SHORT TRAINING PROGRAM

ON

SOIL AND FOUNDATION ENGINEERING ( 17 - 19 April, 2012 )

Prof.(Dr.) SUDHENDU SAHA

Chartered Professional Engineer

Civil Structural Geotechnical Consultant Formerly

Professor and Head of The Dept. of Civil Engineering,

DEAN of Research Consultancy & Industry Institute Interaction,

Bengal Engineering and Science University, Sibpur

CONDUCTED AT

INDIANOIL MANAGEMENT ACADEMY

HALDIA TOWNSHIP, PURBA MEDINIPUR

WEST BENGAL

2

P R E F A C E

For any project, design and construction of foundations – shallow or deep are the

essential requirements. Shallow foundations are sufficient for many light and medium

loaded structures. And piles are widely used for heavy structures in weak and difficult

subsoil conditions. A number of methods have been proposed to predict the load carrying

capacity, and settlement behaviour of shallow and pile foundation. The reliability of these

methods depends on various factors including subsoil conditions, construction technique

and also subsequent construction activities in adjacent areas.

Design and construction of shallow as well as pile foundations are often carried out by

many who may not have the proper understanding of the behaviour of such foundations

in different ground conditions, and also the possible defects that may occur during

construction. Geotechnical Engineering is so complex that it demands proper

understanding of the phenomena associated with soil structure interaction in any

particular situation. Theories only explain idealised problems. Mere application of a

theory might not lead to safe and satisfactory performance of a structure. The Engineers

have to understand the field conditions and apply judgement before adopting any

methodology of design and construction.

IndianOil Management Academy is engaged in organizing training programs on various

subjects of interest for updating the knowledge, skill and expertise of the junior and

midlevel engineers of Indian Oil Corporation Ltd. working in different units of the

company. For the design and construction of various engineering projects, the design and

construction of foundations - shallow or pile foundations are undertaken. Under the

circumstances, it has become important and essential requirements for the designers and

construction engineers of the organisation, to have better and updated understanding of

different aspects of design and construction of different types of shallow and deep

foundations including discussions on case histories highlighting the intricacies of the

projects. In view of this, the undersigned was requested by Mr. A. K. Saha, Chief

Manager (D&T), Indian Oil Management Academy, Haldia. to conduct a short training

course for deliberations on relevant aspects of the subject. The author shall be happy and

remain grateful if his efforts and deliberations help in transfer of knowledge and expertise

of the subject.

IMA, Haldia Township Prof.(Dr.) Sudhendu Saha

17 April, 2012 Chartered Professional Engineer

3

CONTENT

Page

1. INTRODUCTION 4

2. SOIL ENGINEERING AND DESIGN OF SHALLOW FOUNDATION 4

2.1 Some Properties of Soil 5

2.2 Classification of Soil 6

2.3 Shear Strength of Soil 8

2.4 Bearing Capacity of Soil for Shallow Foundation 9

2.5 Contact Pressure Distribution 11

2.6 Stresses Induced in Soils 12

2.7 Settlement Analysis of Soil 12

2.8 Engineering Appreciation 15

2.9 Performance Criteria 16

2.10 Design of Shallow Foundations 18

3. PILE FOUNDATION 21

Classification of Piles 22

Piling Engineering 23

3.2.1 Pile Driving Equipment 24

Construction and Piling Methods 25

Effects of Installation of Piles 28

Behaviour of Piles 29

Vertical Load Bearing Capacity of Pile 31

Lateral Load Capacity of Pile 38

Method of Improving Lateral Load Capacity 45

Pile Testing and Quality Control 45

Types of Load Tests 48

4

1.0 INTRODUCTION

The stability, performance and responses of structures greatly depend upon variety of

factors involving not only the types of structures and foundations, but also types of soils

interacting with the structures and foundations.

There are two phases of design of foundation system : Soil design and structural design

of foundation. The aim of soil design essentially is to arrive at the foundation

proportioning, satisfying two independent requirements from soil side, viz., bearing

capacity and settlement. All foundations may be basically of two types – shallow footing,

and deep piles, on the basis of their depth in relation to their width.

2.0 SOIL ENGINEERING AND DESIGN OF SHALLOW

FOUNDATION

Depending on types of structure, the relevant and appropriate properties of soils have to

be estimated. Normally the properties of subsoils cannot be determined with great

accuracy. The properties of in-situ soils and that of so called undisturbed soil samples

may differ. Moreover, the stress history, the rate of loading and strain or deformation also

influences the subsoil behaviour. Therefore, for realistic values of soil properties, testing

procedures should simulate the field conditions as far as practicable. Even though the

properties are known for one sample of soil beneath an area, the properties of the entire

subsoil affected would be only vaguely known , as because soil materials may vary over a

wide range both horizontally and also with depth.

Soil is a complex mixture of inorganic particles, which may sometimes contain

decomposed organic residues and other substances. The soil particles are formed by the

process of weathering, disintegration and decomposition of rocks and materials through

the action of natural, physical, mechanical and chemical agents into smaller and smaller

particles.

Soil profiles are created by the deposition of soil particles, which have been carried by

the different agencies like glacier, water, wind etc. Depending on the method of

formation, the soil deposits develop their own characteristics, which are normally

different for different soil deposits.

Alluvial soils occur in former and present flood plains, deltas often forming quite thick

deposits. Alluvial deposits are geologically recent materials formed by the deposition of

fine sands, silt and clayey materials in river valleys, estuaries and sea beds. These are

compressible normally consolidated soils showing progressive increase in shear strength

with increasing depth ranging from very soft near ground surface to firm or stiff at depth.

5

e

enPorosity

S

GmeRatioVoid

e

eSGDensityBulk wt

+=

=

+

+=

1

1γγ

2.1 Some Properties of Soils

A soil mass is a three phase system ( Fig.2.1 ) consisting of soil grains, water and air. The

moisture content (m) is defined as the ratio of weight of water to the weight of soil solids

in a given mass of soil. The void ration (e) is defined as the ratio of volume of void to the

volume of soil solids in the given soil mass. The density of soil is defined as the mass of

soil per unit volume. Some of the relationships are given below :

where, S = degree of saturation, G = specific gravity of soil , γw = unit weight of water

The percentage of various sizes of particles in a given soil sample is found by grain size

analysis (Fig. 2.2). For coarse grained soils, certain particle sizes such as d10 and d60

are important. The size d10 is called effective size, which represents a size in mm such

that 10% of the particles of the soil sample are finer than this size.

FIG. 2.1 SOIL AS THREE PHASE SYSTEM

The term ‘Consistency of soils’ (Fig.2.3) relates to fine grained soils and denotes the

degree of fineness of soil varying with moisture content. A set of standard limits such as

liquid limit, plastic limit and shrinkage limit, which are called Atterberg limits, have been

defined to describe the consistency of fine grained soils.

minmax

maxReee

eeRDensitylative

−

−=

mDensityDry t

d +=

1

γγ

6

FIG. 2.2 TYPICAL PARTICLE SIZE DISTRIBUTION CURVE

FIG.2.3 VARIATION OF CONSISTENCY WITH WATER CONTENT

2.2 Classification of Soils

Soil Classification based on particle sizes :

Boulder over 300 mm

Cobble 80 to 300

Gravel 4.75 to 80

Coarse Sand 2.00 to 4.75

Medium Sand 0.425 to 2.00

Fine Sand 0.075 to 0.425

Silt 0.075 to 0.002

Clays < 0.002 mm

7

Soil Classification according to Plasticity Index of clayey soils :

Plasticity Index Classified as

0 Non-Plastic

< 7 Low Plastic

7 – 17 Medium Plastic

> 17 Highly Plastic

when, Plasicity Index = Liquid Limit – Plastic Limit

Broad Classification of Soils

Soils in general may be broadly classified as

(a) Coarse grained soils, composed of more than 50% of soil particles greater than 75

micron sizes, i.e., sands and gravels.

(b) Fine grained soils, composed of more than 50% of soil particles less than 75 micron

sizes, i.e., silts and clays.

