STONE COLUMN: THEORY

& APPLICATIONS

PREPARED BY:

Saraswati Pathariya (MG007)

DEPARTMENT OF CIVIL ENGINEERING

FACULTY OF TECHNOLOGY, DDU (NADIAD)

2

LIST OF CONTENTS

INTRODUCTION

TECHNIQUES FOR SOIL IMPROVEMENT

- Vibroflotation

- Dynamic Compaction

- Stone Column

- Compaction Piles

- Compaction Grouting

STONE COLUMN

- Applicable soil types

- Construction techniques

- Basic design parameters

- Load carrying capacity of stone column

- Failure mechanism

- Field loading tests

ADVANTAGES OF STONE COLUMN

APPLICATIONS OF STONE COLUMN

REFERENCE

3

INTRODUCTION:

Soils are deposited or formed by nature under different environmental conditions.

Man does not have any control on the process of soil formation. As such the soil

strata at a site are to be accepted as they are and any construction has to be adapted

to suite the sub soil condition. The soil at a construction site may not always be

totally suitable for supporting structures such as buildings, dams, highways,

bridges and other important structures. For example, in granular soil deposits, the

in situ soil may be very loose and indicate a large immediate settlement. In such

a case, the soil needs to be densified to increase its unit weight and thus its shear

strength.

Soft saturated clay layers are often encountered at shallow depths below

foundations. Depending on the structural load and the depth of the layers,

unusually large consolidation settlement may occur. Although pile foundation

may be adopted in some situations, they often become too expensive for low to

medium-rise buildings. In such cases, the properties of the soil within the zone of

influence have to be improved in order to make them suitable to support the given

load.

Soil improvement techniques are normally preferred for economic

considerations. This soil improvement technique has been successfully used to

increase bearing capacity and reduce the settlement of constructions such as

storage tanks, earthen embankments, raft foundations, etc. their main advantage

lies in improving the soil properties below a structure and following the reduction

of an irregular settlement.

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TECHNIQUES FOR SOIL IMPROVEMENT:

Vibroflotation

Vibroflotation involves the use of a vibrating probe that can penetrate granular

soil to depths of over 100 feet. The vibrations of the probe cause the grain

structure to collapse thereby densifying the soil surrounding the probe. To treat

an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid

pattern. Vibro Replacement is a combination of vibroflotation with a gravel

backfill resulting in stone columns, which not only increases the amount of

densification, but provides a degree of reinforcement and a potentially effective

means of drainage.

Vibroflotation

Source: - http://www.geoengineer.org/images/VF_construction_sequence.JPG

Dynamic Compaction

Densification by dynamic compaction is performed by dropping a heavy weight

of steel or concrete in a grid pattern from heights of 30 to 100 ft. It provides an

5

economical way of improving soil for mitigation of liquefaction hazards. Local

liquefaction can be initiated beneath the drop point making it easier for the sand

grains to densify. When the excess pore water pressure from the dynamic loading

dissipates, additional densification occurs. As illustrated in the photograph,

however, the process is somewhat invasive; the surface of the soil may require

shallow compaction with possible addition of granular fill following dynamic

compaction.

Dynamic Compaction

Source: - http://www.gci2000.com/images/projects/arial-view-of-ddc.jpg

Stone Column

As described above, stone columns are columns of gravel constructed in the

ground. Stone columns can be constructed by the vibroflotation method. They

can also be installed in other ways, for example, with help of a steel casing and a

drop hammer as in the Franki Method. In this approach the steel casing is driven

in to the soil and gravel is filled in from the top and tamped with a drop hammer

as the steel casing is successively withdrawn.

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Stone column

Source: -

http://www.penninevibropiling.com/Library/Diagrams/Wet_Top_Feed_Stone_Column.jpg

Compaction Piles

Installing compaction piles is a very effective way of improving soil. Compaction

piles are usually made of prestressed concrete or timber. Installation of

compaction piles both densifies and reinforces the soil. The piles are generally

installed in a grid pattern and are generally driven to depth of up to 60 ft.

