in swell increased with increasing the geo

grid stiffness.Relative merits and demerits of

different methods discussed above are compared in

Table1.

Table.1 Merits and Demerits of Commonly used

Expansive Soil Improvement Methods Ground

Improvement

Method

Merits Demerits

Soil

Replacement

When the soil to be

replaced is up to

shallow depth and good

soil is easily/cheaply

available.

If good soil is not

cheaply available,

this method can

be expansive.



Sand Cushion This is simple method

and need no special

methods/equipment.

High permeability

of sand facilitates

easy ingress of water from the

surface runoff and

the swelling

process

accelerates.

CNS Method The CNS concept is

useful for canal lining.

The CNS concept

for buildings has

been very limited.

Mechanical

Stabilization

If area is small the use

of admixtures is good.

If area is large the

cost will be more.

Chemical Stabilization

If area and depth of treatment is small the

use of admixtures is

good.

If the soil is to be treated for greater

depth, the method

is cumbersome.

Soil

Reinforcement

Useful for shallow

depth.

Mixing/laying is

major problem.

3. GRANULAR PILE IN SOFT CLAYS

Weak soil, which has very low shear strength and

high compressibility to support structures require

strengthening to be capable of carrying loads from

structures. Granular piles when installed in soft clays

they improve the load carrying capacity and reduce

settlement. They act as vertical drains and thus

speeding up the process of consolidation. The

replacement of the soft soil by a stronger material and

initial compaction of soil during the process of

installation increases the unit weight of the soil.

Granular piles are ideally suited for structures,

because of reduction of total and differential

settlements, increased bearing capacity of the site to

make it possible to use shallow foundation. Granular

pile have provided an economical method of support

in compressible and fine-grained soils for low-rise

buildings and structures such as liquid storage tanks,

abutments, embankments, and factories that can

tolerate some settlement.

3.1 Construction of Granular Pile

The improvement of a soft soil with granular pile can

be accomplished using various techniques that

involve excavation, replacement and compaction.

Rao (1982) and Ranjan and Rao (1983) developed a

simple method, particularly useful in developing

countries, which is technically viable and uses

indigenously developed equipment. A spiral auger is

used to make the borehole utilizing manual labor.

After reaching the desired depth, the borehole is

thoroughly cleaned and the stone aggregate is placed

in the borehole in layers of 300 500 mm followed

by sand layer of 50 100 mm. A cast iron hammer

weighing 125 kg and diameter less than the diameter

of the borehole, operated by a power winch having a

fall of 750 mm is used to compact the sand/stone

aggregate layer. During the course of compaction, the

sand fills the voids of the stone aggregates followed

by the lateral and downward displacement of the

charged material till full compaction of the

surrounding soil is reached. Various stages of

installation procedure of a granular pile are shown in

Fig.1.

No skilled labor is required. Hence the technique is

economical where man power is cheaply available

such as in the developing countries like India.

3.2. FAILURE MODES OF GRANULAR PILE

Three cases are discussed here.

3.2.1 Single Granular Pile in Homogeneous Soft

Layer Granular pile may be constructed as either end

bearing on a firm stratum underlying soft soil, or as

floating columns with the tip of the column

embedded within the soft layer.

Fig.1.Granular Pile Installation by Simple Auger

Boring Method (After Rao, 1982)

The failure of a granular pile may take place in one of

the following three modes. Either end bearing or

frees floating granular piles having length greater

than about three diameters in length fail in bulging as

illustrated in Fig.2(a). A very short column bearing

on a firm support will undergo either general or local

shear (bearing capacity) type failure at the surface

[Fig.2(b)]. Finally, a floating granular pile less than

about 2 to 3 diameters in length may fail in end

bearing in the weak underlying layer before a bulging

failure can develop [Fig.2(c)]. For the subsurface

conditions generally encountered in practice,

however, bulging is usually the controlling failure

mode.

Fig.2 Failure Modes of a Single Stone Column in

Homogeneous Cohesive Soil (After U.S. Department of

Transportation, Report No. FHWA/RD-83/026, 1983)

3.2.2. Single Granular pile in Non-Homogeneous

Cohesive Soil Fig.4 shows a failure mechanism of a

single Granular pile in non-homogeneous cohesive

soils. A very soft zone at the surface, l-3 m thick, has

a dominating influence on the settlement and ultimate

strength of either granular pile/stone column groups

or single columns. Further, field observations have

indicated that the presence of a very weak layer such

as peat, greater than about one column diameter in

thickness, may also seriously affect granular pile

performance [Fig.3 (b) and (c)].

Fig.3 Failure Mechanisms of a Single Stone Column in

Non-Homogeneous Cohesive Soil



3.2.3. Group Granular Pile Homogeneous Soft

Layer

The ultimate load carrying capacity of Granular pile

in group is slightly more than an isolated single

granular pile due to the confinement of interior

column by surrounding columns. As a result, interior

columns are somewhat stiffened and give rise to

slight increase in the ultimate load carrying capacity

per column. A wide flexible loading such as due to

construction of an embankment over a stone

column improved ground is illustrated in

Fig.4(a).The soil beneath and to the sides of the

foundation move laterally outward due to the

construction of the embankment over the weak

foundation as illustrated in Fig.4(a) and (b). This

phenomenon is called spreading. Lateral spreading

also slightly increases the bulging, the granular pile

undergoes, compared to the condition of no

spreading. A group of stone columns in a soft soil

probably undergoes a combined bulging and local

bearing type failure as illustrated in Fig.4(c). A local

bearing failure is the punching of a relatively rigid

granular pile (or group) into the surrounding soft soil.