Coarse grained soils may be subdivided into gravels (G) and sands (S), which are further

subdivided into four groups of well graded (W), poorly graded (P), with silts (M) and

clay (C) percentages. As such, gravelly soils may be GW, GP, GM and sandy soils may

be SW, SP, SM or SC.

The SPT or N-values are correlated to Relative Density ,

Compactness and Angle of internal friction of cohesionless soil.

N Compactness Relative Angle of Internal

Density R % Friction φ0

0 – 4 Very loose 0 – 15 < 28

4 – 10 Loose 15 – 35 28 – 30

10 – 30 Medium 35 – 65 30 – 36

30 – 50 Dense 65 – 85 36 – 41

> 50 Very Dense > 85 > 41

The SPT values are also correlated to consistency and strength of cohesive soils.

Consistency N Unconfined Compression

Strength qu kPa

Very soft 0 –2 < 25

Soft 2 – 4 25 – 50

Medium 4 – 8 50 – 100

Stiff 8 – 15 100 – 200

Very Stiff 15 – 30 200 – 400

Hard >30 > 400

8

Fine grained soils may be classified into inorganic silts and fine sands (M). inorganic

clays ( C ) and organic silts and clays (O). These may further be subdivided depending on

compressibility Low (L), Medium (I) or high (H). Plasticity chart ( FIG. 2.4 ) is useful

to classify fine grained soils , as ML, CL, OL, MI, CI, OI, MH, CH, OH, and Pt.

The fine-grained soils have the following significant engineering properties :

(a) It often possesses low shear strength, and loses shear strength upon wetting &

disturbance.

(b) It is often plastic & compressible, and deforms plastically under sustained load

particularly when stress is greater than 75% of its shear strength.

(c) It shrinks upon drying and expands upon wetting particularly when rich in

montmorillonite minerals, when it is also commonly called expansive or black

cotton soil.

(d) It is poor material for backfill and embankment, because of low shear strength &

more difficult to compact. Clay slopes are prone to landslide. It is practically

impervious.

FIG. 2.4 PLASTICITY CHART FOR CLASSIFICATION OF FINE GRAINED SOIL

2.3 Shear Strength of Soil

One of the most important properties of soil is its shear strength or ability to resist sliding

along internal surfaces within a soil mass. The stability of foundations of structures, cuts

and embankments depends upon the shear resistance offered by the soil along probable

surfaces of slippage.

The basic concept of friction applies to soils, which are purely granular. But soils which

are not purely granular exhibit an additional strength which is due to the cohesion

between the particles. The fundamental shear strength of soils is expressed by Coulomb’s

equation as follows :

9

S = C + (σσσσ - u) tanφφφφ

where, C = cohesion of soil, σ = total stress, u = neutral stress,

φ = angle of internal friction of soil.

The shear strength parameters of cohesion C and angle of internal friction φ depend upon

several factors as past history of soil, degree of saturation, rate of loading, or drainage

etc. The failure of a soil mass is more truly explained by Mohr-Coulom failure theory .

The Mohr theory is based on the postulate that a material will fail when the shearing

stress on the plane along which the failure is presumed to occur, is a unique function of

normal effective stress acting on that plane. The conditions of failure will be attained

when

τ ≥≥≥≥ S = C + (σσσσ - u) tanφφφφ ,

where, τ is the shear stress induced on the plane due to superimposed load.

2.4 Bearing Capacity of Soil for Shallow Foundation

The stability of a foundation resting on soil depends on two factors, which are

( i ) Shear failure of soil,

( ii ) Settlement of foundations

The ultimate bearing capacity of soil may be defined as the maximum intensity of loading

that can be applied at the base of the foundation without causing failure by shear or

excessive settlement. There are a number of theories available which may be used for

estimation of ultimate bearing capacity of soil. These theories are appropriate, so long the

assumptions used for derivation of a particular theory truly represent the field conditions.

Shear failure of soils below shallow foundations are shown in FIG. 2.5.

FIG. 2.5 MODES OF SHEAR FAILURE BELOW FOOTING

The safe bearing pressure on soil may be taken as the load intensity at the base of

foundation, which will not cause settlement exceeding the permissible values specified

for particular structure and type of soil. For soils with cohesion and angle of internal

friction φ , the net ultimate bearing capacity may be calculated as

10

Bearing Capacity Factors

φ degrees Nc Nq Nγ

0 5.14 1.0 0

10 8.35 2.47 1.22

15 10.98 3.94 2.65

20 14.83 6.40 5.39

25 20.72 10.66 10.88

30 30.14 18.40 22.40

35 46.12 33.30 48.03

40 75.31 64.20 109.41

Shape Factors

Sc = 1 + 0.2 B/L , B = width or diameter of footing

= 1.3 for Circle, L = length of footing

Sq = 1 + 0.2 B/L , 1.2 for circle,

Sγ = 1 – 0.4 B/L , 0.6 for Circle

Depth Factors

dc = 1 + 0.2 D/B √Nφ D = depth of foundation

dq = d γ = 1 for φ < 100

Nφ = tan2 (45

0 + φ/2 )

dq = d γ = 1 + 0.1 D/B√Nφ for φ > 100

For cohesionless soils, bearing capacity can also be determined using SPT or N-values.

The correlations between N and static cone resistance against φ can be used, and the

value of φ so obtained can be used to have corresponding values of bearing capacity

factors. The net ultimate bearing capacity for shallow foundations can be estimated as

discussed above.

.,

,,

,

,

,

)(,

5.0)1(

soilofweightuniteffectivecohesionc

foundationofdepthddqfootingstripofwidthB

factorsninclinatioareiii

andfactorsdepthareddd

factorsshapeareSSS

TableingivenfasfactorscapacitybearingareNNNwhere

idSNBidSNqidScNq

ff

qc

qc

qc

qc

qqqqccccult

==

===

−

+−+=

γ

γ

φ

γ

γ

γ

γ

γ

γγγγ

11

2.5 Contact Pressure Distribution

Estimation of vertical stress at any point in a soil mass due to external loading is of great

significance in the prediction of settlements. The loads at the surface may act on flexible

or rigid footings. The stress conditions in the elastic layer below vary according to the

rigidity of the footing and the thickness and nature of soil. The variation of contact

pressures beneath flexible and rigid foundations on a clay , sandy and intermediate soil

types are shown FIG.2.6. When the bearing pressures are increased to the point of shear

failure in the soil, the contact pressure is changed tending to an increase in pressure over

the centre of the loaded area in each of these cases. A fully flexible foundation such as

the steel floor of an oil storage tank, assumes the characteristic bowl shape as it deforms

with the consolidation of the underlying soil.

In the calculation of settlement, it is important to be concerned with the pressure

distribution for a contact pressure which has a reasonable safety factor against shear

failure of the soil. Also, it is impracticable to obtain complete rigidity in a normal

foundation structure. Consequently, the contact pressure distribution is intermediate

between that of rigid and flexible foundations, and for all practicable purposes it is

regarded as satisfactory to assume a uniform pressure distribution beneath the loaded

area.

FIG.2.6 CONTACT PRESUURE DISTRIBUTION BELOW FOOTINGS

12

2.6 Stresses Induced in Soils

The pressure transmitted through grain at the contact points through a soil mass is termed

as intergranular or effective pressure. If the pores of soils are filled with water, and

pressure is induced in it, which tries to separate out the grains, then this pressure is

termed as pore water pressure or neutral pressure. Due to flow of water intergranular

pressure changes. The effective pressure reduces to zero when the hydraulic gradient

attains a value which is equal to the ratio of submerged unit weight of soil and unit

weight of water.

Estimation of vertical stress at any point in a soil mass due to external loading is of great

significance in the prediction of settlements. The loads at the surface may act on flexible

or rigid footing or piles. The stress condition in the elastic layer below vary according to

the rigidity of the footings and thickness of elastic layers. The verical stress at a point at

depth z in a semi-infinite soil mass, due to a point load on the ground surface at

horizontal distance r is given by Boussinesq formula as

Vertical stress caused by a point load.