Compaction Grouting

Compaction grouting is a technique whereby a slow-flowing water/sand/cement

mix is injected under pressure into a granular soil. The grout forms a bulb that

displaces and hence densifies, the surrounding soil. Compaction grouting is a

good option if the foundation of an existing building requires improvement, since

it is possible to inject the grout from the side or at an inclined angle to reach

beneath the building.

7

Compaction Grouting

Source: - http://www.sparksconcretelifting.com

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STONE COLUMN:

A suitable technique of ground improvement for foundations on soft clay is to

install vertical stone columns in the ground. Stone columns are essentially a

method of soil reinforcement in which soft cohesive soil is replaced at discrete

points by gravel or crushed rock in pre-bored vertical holes to form ‘columns’ or

‘piles’ within the soil. The gravel used for the stone column has a size range of 6

to 40 mm. The stone columns serve two basic functions, namely

(a) Providing strength reinforcement to the soil and

(b) Acting as vertical drains to allow subsoil consolidation to occur quickly under

any given loading.

Greater stiffness of stone columns compare to that of the surrounding soil causes

a large portion of the vertical load to be transferred to the columns. The entire soil

below a foundation, therefore, acts as a reinforced soil with higher load carrying

capacity than the virgin ground. Further, pore-pressure dissipation by radial flow

accelerates the consolidation of the subsoil. Engelhardt et al (1974) demonstrated

the beneficial effects of stone columns by carrying out load tests in soft clay with

and without stone column reinforcement. Fig. is a graphical representation of the

same.

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Effectiveness of stone columns

Source: - Theory & Practice of Foundation Design

APPLICABLE SOIL TYPES:

Soft, Non-compactible, Weak soil

Granular with high fines content

Organic soils

Marine/ Alluvial clays

Liquefiable soils

Cohesive soils

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CONSTRUCTION TECHNIQUES:

Installation of stone columns in soft clay may be done in two ways:

a) Vibratory technique using vibroflot and

b) Rammed stone column technique.

Vibroflotation

The basic tool used in these technique is a poker vibrator or vibroflot, as shown

in fig, which is 2 to 3m long with a diameter varying between 300 mm to 500mm.

extension tubes are attached to the vibroflot whenever greater depth of treatment

is needed. The vibroflot is a hollow steel tube containing an eccentric weight

mounted at the bottom of a vertical shaft; the energy is imparted by rotational

motion through the shaft while the eccentric weight imparts vibration in a

horizontal plane. Vibration frequencies are fixed at 30 Hz or 50 Hz to suit electric

power cycles.

Vibroflot

Source: - Theory & Practice of foundation Design

11

Mechanism of Vibrator

The free fall amplitude varies between 5-10 mm the machine is suspended from

a vibration damping connector by follower tubes through which power lines and

water pipes pass. These allow simultaneous release of water jets to remove the

soil around the vibroflot as the latter makes its way into the hole under vertical

pressure from the top. When the vibroflot reaches the desire depth, the water jet

at the lower end is cut off and granular backfill is poured through the annular

space between the hole and the vertical pipe by head load or conveyor as the

vibratory poker is withdrawn. Well graded stone backfill of size 75 mm to 2mm

is used and compaction, the stones are pushed sideways into the soft soil to

produce a stone column of diameter larger than the diameter of the borehole.

Normally, 600-900 mm diameter stone column can be obtained for 300-500 mm

diameter vibroflot.

Extension Tube

Coupling

Air or

Water

Supply

Motor

Excentric

Weight

Tip

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Stone column installation by vibroflotation

Source: - http://www.polbud-pomorze.ru/zdjecia/wibrowymiana/wibrowymiana+beton.jpg

Rammed stone column

This installation technique was proposed by Datye and Nagaraju (1977) and

developed further by Nayak (1983). In this technique, the granular fill is

introduced into a pre-bored hole and compacted by operating a heavy rammer

through the borehole. The hole is made by using normal bored piling rig with

winch, bailer and casing. The method of installation is illustrated in fig. To

facilitate charging of granular aggregate into the borehole, windows with hinged

flap valve opening outside are provided to the casing at interval of 2m or so.