Granular pile groups having short column lengths

may fail in end bearing [Fig.4 (d)] or perhaps

undergo a bearing capacity failure of individual

granular pile similar to the failure mode of short

single granular pile.

Fig.4 Failure Modes of Granular Pile in Group

(After U.S. department of Transportation, Report No.

FHWA/RD-83/026, 1983)

Mugerson,S. et al (2006), have brought out

conceptual performance of granular piles encased in

geosynthetic material and recommended it for

supporting a wide variety of structures including

buildings and flexible structures. The stone columns

derive their load capacity from the confinement

offered by the surrounding soil. In very soft soils this

lateral confinement may not be adequate and the

formation of the stone column itself may be doubtful.

Wrapping the individual stone columns with suitable

geotextile/geosynthetic is one of the ideal forms of

improving the performance as it makes the stone

columns stiffer and stronger. In addition, encasement

prevents the lateral squeezing of stones in to the

surrounding clay soil and vice versa, preserves

drainage function of the stone column and frictional

properties of the aggregates.

4. MODEL STUDIES ON GRANULAR PILES IN

EXPANSIVE SOIL

Rathod (2012) has carried experimental investigation

to study the behavior of granular pile in expansive

soil (Fig.5). The experiments were carried out in a

cylindrical tank of diameter 300mm and height

750mm. Tests were carried out with two types of

loading

1. Only granular pile/stone column loaded to

estimate the limiting axial capacity.

2. The entire area of tank loaded

Fig.5 Test Setup used in Model study by Rathod(2012)

The black cotton soil was taken from Raisen road

Bhopal, India. The soil is clay of high

compressibility.

The properties of the soil are given in Table 2.

Table.2 Properties of Expansive Clay used in the

Study Property Value

Specific Gravity

Clay Content

Silt Content

Liquid Limit

Plastic Limit

Plasticity Index

2.62

48 %

52 %

60 %

28 %

32 %

Granular materials consisting of crushed stone having

size between 2mm to 10mm have been used to form

the stone column. To load the stone column area

alone, a loading plate equal to the diameter of the

column was placed over the stone column. The load

was applied through a proving ring up to15 minutes.

Settlements were monitored for equal intervals of

loads up to failure. In the case where load is applied

over the entire area, a 30 mm thick sand layer was

placed over the entire surface. A steel plate of 12 mm

thickness and 10 mm diameter less than the inside

diameter of the tank was placed over the sand

blanket. The loading was applied in the similar way

until the settlement exceeded 10 mm. The granular

piles of diameter 40mm, 60mm and 80mm were

taken in the study. The load-settlement behaviour of

soil with granular pile of different diameters is shown

in Fig.6 and Fig. 7.

Fig. 6. Load-Settlement Curve for Granular Pile of

Different Diameters (Only Stone Column Loaded)

Fig.7. Load-Settlement Curve for Granular Pile

Encased with Geo grid for Different Depths (Only Stone

Column Loaded)



The load settlement behaviour of geo grid encased

stone columns with different lengths of geo grids is

shown in Fig8 and Fig. 9.

Fig.8. Load-Settlement Curve for of Granular Pile of

Different Diameter (Entire Area Loaded)

Fig.9. Load-Settlement Curve for of Geo grid

Encased Granular Pile to Different Depths (Entire Area

Loaded)

The findings of the studies are summarized as given

below:

Stone columns play an effective role in

reducing the settlements of expansive soil and

increase the bearing capacity.

As the diameter stone columns increases the

bearing capacity increases and settlement

decreases.

The bearing capacity of the stone column

increases by introducing circumferential geo-

grid reinforcement. And as the depth of

circumferential reinforcement increases

settlement decreases and the bearing capacity

increases.

Further reduction in settlements is noticed with

the increasing depth of geo grid-encasement.

On full depth encasing, reduction in total

settlements of up to 79.13% is noticed.

Smaller diameter geo grid-encased stone

columns show better performance than large

diameter geo grid-encased stone columns.

The tests also reveal that the larger diameters

of ordinary stone columns can be replaced by

smaller diameters of geo grid-encased stone

column.

5. CONCLUSIONS

The unsuitable and unfavorable sites can be utilized

by using ground improvement techniques. A variety

of ground improvement techniques are available and

have to be adopted according to necessity of structure

and economy. The use of granular pile as a technique

of soil reinforcement is frequently implemented in

soft cohesive soils and has been successfully used to

support isolated footings, large raft foundations and

embankment. Their use in soft clays has been found

to provide increases in load carrying capacity

accompanied by significant reduction in settlement.

Being granular and freely drained material,

consolidation settlement is accelerated and post

construction settlement is minimized. The possibility

of using them for improving behavior of expansive

black cotton soil has been explored through a model

study conducted art MANIT, Bhopal (INDIA). The

results of model study carried by Rathod (2012)

showed that granular pile/ stone column increases the

load bearing capacity and decreases the settlement of

soft expansive soil. Further by using geogrid as a

circumferential reinforcement to stone column, added

improvement in bearing capacity and reduction in

settlement can be obtained. Besides the strength, the

aspect of swelling and shrinkage of granular pile

improved expansive soil need to be studied in detail.

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317.

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13. U.S. Department of Transportation (1983) Design and

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