The vertical stress at a point at depth z in a semi-infinite soil mass, due to a point load (Q)

on the ground surface at horizontal distance r is given by Boussinesq Formula as

2.7 Settlement Analysis of Soils

Structures transfer loads to the subsoil through the foundations. The effect of the load in

shallow foundation is felt significantly by the soil normally upto a depth of about twice

the least width of the foundation. The soil within this depth gets compressed due to the

imposed stresses. The compression of the soil mass leads to the decrease in the volume of

the mass which results in the settlement of the structure. The compression of the soil

mass due to the imposed stresses may be almost immediate for coarse grained soil

according to relative density, or time dependent according to permeability characteristics

of soils.

Consolidation Settlement of Cohesive Clay soils

Consolidation settlement of compressible clay soils can be estimated from e – p curve

(Fig.2.7 )

Sc = λ . Rf Σ h . mv . ∆p

( )

25

22

1

1

2

3

+=

zrz

Qz π

σ

13

FIG. 2.7 SETTLEMENT CALCULATION FROM e – p CURVE

FIG. 2.8 SETTLEMENT CALCULATION FROM e – logp CURVE

From e – logp curve Settlement may be estimated ( Fig.2.8) as,

where, mv = co-efficient of volume compressibility, corresponding to the pressure range

at mid-depth of respective layer, mv = av / ( 1 + e0 ) and av = ∆e/∆p , h = thickness

0

010

0

log1 p

ppC

e

hRS cfc

∆+

+= λ

14

of compressible layer ; if thickness is more than 3 m, the total thickness may be divided

into several layers, ∆p = the increase in effective overburden pressure at mid-depth of

corresponding layer,

p0 = initial effective overburden pressure

e0 = existing initial void ratio, and ∆e = change of void ratio.

Cc = Compression Index ( slope of e – log10p curve )

λ = settlement co-efficient depending on pore pressure

co-efficient, and relative thickness of cohesive layer

Rf = Rigidity factor due to stiffness of foundation.

Settlement of Foundations on Cohesionless Soils

Settlements of structures on cohesionless soils such as sands take place immediately as

the foundation loading is imposed on them. Because of difficulty of sampling these soils,

there are no practicable laboratory procedures for determining their compressibility

characteristics. Consequently, settlements of cohesionless soil deposits may be estimated

by semi-empirical method based on results of standard penetration tests or static cone

penetration test.

Method based on SPT Values IS : 8009 ( Part-I )

Settlement of footing with width B under unit intensity of pressure resting on dry

cohesionless deposit with known standard penetration resistance value N , may be read

from Fig..2.9. The settlement under any other pressure may be computed by assuming

that the settlement is proportional to the intensity of pressure. If water table is at a

shallow depth, the settlement read from Fig. 2.9 shall be multiplied by the correction

factor W’ .

FIG. 2.9 SETTLEMENT PER UNIT PRESSURE USING N-VALUE

15

Settlement for uniformly loaded flexible rectangular area of size L x B

L/B I3 Se (average) = 0.85 Se (centre)

1 1.222 E (sandy silt) =10,000 kN/m2 , and φ = 30

0

2 1.532 E = 40 +C (N - 6) kg/cm2 for N > 15

3 1.783 E = C (N + 6 ) kg/cm2 for N < 15

5 2.105 E = 2 qc , qc = static cone penetration resistance

Ec = 7qc , for clays, or Ec = 200 qu , qu = unconfined compression strength

All settlements that can be estimated for cohesionless soils, using assumed values of E &

µ and other parameters correlated to SPT or N values, are only immediate settlements,

and occur immediately on application of loads during constructions, and subsequently on

application of live loads. As such, part settlements for stages of loading may be

estimated, and compared with permissible settlements at different stages of constructions.

2.8 Engineering Appreciation

Engineering appreciation of different aspects of design and construction of any project is

very important. Understanding the influences of various interdependent parameters of

soil structure interaction on the satisfactory performance of structures in the light of

subsoil characteristics and design criteria is extremely essential. The techniques, stages

and the time taken for constructions of buildings should be finalised based on

considerations related to subsoil conditions.

Geotechnical considerations play an important role in the planning, design and

construction of foundations and structures. It is essential that subsurface conditions be

explored and tested for both design and construction requirements. The subsurface data

are generally gathered for design, may not necessarily be adequate for construction

operations. It is important to recognise that soil properties and behaviours are also

function of construction techniques and procedures.

Major planning decisions for foundations of buildings in urban areas are strongly

influenced by the cost, type and design of structural support system. The feasibility type

and cost of foundations are controlled to a significant extent by the character of the

subsurface soil materials and the construction procedures.

Geotechnical problems involving foundations and structures are complex, because of

influence of many inter-related factors. The close relationship between design and

construction of foundations is a result of dependence of behaviour, i.e. loads and

movements. Further dependencies result from intrinsically heterogeneous nature of

3

21I

EqbSe

µ−=

16

natural soil deposits, where geologic details may not be fully known. Ground settlements

are especially influenced by factors that are very difficult to predict correctly. Therefore,

in many cases, engineering analyses and predictions have to be based almost entirely on

observational data and prior experiences. Observational data obtained are frequently

either incomplete or too inaccurate.

The assessment of soil behaviour for design is based on exploration and selected tests on

undisturbed soil samples prior to construction. However, during installation of a

foundation and facility, the soil is disturbed by the procedures and sequences of

operations during constructions. The extent of soil disturbances by construction depend

on the type of foundations, to be built and numerous other factors normally not

considered. This disturbance is neglected in many foundation design and in the

evaluation of the soil structure interaction. This disturbance influences soil behaviour,

and structure interactions quite different from those assumed. The certainty of predictions

decrease with complexity of soil, difficulty of construction and indeterminate nature of

structure.

The theories for displacements and settlements which are of prime concern, often misled

the predictions of the behaviour of foundations not because the theories are unsound, but

because the resulting answer is only as good as the parameters used. Normally tolerable

limits of settlements are set from prior practice. It is important for each structure that the

significance of movements and settlements be evaluated in relation to the desired

performance of the structures. This means that the same degree of conservation should

not be applied to all kinds of structures. Generally, movement exceeds its tolerable limit

long before the forces causing failure develop.

2.9 Performance Criteria

Allowable bearing pressure is the maximum allowable net loading intensity of ground in

any case, taking into considerations of the bearing capacity, the estimated amount and

rate of settlement that is likely to occur, and the ability of the structure to accommodate

the settlement. It is therefore a function both of the site subsoil and the structural

conditions.

The principle of design according to limit condition has been generally adopted for all

types of engineering structures. By limit condition is meant such a condition of the

structure at which it ceases to meet functional requirement, i.e. it loses the capacity to

resist external forces or undergoes non-allowable deformations or local damage. The

limit condition of the superstructure under certain conditions is reached even without

substantial deformation of the subsoil. On this basis, the subsoil of the structure may have

two limit conditions :

(a) depending on its stability, and

(b) depending on its deformation / settlement criteria.

17

If the foundation loses its stability due to exceeding the strength of subsoils, the structure

erected on it will definitely cross the limit condition and fail. But the limit condition of

subsoil depending on deformation is a different phenomenon. The subsoil may deform to

an extent that the superstructure reaches the limit condition, although the bearing capacity

of the foundations is by no means exceeded.

Building practice shows that the latter case occurs more frequently. Therefore, the design

of foundations depending on deformation is a primary importance. Calculation according

to deformation comprises an integrated and complicated problem including numerous

problems which are naturally divided into two groups. The first group include methods of

settlement calculation in consideration of the combined effect of the structure and the

subsoil determination and choice of design properties of the soil. The second group

includes problems relating determination of allowable ultimate subsoil deformation of

structures.

The actual settlement of any foundation depends on a large number of factors which are

taken into account by the given method of calculation only with some degree of

approximation. The most important of these factors are the following :

(a) the load on the foundation which should be actual loads or an estimated average

imposed loading reflecting the actual occupancy of the building.