These windows are kept in closed position during driving or withdrawal of casing

by screwing nuts to prevent ingress of soil into the granular backfill. For installing

stone columns to greater depths, more than one piece of casing is used with the

help of special quick release couplings. The casing maintains the stability of

borehole. The stone columns are required to function as drain wells and it is

advice not to use bentonite slurry for maintain the stability of the borehole.

Backfill material should be such that it gives high angle of internal friction under

given energy of compaction. Sometimes the mixtures of stone aggregate and

sand, generally in proportion of 2:1 are used as backfill material. It is observed

that sand is utilized mainly in filling the voids in gravel skeleton. Gravel backfill

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of aggregate size 75mm to 2 mm is generally recommended. The gravel should

be well graded and preferably angular shaped for good interlock. The main

purpose of compaction is to rearrange the stone particles so that very good

interlocking between particles is obtained to give high angle of internal friction.

Too much ramming may crush the aggregates.

Rammed stone column

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BASIC DESIGN PARAMETERS:

1. Stone column diameter, D

2. Depth of stone column

3. Pattern

4. Spacing

5. Equivalent diameter

6. Replacement ratio, as

7. Stress concentration factor, n

Stone column diameter (D):

Installation of stone columns in soft cohesive soils is basically a self-

compensating process that is softer the soil, bigger is the diameter of the stone

column formed. Due to lateral displacement of stones during vibrations/ramming,

the completed diameter of the hole is always greater than the initial diameter of

the probe or the casing depending upon the soil type, its undrained shear strength,

stone size, characteristics of the vibrating probe/rammer used and the

construction method.

Approximate diameter of the stone column in the field may be determined from

the known compacted volume of material required to fill the hole of known length

and maximum and minimum densities of the stone.

Nayak has given a chart to correlate the diameter of stone column and the

undrained shear strength of soil, as in fig. The diameter obtained from fig. is the

nominal diameter to be consider in design.

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Shear strength of soil versus diameter of stone column

Source: ‘Theory and practice of foundation design’ by N.N Som, S.C Das

Depth of stone column:

The stone column is installed below a foundation up to the depth of soft

compressible strata within the zone of influence in the subsoil.in addition to

carrying vertical load, stone columns function as drainage path to dissipate excess

pore water pressure and hence, accelerate the rate of consolidation. This requires

the stone columns to be taken down to the depth of major compressible strata

which makes significant contribution to the settlement of the foundation.

This point can best be understood by determining the contribution of each layer

of soil towards the settlement of the foundation. However, in some stratified

deposits, the nature of stratification more or less determines the depth of stone

column.

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Pattern:

Stone columns should be installed preferably in an equilateral triangular pattern

which gives the densest packing although a square pattern may also be used. A

typical layout in an equilateral triangular pattern is shown in below figure.

Spacing:

The design of stone columns should be site specific and no precise guidelines can

be given on the maximum and the minimum column spacing. However, the

column spacing may broadly range from 2 to 3 depending upon the site

conditions, loading pattern, column factors, the installation technique, settlement

tolerances, etc.

For large projects, it is desirable to carry out field trials to determine the most

optimum spacing of stone columns taking into consideration the required bearing

capacity of the soil and permissible settlement of the foundation.

Triangular arrangement of stone columns

Source: ‘IS- 15284 (part 1): 2003

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Square arrangement of stone columns

Source: ‘IS- 15284 (part 1): 2003

Mitchell and Katti have suggested typical pile spacing for rectangular and square

grid depicted in fig.

Effect of stone column on anticipated settlement

Source: ‘Theory and practice of foundation design’ by N.N Som, S.C Das

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Equivalent diameter:

The tributory area of the soil surrounding each stone column forms regular

hexagon around the column. It may be closely approximated by an equivalent

circular area having the same total area, see the above fig.

The equivalent circle has an effective diameter (De) which is given by following

equation:

De = 1.05 S for an equilateral triangular pattern, and

= 1.13 S for a square pattern

Where,

S = spacing of the stone columns.

The resulting equivalent cylinder of composite ground with diameter (De)

enclosing the tributory soil and one stone column is known as the unit cell.

Replacement ratio (as):

For purpose of settlement and stability analysis, the composite ground

representing an infinitely wide loaded area may be modeled as a unit cell

comprising the stone column and the surrounding tributory soil. To quantify the

amount of soil replaced by the stone, the term replacement ratio, as is used.

Replacement ratio (as) is given by:

as = As / A = As / (As + Ag)

Where,

As = area of the stone column,

Ag = area of ground surrounding the column, and

A = total area within the unit cell.

The area replacement ratio may also be expressed as follows:

as = 0.907 (D/S)2

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Where, the constant 0.907 is a function of the pattern used which, in this case, is

the commonly employed equilateral triangular pattern.

Stress Concentration factor (n):

Stress concentration occurs on the stone column because it is considerably stiffer

than the surrounding soil. From equilibrium considerations, the stress in the stiffer

stone columns should be greater than the stress in the surrounding soil.

The stress concentration factor, n, due to externally applied load σ ,is defined as

the ratio of average stress in the stone column, σs to, the stress, σg ,in the soil

within the unit cell,

n = σs/ σg

The value of n generally lie between 2.5 and 5 at the ground surface. The stress

concentration factor (n) increases with time of consolidation and decreases along

the length of the stone column. Higher n value at ground surface may result if

load is applied to the composite ground through a rigid foundation as compared

to the flexible foundation.

The stress concentration factor, n, may be predicted using elastic theory as a

function of the modular ratio of the stone and the clay assuming equal vertical

displacements. However, as the modular ratio can vary within wide limits, it

should be selected from the above formula.

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LOAD CARRYING CAPACITY OF STONE COLUMN:

There is no theoretical procedure for predicting the combined improvement

obtained, so it is usual to assume that the foundation loads are carried only by the

several stone columns with no contribution from the intermediate ground. The

stone columns are more than about 10 times as stiff as the surrounding soil. Also

a compacted layer of granular material should be placed over the site prior to

placing the footings.

An approximate formula for the allowable bearing pressure qa of stone columns

is given by Hughes et al. (1975)

qa= Kp (4c + σ`r) / (SF)

Where, Kp = tan2 (45° + Φ/2)

Φ` = drained angle of internal friction of stone

c = either drained cohesion (suggested for small column

spacing’s) or the undrained shear strength su when the column spacing

is over about 2 m

σ`r = effective radial stress as measured by a pressuremeter (but may use

2c if pressuremeter data are not available)

SF = safety factor—use about 1.5 to 2

The allowable load Pa on the stone column of average cross-sectional area,

Ac =0.7854D2col is

Pa = qa * Ac

We can also write the general case of the allowable column load Pa as

Pa = (cs As + Ac cp Nc)* 1/ SF

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Where, cs = side cohesion in clay—generally use a "drained" value if available;

cs is the side resistance (γ z K tan δ) in sand

cp = soil cohesion at base or point of stone column

As = average stone column perimeter area

To compute As, use the in-place volume of stone Vc and initial column depth Lc

as follows:

Ac Lc = 0.7854D2col Lc = Vc and

As = П D Col Lc

Observe that, by using the volume of stone Vc, the diameter Dcol computed here

is the nominal value.

Nc = bearing capacity factor, but use 9 for clay soils if

Lc /Dcol ≥ 3 (value between 5.14 and 9 for smaller L/D)

The allowable total foundation load is the sum of the several stone column

contributions beneath the foundation area (perhaps 1, 2, 4, 5, etc.).