(b) Thickness of compressible layers in the subsoil,

(c) Compressibility of the soil which is function of stress history and pore pressure

coefficient.

The allowable subsoil deformation or settlement ( total and differential ) may be chosen

by the designer on the basis of analyses of the design of the structures and operation

conditions. The amount of settlement a structure can tolerate is considered to be

allowable or permissible settlement, which depends on many factors including the type,

size, and intended use of the structure, and the pattern, rate, cause and source of

settlement.

The settlement may be of various type – uniform , tilt or dish type. A building with a very

rigid structural material undergoes uniform settlement. Non-uniform or differential

settlement results from :

(i) uniform stress acting upon homogeneous soils,

(ii) non-uniform bearing stress,

(iii) non-homogeneous subsoil conditions, and

(iv) flexibility of the structure.

Settlements are of significance from two aspects. First, differential settlements may be so

large as to distort the structure, such that the collapse limit state is exceeded, and

secondly, the settlements may include cracking or distorsion of the structure such that the

serviceability limit is exceeded.

18

Generally, the magnitude of total uniform settlement is not a critical factor. No damage

will be done to a structure if it settles uniformly as a whole regardless of how large the

settlement may be. It is usually the differential settlement that is important in the

designing of a foundation and structure. The magnitude of differential settlement is

affected greatly by the non-homogeneity of natural soil deposits, and also by the ability

of structure to bridge over soft soil in the foundation. Soil characteristics are never

uniform even in an apparantly uniform soil deposit. The differential or relative settlement

between one part of ac structure and another is of great significance to the stability of

superstructure than the magnitude of total settlement. Serious cracking and even collapse

of the structure may occur if the differential movements are excessive. The degree of

damage caused by settlement is to some extent dependent on the sequence and time of

construction operations.

2.10 DESIGN OF SHALLOW FOUNDATIONS

Shallow foundations are those that are placed at a depth D, not exceeding the width B of

the foundation. From the point of view of design, the shallow foundations are classified

into four types :

(a) Spread footing. (b) Combined footing., (c ) Strap footing.

(d) Raft or Mat foundation., (e) Floating foundation.

A foundation is an integral part of the superstructure. The stability of a structure depends

upon the stability of supporting soil. In designing foundations, the following basic

requirements must be satisfied :

(a) The foundation structure must be properly located with respect to any

future influence which could adversely affect its performance.

(b) The foundation including the earth beneath must be stable or safe from any

mode of failure.

(c) The foundation must not settle or deflect sufficiently to damage the

structure or impair its usefulness.

The location and depth of foundation of a structure play an important point in the overall

stability of the foundation. The location of the foundation in an area should not affect

either its future expansion, or its foundation should not be affected by the constructions in

the adjoining area. The depth of foundation depends upon the type of soil, size of

structure, the magnitude of loads, and the environmental conditions. The factors that

govern the minimum depth of shallow foundations are

(a) Local erosion, (b) Underground defects,

(c ) Unconsolidated filled up soil,

(d) Presence of adjacent structure, property line etc.

(e) Ground water level,

(f) Expansive soil,

19

FIG. 24 NEW FOUNDATION NEAR EXISTING FOUNDATION

(a)

(b)

FIG. 2.10 EFFECT OF STRESS FROM ONE FOUNDATION TO OTHER

The selection of type of foundation depends on many factors like

(a) The function of the structure and the loads it must carry,

(b) The subsurface condition,

(c) The cost of the superstructure.

In selecting the type of foundation, the design load plays an important role which again

depends on the subsoil conditions. The various loads that are likely to be considered are

20

(a) Dead loads, (b) Live loads, (c ) Wind and earthquake loads,

(d) Lateral earth pressure on structure,

(e) Impact equivalents related to moving and dynamic loads,

(f) Uplift forces, (g) Swelling pressures.

Special Foundations

Special typical foundations are required to be provided for special structures like

overhead water tanks, silos, chimneys, cooling towers, transmission towers, different

industrial structures, and ground storage tanks.

FIG. 2.11 GROUND STORAGE TANK FOUNDATIONS TYPE a & b

FIG. 2.12 GROUND STORAGE TANK FOUNDATIONS TYPE c , d , e, f, g & h

21

3.0 PILE FOUNDATION

Shallow foundations are normally used where the soil close to the ground surface and

upto significant depth possesses sufficient bearing strength to carry the superstructure

load without causing any distress. However, when the subsoil is weak, the load from

superstructure needed to be transferred to the deeper strata, pile foundation is the obvious

choice. Pile foundation shall be designed in such a way that the load from the structure

can be transmitted to the soil without causing any soil failure and without causing such

settlements, differential or total, under permanent / transient loading which may result in

structural damage and/or functional distress. The pile shaft should have adequate

structural capacity to withstand all loads and moments which are to be transmitted to the

subsoil.

Pile foundation is particularly used where the upper soil strata are normally weak or

compressible, and the load has to be transferred to deeper layer or to a firm stratum. The

behaviour of piles and their load transfer mechanism are completely different from that of

shallow foundations. The piles are commonly used :

a) To carry vertical compressive, uplift, lateral or overturning forces from the

structure.

b) To control settlements when spread footings are underlain by highly compressible

stratum.

c) To stiffen the soil beneath machine foundation to control both amplitude of

vibration and the natural frequency of the system.

d) As an additional safety factor beneath bridge piers, particularly if scour is a

potential problem.

e) In offshore construction to transmit loads from waves, through water into the

underlying soil.

f) To control earth movement.

Theory, understanding of complex load transfer mechanism and practical experience in

piling works are extremely important in safe and satisfactory design and construction of

pile foundation. The available codes of practice for design, construction and testing of

piles cannot be totally relied upon for any blind use.

The construction of piles is normally undertaken by any constructors, many of whom

may not be necessarily a civil engineer having minimum understanding of the behaviour

of piles in various ground conditions, and the consequences of the methodology of piling

adopted, on the performance of pile foundation. The quality construction of piles depends

on many factors. In modern piling practice, subsoil investigation is extremely important

in making decisions both regarding pile design and the choice of appropriate construction

methods. The major requirement of soil investigation in terms of design is to provide

comprehensive information over the full depth of the proposed foundations, and well

below any possible pile toe level.

22

The behaviour of piles depends on many factors, including subsoil conditions, which

should be well investigated, materials and types of piles and also methods of

construction. As such, piles are required to be classified properly, for realistic

understanding of pile behaviour.

3.1 Classification of Piles

Piles may be classified in a number of ways, based on

(a) material,

(b) method of installation, and

(c ) method of load transfer.

The materials of piles may be timber, steel, concrete or composite materials. Piles

entirely submerged in water last longer without decay, provided marine borers are not

present. When pile is subjected to alternate wetting and drying, the useful life is relatively

short unless treated with wood preservatives. Steel piles are usually rolled H-shapes or

pipe piles. Pipe piles are often filled with concrete after driving.

Steel H-Piles. Steel H-piles have significant advantages over other types of piles. They

can provide high axial working capacity, exceeding 200 tons. They may be obtained in a

wide variety of sizes and lengths and may be easily handled, spliced, and cut off. H-piles

displace little soil and are fairly easy to drive. They can penetrate obstacles better than

most piles, with less damage to the pile from the obstacle or from hard driving. The major

disadvantages of steel H-piles are the high material costs for steel and possible long

delivery time. H-piles may also be subject to excessive corrosion in certain environments

unless preventive measures are used. Pile shoes are required when driving in dense sand

strata, gravel strata, cobble-boulder zones, and when driving piles to refusal on a hard

layer of bedrock.

Steel Pipe Piles. Steel pipe piles may be driven open- or closed end and may be filled

with concrete or left unfilled. Concrete filled pipe piles may provide very high load

capacity, over 500 tons in some cases. Installation of pipe piles is more difficult than H-

piles because closed-end piles displace more soil, and open-ended pipe piles tend to form

a soil plug at the bottom and act like a closed-end pile. Handling, splicing, and cutting are

easy. Pipe piles have disadvantages similar to H-piles (i.e., high steel costs, long delivery

time, and potential corrosion problems).