Stone columns should extend through soft clay to firm strata to control

settlements. If the end-bearing term is included when the column base is on firm

strata, a lateral bulging failure along the shaft may result. The bulge failure can

develop from using a column load that is too large unless the confinement

pressure from the soil surrounding the column is adequate. The failure is avoided

by load testing a stone column to failure to obtain as Pult from which the design

load is obtained as Pult /SF

Taking this factor into consideration gives a limiting column length Lc (in

clay based on ultimate resistance) of

Pult ≤ П Dcol Lc cs + 9 cp Ac Ac = 0.7854 D2col

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Solving for Lc, we obtain

Lc ≥ ( Pult – 7.07 dp D2col ) / П Dcol cs

Where all terms have been previously identified.

FAILURE MECHANISMS:

Failure Mechanisms of a Single Stone Column in Homogenous Soft Layer:

Failure mechanism of a single stone column loaded over its area significantly

depends upon the length of the column. For columns having length greater than

its critical length (that is about 4 times the column diameter) and irrespective

whether it is end bearing or floating, it fails by bulging (fig. a).

(a) Long stone column with firm or floating support- Bulging failure

Source: ‘IS- 15284 (part 1): 2003

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However, column shorter than the critical length are likely to fail in general shear

if it is end bearing on a rigid base (fig. b).

In end bearing if it is a floating column as shown in fig. c

Different type of loading applied to stone column:

A stone column is usually loaded over an area greater than its own (fig. d) in

which case it experiences significantly less bulging leading to greater ultimate

(a) Short column with rigid base- shear failure

Source: ‘IS- 15284 (part 1): 2003

(c) Short floating column- punching failure

Source: ‘IS- 15284 (part 1): 2003

24

load capacity and reduced settlements since the load is carried by both the stone

column and the surrounding soil.

Stone Column Failure Mechanisms in Non-Homogeneous Cohesive Soil:

Wherever interlayering of sand and clay occurs, and if the sand layer is thick

enough as compared to the size of the loaded area, the general compaction

achieved by the action of the installation of the stone columns may provide

adequate rigidity to effectively disperse the applied stresses thereby controlling

the settlement of the weak layer. However, effective reduction in settlement may

be brought about by carrying out the treatment of stone columns through the

compressible layer. When clay is present in the form of lenses and if the ratio of

the thickness of the lense to the stone column diameter is less than or equal to 1,

the settlement due to presence of lenses maybe insignificant.

(d) Loading applied to stone column

Source: ‘IS- 15284 (part 1): 2003

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In mixed soils, the failure of stone columns should be checked both for

predominently sandy soils as well as the clayey soil, the governing value being

lower of the two calculated values.

By assuming a triaxial state of stress in the stone column and both the column

and the surrounding soil at failure, the ultimate vertical stress, σ1, which the stone

column can take, may be determined from the following equation:

σ1 / σ3 = (1+ sin Фs) / (1- sin Фs)

Where,

σ1 = lateral confining stress mobilized by the surrounding soil to resist the

bulging of the stone column;

σ3 = angle of internal friction of the stone column;

σ1 / σ3 = coefficient of passive earth pressure kp of the stone column.

This approach assumes a plane strain loading condition (such as passive

resistance mobilized behind a long retaining wall) and hence does not realistically

consider the three dimensional geometry of a single stone column.

The bearing capacity of an isolated stone column or that located within a group

maybe computed using the other established theories also. Besides the passive

resistance mobilized by the soil, the increase in capacity of the column due to

surcharge should be taken into consideration, In addition, capacity increase due

to soil bearing should also be taken into account.

Particular attention should be paid to the presence of very weak organic clay

layers of limited thickness where local bulging failure may take place (fig. e).

Therefore, capacity of column in such weak clays should also be checked even if

they are below the critical depth.