Precast Concrete Piles : Precast concrete piles are used for variety of structures, when

soil conditions may be unfavourable for cast-in-situ piles. The structural design of precast

concrete piles is governed by the stresses caused by handling and driving. Concrete piles

are precast or cast in situ. Normally precast piles of square or octagonal sectioned are

manufactured. Necessary reinforcements are provided by taking care of handling and

driving stresses.

23

Fig. 3.1 Typical Details of Precast Reinforced Concrete Pile

According to method of installation, piles may broadly be classified into

(i) Displacement piles or driven piles, and

(ii) Non-displacement piles or replacement piles or bored piles.

Displacement piles may be totally preformed or driven cast in situ. For driven cast in situ,

normally a steel tube is driven into the ground to form a void, which may be filled with

concrete. The steel tube may be withdrawn during concreting with reinforcements. In

non-displacement piles, normally a void is formed by boring, and soil is replaced by

preformed or cast in situ concrete piles.

Based on load transfer mechanism, piles are divided into

(a) Friction Piles, and

(b) End Bearing Piles

If the bearing stratum at the pile tip is a hard and relatively impenetrable material such as

rock or very dense sand and gravel, the piles derive load carrying capacity mostly from

tip resistance. Such piles are called end-bearing piles. On the other hand, if the piles do

not reach the hard stratum, their carrying capacity is mostly derived from the skin friction

along the embedded length.Such piles are floating or friction piles.

3.2 Piling Engineering

It would be logical to realise and distinguish “Piling Engineering” from “ Pile

Engineering”. The latter represents a typical structural / soil design route, which employs

subsoil parameters from standard investigation report.; while the former includes the

recognition of installation methodology along with due consideration for its response to

the subsurface conditions.

24

3.2.1 Pile Driving Equipment

Piles are installed or driven into the ground by a rig which supports the leads, raises the

pile, and operates the hammer. Rigs are usually prefabricated in units and assembled in

the field. Modern commercial rigs use vibratory drivers while most older and expedient

rigs use impact hammers. The intent is the same, that is to drive the pile into the ground

(strata).Pile-driving rigs are mounted in different ways, depending on their use.

Specialized machines are available for driving piles. Most pile driving is performed using

a steel-frame, skid-mounted pile driver or power cranes, crawlers, or truck-mounted

units, with standard pile-driving attachment. The leads and catwalk assembly support

drop hammers. .

FIG. 3.2 Typical Driven Piling Rig

FIG.3.3 Drop Hammer and Pile Tube with Cap & Cushion

25

FIG.3.4 Winch for Controlling Piling Operation & Hammer Drop

3.3 Construction and Piling Methods

The selection of type, length and capacity of pile is ususlly made from estimation based

on the soil conditions and magnitude of loads and design criteria. The method of

construction of a pile at the site depends upon the type of pile. Pile of any particular type

may be considered on various considerations, such as, availability, handling, driving,

strength, quality and flexibility. The following types of piles may be considered :

FIG.3.5 Premix Concrete is being poured through Tremie Pipe

1. Driven Piles - The piles that come under this category are Timber, Steel, Pipe

piles and precast concrete piles. There are advantages and disadvantages of the

system. Except in special circumstances, use of precast piles are not common.

26

2. Driven Cast in Situ Piles – This involves driving a steel tube in diameters ranging

from 300 to 600 mm, to the required depth with end closed by a shoe. The tube is

normally driven by a drop hammer striking at the top. Enlarged bases can be formed

by using an internal drop hammer to force out a plug of concrete from beneath the

toe of the tube. After putting the reinforcement cage inside the tube, concrete is

poured into the tube in stages. The steel tube is withdrawn simultaneously with

concreting. Care must be taken to see that the bottom of the casing is always kept

sufficiently below the level of concrete within the casing to prevent water entering

the casing tube.

FIG.3.6 Installation Method of franki Piles

3. Bored Cast in Situ Piles – Boring of pile hole is done either by augering or drilling

with direct mud circulation. The bottom of the bore may be under-reamed, if

necessary. In case, a bored pile is stabilised by drilling mud or by maintaining water

heads within the hole, the bottom of the hole shall be cleaned very carefully before

concreting work is taken up. Borehole shall be flushed with fresh bentonite solution

under pressure through tremie pipe.

After boring is completed and properly cleaned, the hole is concreted after putting the

reinforcement cage. Concreting under water shall be done with the use of tremie method.

In addition to the normal precautions to be taken in tremie concreting, the following

requirements are particularly applicable to the use of tremie concrete in pipes :

a) The concrete should be coherent, rich in cement (not less than 400 kg/m2 ) and

slump between 150 to 200 mm.

b) When concreting is required to be done under water, a temporary casing should

be used for full length of bore hole or a partial casing, so that the fragments of

ground cannot drop from the sides of the hole into the concrete as it is placed.

c) The hopper and tremie should be water tight closed system embedded in the

placed concrete, through which water cannot pass.

27

d) The tremie should be large enough with due regard to the size of the

aggregates. For 20 mm aggregate, the tremie pipe should be of diameter not

less than 200 mm.

e) Where cutoff level is less than 2.5 m below the ground, concrete shall be cast

to minimum of 600 mm above cutoff level.

FIG. 3.7 Stages of Construction of Bored Under-reamed Pile

Large Diameter Bored Piles - Typically a bored pile with diameter more than 600 mm

is categorised as large diameter bored pile. Emphasis and concern may be directed to the

subsurface formations, such as Alluvium, Deltaic, Marine, Off-shore, Back swamp,

Residual and geomorphologically altered formations. Further, the prime soil mechanics,

to the extent they are valid wihin the boundaries of theoretical assumptions and within the

precincts of laboratory, must be realised, modified / modulated to encompass the natural

field boundaries with the site-specific characterisation evolved from many naturally

occuring features.

The basic need is forming a large diameter hole to depths as required; keeping the hole

temporarily stable against all varieties of issues, like side collapse, squeezing, occurrence

of unfavourable hydraulic gradient, temporary liner, permanent liner or fluid stabilisation.

Specifications of Drilling Mud (Bentonite)

The bentonite suspension used in bore holes is basically monmorillonite clay, and helps

in stabilising the sides of bore holes by forming a membrane on the bore hole wall. In the

case of granular soil , the bentonite suspension penetrates into sides under positive

pressure and after a while forms a jelly deposited on the sides of the pores and makes the

surface impervious and imparts a plastering effect. In impervious clay, the bentonite does

not penetrate into the soil, but deposits only as thin film on the surface of hole. Under

such condition, stability is achieved from the hydrostatic head of suspension.

28

The bentonite shall have liquid limit not less than 400%. The density of suspension shall

be about 1.10 g/ml The density of suspension after contamination with deleterious

material in the bore hole may rise upto 1.25 g/ml, and should be brought down to at least

1.12 g/ml by flushing before concreting. The pH value of the bentonite suspension shall

be between 9 and 11.5.

3.4 Effects of Installation of Piles

Techniques of installation of piles have a very important effect on the carrying capacity

of piles. Th advantage of the soil design method of calculating carrying capacity is that it

enables allowable loads to be assessed from considerations of the characteristics of the

soil and the type of pile. However, confirmation of the design assumption must be made

at some stage by load tests on piles.

The effects of pile driving in clays have been classified into four major categories :

(a) Remolding or partial structural alteration of the soil surrounding the pile.

(b) Alteration of the stress state in the soil in the vicinity of the pile.

(c) Dissipation of the excess pore pressures developed around pile.

(d) Long term phenomena of strength regain in soil.

When a pile is driven into sands and cohesionless soils, the soil is usually compacted by

displacement and vibration, resulting in permanent rearrangement of particles. Thus in

loose sands, the load capacity of a pile is increased as a result of the increase in relative

density caused by pile driving. In such case, installation by driving rather than boring has

distinct advantages. When groups of piles are driven into loose sand, the soil around and

between the piles becomes compacted. If the pile spacing is close, the ultimate capacity

of the group may be greater than the sum of the capacities of the individual piles.

The effects of installing bored piles in clay have been studied largely in relation to the

adhesion between pile and soil. The adhesion has been found to be less than the

undrained cohesion before installation, mainly because of softening of the clay

immediately adjacent to the soil surface. This softening may arise from three causes :

(a) Absorption of moisture from the wet concrete.

(b) Migration of water from the surrounding soil into the bore hole.

(c) Water poured into the boring to facilitate operation of cutting tool.

A further effect of installing a bored pile is that the clay just beneath the pile base may be

disturbed and softened by the action of the boring tools. Soil sludge also gets deposited at

the base of bore hole. It is very important to clear out the base thoroughly. The effect of

this disturbance may result in increased settlement.

Construction problems may also arise with bored piles, such as

(a) Caving of the bore hole, resulting in necking or misalignment of pile.

(b) Aggregate separation within the pile.

(c) Buckling of the pile reinforcement.

29

Such structural defects may be difficult to detect, since a load test may not reveal any

abnormal behaviour, especially if the load is only taken to the design load. In favourable

conditions, bored piles can be constructed without casing, except for a short guide casing

length. Under less satisfactory conditions, temporary casing shall be used to support the

wall of bore hole, or bentonite drilling fluid may be resorted to.

A variety of defects may arise when forming cast in situ piles in very soft alluvium. The

lateral pressure of concrete can easily exceed the passive resistance of soft soils, and

bulges on the pile shaft will almost certainly occur. Such defects may be detected by

close check on the volume of concrete used or by sonic integrity test. Near the head of the

pile, the lateral pressure of concrete may be low, and further reductions in pressure can be

caused by friction as the casing is withdrawn. In such situations, it is possible for soft soil

to squeeze the pile section, leading to local wasting of the concrete.

Concreting cast in situ piles is a special operation calling for considerable skill and the

correct design of concrete mix. More problems are caused by using a mix which is too

stiff than by using one with high slump. For bore holes filled with water or drilling mud,

the tremie method of concreting is necessary. After concreting, extraction of the

temporary casing can create problems, particularly if delays occur and partial separation

of the pile shaft may result. The casing, if it has to be withdrawn it must be withdrawn

simultaneously with concreting with external tamping.

3.5 Behaviour of Piles

The behaviour of piles are greatly influenced by the load transfer mechanism. If the

bearing stratum at the pile tip is a hard and relatively impenetrable material such as rock

or very dense sand and gravel, the derive most of of their carrying capacity from the

resistance of the stratum at the tip or toe of the piles. Such piles are called end-bearing

piles. On the other hand, if the piles do not reach an impenetrable stratum, but are driven

for some distance into penetrable soils, their load carrying capacity is mostly derived

from the skin friction along the embedded pile surface. Such piles are called floating or

friction piles.

The load bearing capacity of a pile depends on the properties of the soil in which it is

embedded. A pile subjected to vertical axial load will carry the load partly by frictional

resistance along shaft and partly by the resistance at the base. A horizontal load on a

vertical pile is transmitted to the subsoil primarily by horizontal subgrade reaction

generated in the upper part of the shaft. Lateral load bearing capacity of a single pile

depends on the soil reaction developed and the structural capacity of the shaft under

bending.

The ultimate bearing capacity of a pile may be estimated by static formula on the basis of

soil investigation data, or by using a dynamic formula using data obtained during pile

driving. However, dynamic formula should be used only as a measure to control the pile

driving at site. Pile capacity shall always be confirmed by appropriate load tests. The

30

settlement of pile obtained at safe / working load from load-test data on a single pile shall

not be directly used for estimating the settlement of the structure.

FIG. 3.8 Soil Resistances supporting the pile load Q

Total load Q on pile is supported by soil frictional resistance (τs ) along pile shaft, and

pile tip resistance qb .

The ultimate bearing capacity of a pile used in design may be one of three values: the

maximum load Qmax, at which further penetration occurs without the load increasing; a

calculated value Qf given by the sum of the end-bearing and shaft resistances; or the load

at which a settlement of 0.1 diameter occurs For large-diameter piles, settlement can be

large, therefore a safety factor of 2-2.5 is usually used on the working load. A pile loaded

axially will carry the load: partly by shear stresses ( τs ) generated along the shaft of the

pile and partly by normal stresses (qb) generated at the base. The ultimate capacity Qult of

a pile is equal to the base capacity Qb plus the shaft capacity Qs.

Qult = Qb + Qs = Ab . qb + (As τs. )

where Ab is the area of the base and As is the surface area of the shaft within a soil layer.

Full shaft capacity is mobilised at much smaller displacements than those related to full

base resistance. This is important when determining the settlement response of a pile. The

same overall bearing capacity may be achieved with a variety of combinations of pile

diameter and length. However, a long slender pile may be shown to be more efficient

than a short stubby pile. Longer piles generate a larger proportion of their full capacity by

skin friction and so their full capacity can be mobilised at much lower settlements.

31

The proportions of capacity contributed by skin friction and end bearing do not just

depend on the geometry of the pile. The type of construction and the sequence of soil

layers are important factors.

The pile capacity can be written as

Qu = Qp + Qf

where, Qp = ultimate point resistance, and

Qf = ultimate shaft or frictional resistance.

The shaft or frictional capacity of a pile is mobilised at much smaller displacements of

about 0.5% to 1.0% of pile diameter, compared to displacement required of about 10% of

pile diameter for full mobilisation of base resistance Qp . Choice of factor of safety for

such a pile must be made in the light of different response of pile shaft and base.

Load-settlement characteristics, and load measurements using strain gauges along pile

shaft and also at pile tip, will show that initially shaft takes an increased amount of skin

friction load, but the load carried by the shaft will not equal the total load on the pile,

indicating that some proportion of the load is gradually being carried in end-bearing.

When the load approaches failure value, the settlement increases rapidly with little

further increase of load. Gradually, the proportion of base capacity increases. If the total

load on the shaft and the load on the base of a pile are measured separately, the load-

settlement relationship for each of these components will reveal that the skin friction on

the shaft increases to a peak value, then fails with increasing settlement. On the other

hand, the base load increase progressively until complete failure occurs.

When piles are arranged in close-spaced groups, the mechanism of failure is different

from that of a single pile. The piles and the soil contained within the group act together as

a single unit. A slip plane occurs along the perimeter of the group and block failure takes

place when the group sinks and tilts as a unit. The failure load of a group is not

necessarily that of a single pile multiplied by the number of piles in he group. In sands, it

may be more than this ; in clays it is likely to be less. The efficiency of a pile group is

taken as the ratio of the average load per pile, when failure of the group occurs, to the

load at failure of a comparable single pile.

So far, the failure load has been taken as the load causing ultimate failure of piles.

However, in engineering sense, failure may have occurred long before reaching the

ultimate load, since the settlement of the structure would have exceeded tolerable limits.

In almost all cases, where piles are acting as structural foundations, the allowable load is

governed solely from considerations of tolerable settlement at working load.

3.6 Vertical Load Bearing Capacity of Piles

Dynamic pile formulas are based on the theory that the allowable load on a pile is closely

related to the resistance encountered during driving. The concept assumes that the soil

resistance remains constant during and after driving operations. This may be true for

coarse-grained soils, but may be in error for fine-grained soils because of the reduction in

32

strength due to remolding caused by pile driving. The allowable load on a pile may be

estimated by the Engineering News formula.

The determination of ultimate load capacity using static formulae is based on the

principles of soil mechanics. It has already been discussed that the ultimate point and skin

friction resistances are not mobilised simultaneously. The ultimate magnitudes and the

strain dependent resistances greatly depend on the types of soils and the method of

installation. These aspects are to be properly taken care of, while estimating the pile

capacity by static formulae.