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(e) Failure mechanisms in non-homogenous cohesive soil

Source: ‘IS- 15284 (part 1): 2003

27

FIELD LOADING TESTS:

Irrespective of the method used to construct the stone columns, the

initial load tests should be performed at a trial test site to evaluate the

load settlement behaviour of the soil-stone column system. The tests

should be conducted on a single and also on a group of minimum three

columns.

For the initial load tests, in order to simulate the field conditions of

compaction of the intervening soil, a minimum of seven columns for a

single column test and twelve columns for three column group test may

be constructed for triangular pattern as shown in fig. f.

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The diameter of the circular concrete footing or equivalent steel plate

of adequate thickness and rigidity may be based on the effective

tributory soil area of stone column for a single column test and three

times the effective area of single column for a three column group test.

In each case, the footing may cover the equivalent circular effective

area centrally.

The initial and final soil conditions at the trial site should be

investigated by drilling at least one borehole and one static cone test /

pressure meter test / dynamic cone test prior and subsequent to the

(f) Arrangement of columns

Source: ‘IS- 15284 (part 1): 2003

29

installation of columns. All these tests including the standard

penetration test, field vane shear tests and collection of

undisturbed/disturbed samples and laboratory testing on the samples

should be as per relevant Indian Standards.

A granular blanket of medium to coarse sand having thickness not less

than 300 mm should be laid over the test column(s). Over the blanket,

a properly designed footing should be laid. The footing may be cast

away from the test site and transported to the test location so as to fix

it properly over the sand blanket.

In case high water table conditions exist at site, the water level during

the tests should be maintained at the footing base level by dewatering.

Following procedure should be followed for application of load:

a) The load should be applied to the footing by a suitable kentledge

(fig. g), taking care to avoid impact, fluctuations or eccentricity.

The kentledge should be minimum 1.30 times the maximum test

load.

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b) Load settlement observations should be taken to 1.5 times the

design load for a single column and three column group test

respectively.

c) The settlements should be recorded by four dial gauges

(sensitivity less than or equal to 0.02 mm) fixed at diametrically

opposite ends of the footing.

d) Each stage of loading should be near about 1/5 of the design load

and should be maintained till the rate of settlement is less than

0.05 mm/h at which instant the next stage of loading should be

applied.

(g) Section showing loading arrangement for a single column test

Source: ‘IS- 15284 (part 1): 2003

31

e) The design as well as the maximum test load should be

maintained for a minimum period of 12 h after stabilization of

settlement to the rate as given in (d).

f) Load settlement and time settlement relationships should be

plotted from the settlements observed for each increment of load

at intervals of 1 min, 2 min, 4 min, 8 min,16 min, ½ h, l h, 1 ½ h,

2 h,3 h,4 h, and so on till the desired rate of settlement has been

achieved. The time intervals may be suitably modified if so

desired.

g) The test load should be unloaded in five stages. At each stage

enough time should be allowed for settlements to stabilize.

h) The load test should be considered acceptable if it meets the

following settlement criteria:

- 10 to 12 mm settlement at design load for a single column test,

and

- 25 to 30 mm settlement at the design load for a three column

group test.

i) For routine load test few job columns (say 1 test for 625m2 area)

maybe tested up to 1.1 times the design load intensity with minimum

kentledge of 1.3 times the design load.

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ADVANTAGES OF STONE COLUMN:

Increase the bearing capacity of in-situ soils

To reduced total and differential settlements

Expedites consolidation settlement.

Mitigates the risk of liquefaction.

APLLICATIONS OF STONE COLUMN:

TANK FOUNDATION FOOTINGS (RAFT/ ISOLATED)

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REINFORCED EARTH WALLS

RAILWAY EMBANKMENT

34

HIGHWAY EMBANKMENT

PORTS

35

REFERENCES:

IS 15284 (part- I) : 2003, “Design & Construction for ground improvement

- Guidelines”.

“Theory & Practice of foundation design” by N.N. Som & S.C. Das.

“Foundation analysis and design” (5th edition) by Joseph E. Bowles.

“Foundation design principles and practices” by Donald P. Coduto.