The ultimate load carrying capacity of a pile is: Qult = Qb + Qs

Therefore, the ultimate pile capacity in any soil medium is:

Qult = Ab (9Cb + pd Nq ) + ππππd Σ Σ Σ Σ (ααααiCi + Ki. pdi .tanδδδδιιιι)li

Values of earth pressure coefficient K and and angle of soil pile friction δ may be related

to the angle of internal friction (φ´).

where, Qp = ultimate soil resistance at the level of the pile tip/base ,

Qf = ultimate frictional resistance along pile shaft ,

Ab = cross sectional area of pile tip,

Nc = 9 for all piles, As = area of pile shaft = Σ π d l,

Cb & Cs = undrained cohesion at pile tip and average value along

shaft respectively,

d = diameter of piles, γ = effective unit weight of soil,

pd = effective overburden pressure at pile tip level = 15 to 20

times of d.γ

pdi = effective overburden pressure at mid depth of layer i

K = earth pressure coefficient depends on the nature of soil

strata, type of pile and its method of construction. For

driven piles in loose to dense sands, k values varies from

1 to 2 . For bored piles, K = 1 to 1.5

δ = angle of friction between pile surface and side soil = 0.75φ

α = adhesion or reduction factor for piles through cohesive soils .

The following values of α may be taken depending upon the consistency of soils :

Consistency N value Cu Values of αααα

kN/m2 Bored Piles Driven Piles

Soft to very soft < 4 25 0.7 1.0

Medium 4 to 8 25 to 50 0.5 0.7 to 0.4

Stiff 8 to 15 50 to 100 0.4 0.4 to 0.3

Stiff to hard > 15 > 100 0.3 0.25

33

FIG. 3.9 Bearing Capacity Factor Nq for Pile

3.6.1 Use of Static Cone Penetration Data

When full static cone penetration data are available for entire depth, the following

correlation may be used as a guide for the determination of the pile capacity. The ultimate

end bearing resistance qu in kN/m2 may be taken as

where, qc 0 = average static cone resistance in kN/m2 over a depth of 2d

below pile toe.

qc 1 = minimum static cone resistance in kN/m2 over a depth of 2d

below pile toe. qc 2 = average of the envelope of minimum static cone resistance

values over length of 8d above pile toe.

qc = cone resistance at any depth in kN/m2

The ultimate skin friction resistance can be approximated to local side friction in kN/m2

obtained from static cone resistance as

2

22

10

c

cc

u

qqq

q

++

=

34

Type of soil Local side friction fs in kN/m2

For qc less than 1000 kN/m2 qc/30 < fs < qc/10

Clays qc/25 < fs < 2qc/25

Silty clay and Silty sands qc/100 < fs < qc/25

Sands qc/100 < fs < qc/50

3.6.2 Meyerhof Formula for Cohesionless Soil

The correlation suggested by Meyerhof using standard penetration test data N in saturated

cohesionless soil to estimate the ultimate capacity of driven pile as

The first part gives the end bearing resistance Qp (not exceeding 400N Ap ), and the

second part gives the frictional resistance Qf .

where, N = average N-value at pile toe.

Lb = length of penetration of pile in the bearing strata in m.

N = average N-value along the pile shaft.

For non plastic silt or very fine sand , the equation has been modified as

3.6.3 Piles in Weathered / Soft Rock

For pile founded in weathered / soft rock different empirical approaches are used to arrive

at the socket length necessary for utilising the full structural capacity of pile. Since it is

difficult to collect cores in weathered / soft rocks, the method suggested by Cole and

Stroud using N-values is more widely used. The allowable load on the pile is given by

Where, Cu 1 = Shear strength of rock at base of pile in kN/.m2

Nc = Bearing capacity factor taken as 9

Fs = Factor of safety usually taken as 3.

α = 0.3 as recommended value.

Cu 2 = Average shear strength of rock in the socketed length.

B = Diameter of Pile in m, L = Length of Socket in m.

For N > 60 , the stratum to be treated as weathered rock rather than soil.

50.040)( s

pb

u

ANA

d

LNkNQ +=

60.030 s

pb

u

ANA

d

LNQ +=

s

u

s

cuaF

LBC

F

BNCQ

παπ2

2

14

+=

35

Consistency and Shear Strength of Weathered rock.

Strength /

consistency

Grade Shear

strength

in kN/m2

Breakability Scratch

Very strong A 40000 Difficult to break Cannot be scratched with

knife

Strong B 20000 Broken against solid

object with hammer

Can just be scratched

Moderate C 4000 Broken in hand with

hammer

Can just be scratched by

thumb nail

Weak D 2000 Broken by leaning on

sample with knife

No Penetration by thumb

nail

Weak E 1000 Broken by hand Penetration with knife

3.6.4 Load Carrying Capacity using Dynamic Formula

For driven precast or cast in piles, the following modified Hiley formula may be used.

Qu = W h η / ( S + 0.5 C )

where, Qu = ultimate driving resistance in kN,

W = weight of monkey or rammer in kN,

h = effective height of free hall of drop hammer in m,

η = efficiency of blow = ( W + P e2 ) / ( W + P ) for W > P e

= ( W + P e2 ) / (W + P ) - ( ( W - P e ) / (W + P ) )

2 for W<P.e

e = coefficient of restitution of the materials under impact may

be taken as

(a) For drop hammer striking on the head of reinforced concrete pile , e = 0.4

(b) For drop hammer striking a well conditioned driving cap and helmet

with hard wood dolly in driving reinforced concrete piles , e = 0.25

(c) 0.25 to 0.4 for single acting steam hammer,and 0.4 to 0.5 for double

acting steam hammer,

S = final set or penetration per blow in cm,

C = sum of temporary elastic compressions in cm of pile, dolly, packings and

ground that can be conveniently measured during driving of pile.

C = C1 + C2 + C3

For concrete pile, elastic compression of pile C1 = 0.2 – 0.25 cm.,

Elastic compression of head assembly C2 = Qu L/ApEp ,

and elastic compression of soil C3 = 0.25 cm.

36

Simplex Formula

The skin friction component of pile capacity is brought into the empirical expression by

means of resistance measured in hammer blows for the full driving of pile. It is necessary

to maintain a uniform fall of hammer throughout the driving of pile and recording the

total number of blows Np required for full penetration of the pile.

The ultimate capacity Ru ( kN ) expressed as

where, W = weight of pile hammer in kN.

H = height of free fall in m, L = length of pile,

P = average set in cm for last four blows.

3.6.5 Negative Skin Friction on Pile

When the soil surrounding the pile shaft moves downwards relative to the pile, downdrag

stresses are developed along pile shaft. This is known as negative skin friction . The

phenomenon occurs when the pile tip rests on relatively stiffer or rigid stratum. The

downdrag on pile may be caused by

(a) Consolidation of soil under the weight of recent fill

(b) Land subsidence due to lowering of ground water table

(c) Reconsolidation of soil around pile disturbed by driving.

Negative skin friction imposes additional load on pile, and decreases the ultimate load

capacity of the pile. The settlement of pile foundation also becomes excessive due to

downdrag.

3.6.6 Load Bearing Capacity of Pile Group

Very rarely structures are founded on single piles. The spacing of piles in group is

decided in consideration of several factors. Which include

(a) Overlapping of stresses of adjacent piles

(b) Method of installation

(c) Type of soil and load transfer mechanism, and

(d) Efficiency of pile group.

36.254.2

Lx

P

WHx

L

NR

p

u +=

37

FIG. 3.10 Effect of Spacing on Load Transfer Mechanism

There is no acceptable efficiency formula for pile group capacity. The capacity of pile

group may or may not be equal to single pile capacity For floating pile groups, the

efficiency is unity at relatively large spacing; but decreases as spacing decreases. For

point bearing piles, the efficiency is usually considered to be unity for all spacings. For

piles that derive their load capacity from both side adhesion and bearing, it ia often

recommended that group effect be taken into consideration for side adhesion component

only.

Out of several empirical formulae, the one known as Converse-Labarre formula is often

used which is expressed as

Group Efficiency mn

mnnm )1()1(

901

−+−−=

θη

where, m = number of rows n = number of columns, d = diameter of pile

θ = tan-1

d/s in degrees, s = spacing of pile

Since there appears to be little field evidence to support the consistent use of any

empirical formula, an alternative means of estimation group efficiency has been widely

attempted, whereby group capacity is the lesser of the

(a) the sum of ultimate capacities of the individual piles in the group, and

(b) the bearing capacity for the block failure of the group which is expressed as

Qgu = BL CbNc + 2 (B+L). l. Cu

38

where , Cb = undrained cohesion at pile base

Cu = undrained average cohesion along depth

l = length of piles B = width of pile group

L = length of pile group Nc = bearing capacity factor

FIG. 3.11 Overlapping of stresses in Pile Group

3.6.7 Settlement of Pile Group in Clay

The consolidation settlement of a compressible clay stratum along pile depth and also

below the pile tip can be obtained approximately using the one dimensional consolidation

theory, once the stress increase due to pile group for a layer is known. The problem

involved here is to estimate the increase in stress ∆p beneath a pile group, when the group

is subjected to vertical load Qg . The increase in effective stress ∆p can be obtained

approximately by assuming an equivalent footing at the two-third depth of pile in the

compressible layer. Then the settlement can be easily estimated as

S = Σ hi mvi ∆pi

where, h = thickness of ith. layer

∆pi = increase in effective stress at mid-depth of ith. layer

mvi = coeff. of volume compressibility of ith. layer

3.7 LATERAL LOAD CAPACITY OF PILES

The piles are often subjected to lateral forces under different conditions as indicated

above. In designing such piles, two criteria need be satisfied : first, an adequate factor of

safety against ultimate failure; and second, an acceptable deflection at working loads.

39

Sources of Lateral Loading

• Earth pressures on retaining walls

• Wind Loads and Seismic Loads

• Impact Loads from Ships (Berthing, Pier Collision, waves etc. )

• Eccentric Loads on Columns

• Slope movements

• Cable forces on transmission towers

Batter Piles

• Basically turn lateral loads into axial loads

• Present challenges in driving and testing

• Form a very stiff system than can pose problems in seismic situations

• Very common solution to lateral loading

Fig. 3.12 Raker Piles used to resist high Lateral Loads

The ultimate resistance of a vertical pile to lateral load and the deflection of the pile as

the builds up to its ultimate value are complex matters involving the interaction between

a semi-rigid structural element and soil which deforms partly elastically and partly

plastically. The failure mechanism of an infinitely long pile and that for a short pile are

different. The failure mechanism also differ for a restrained and nonrestrained pile head

conditions.

40

FIG.3.13 Lateral Failure Short vs. Long Foundations

FIG.3.14 Compression of Soil in Lateral Loading

Because of complexity of the problem, only an approximate solution is adequate in most

of the cases. To determine if the pile is a short or long, calculate the stiffness factor R or

T for particular combination of pile and soil. The embedded length of pile is Le.

Determine the depth of Fixity and equivalent length of cantilever (IS :2911, Part-1/sec-2)

where Stiffness

T = (EI/k1)0.2

for coarse grained soil

R = (EI/k2 )0.25

for fine grained soil

E = Young’s modulus of pile material in kg/cm2

I = moment of Inertia of pile cross-section in cm4 = π d

4 /4

If embedded length Le > 4T or 4R, the pile is long flexible pile

and if embedded length Le < 4T or 4R , the pile is rigid.

41

FIG.3.15 Rigid and Flexible Pile

Values of Constant k1 (IS:2911)

Soil Type

(coarse grained soil)

N-value Values of k1

Dry Submerged

Loose Sand

Medium Sand

Dense Sand

Very Loose Sand

2 – 4

4 - 10

10 - 35

< 2

o.26 0.146

0.775 0.525

2.075 1.425

-- 0.0406

Values of Constant k2 (IS:2911)

Soil Consistency (fine grained soil)

Unconfined

compression strength

qu in kg/cm2

Values of k2

kg/m2

Soft

Medium Stiff

Stiff

Very Stiff

0.20 to 0.40

1 to 2

2 to 4

More than 4

7.75

48.80

97.50

195.50

42

( )IE

zeHy

f

12

3+=

In designing piles for lateral load, two criteria need be satisfied : first, an adequate factor

of safety against ultimate failure; and second, an acceptable deflection at working loads.

The safe lateral load capacities as recommended may be moderated in design, keeping

compatibility with structural design and acceptable horizontal deflection.

The following assumptions are made in the analysis for determination of ultimate lateral

load carrying capacity of bored cast in situ RCC pile :

The active earth pressure acting on the back of the pile is neglected.

The distribution of passive pressure along the front of the pile is equal to three times the

Rankene passive pressure.

1. The shape of the pile section has no influence on the distribution of ultimate soil

pressure or the ultimate lateral resistance.

2. The full lateral resistance is mobilised at the movement considered.

3. The piles are long piles in cohesionless soils and pile heads are restrained.

Safe Lateral Capacity of fixed head pile for permissible lateral defection of 5 mm

( IS : 2911 Part I/Sec 2 ) is given by

where

y = Deflection of pile head in cm. = 0.5 cm

H = Safe Lateral load capacity in kg, for 5 mm lateral deflection at pile head.

E = Young’s Modulus of pile material

I = Moment of Inertia of pile cross section in cm4

zf = Depth of fixity

FIG. 3.16 Depth of Fixity of Pile

43

The safe lateral load capacity of pile will increase with increase of concrete grade, pile

diameter and lateral subgrade modulus of soil, and also when piles are working in a

group. Lateral capacity can be increased also by backfilling the excavation up to depth of

pile cap using compacted stone aggregates.

Safe lateral load capacity shall be checked by suitable lateral load test.

The ultimate resistance of a vertical pile to a lateral load and the deflection of the pile as

the builds up to its ultimate value are complex matters involving the interaction between

a semi-rigid structural element and soil which deforms partly elastically and partly

plastically. The failure mechanism of an infinitely long pile and that for a short pile are

different. The failure mechanism also differ for a restrained and non-restrained pile head

conditions. The ultimate lateral resistance may be estimated using standard curves given

below.

FIG. 3.17 Long Flexible Restrained pile In Cohesive Soil

f = Hu / (9Cu d)

FIG.3.18 Ultimate Lateral Capacity of Pile in Cohesive Soil

44

FIG. 3.19 Ultimate Lateral Capacity of Long Pile in Cohesionless Soil

FIG. 3.20 Long Flexible Restrained pile In Cohesionless Soil

45

3.7.1 Methods of Improving Lateral Load Capacity

FIG.3.21 Methods of Improving Lateral Load Capacity

3.8 Pile Testing and Quality Control

Static pile load testing is the most definitive method for determining pile capacity. To

facilitate the testing procedure, the arrangement for testing is important. Suitable

kentledge or anchor piles to be provided to take the reaction of the load applied on the

pile. The hydraulic jack to be used to apply load must be of sufficient capacity. A set of

minimum three dial gauges should be used with a datum bar which must not be disturbed

during the conductance of the test.

46

FIG. 3.22 Conventional Arrangement for Pile Load Test

Testing of piles for load carrying capacity and construction quality is of utmost

importance. Generally, load tests are conducted to determine the bearing capacity and to

establish the load settlement relationship under compression load. Load tests may be

carried out either on a working pile or a test pile. The routine test on working pile is

conducted normally for one and half times the design load and checked for total and net

settlement. For any project, it is always advisable to carry out an initial test on a test pile

for load of 2.5 times the design load, or the load imposed must be such as to give a total

settlement not less than one-tenth the pile diameter.

Reaction Piles

Ground conditions, pile type and site constraints often make the use of reaction piles

economical. A number of reaction (anchor) piles can be placed surrounding the test pile

and will provide the required tensile capacity and act as reaction against the compression

test pile. Transfer of the forces involved is carried out by a series of beams, bars and

couplers as illustrated in Figure 2, 3 , 4 & 5. The beams are placed over the reaction piles

and securely connected by the couplers to high strength threaded bars cast into the

reaction piles and specifically designed for the purposes of the test. Reaction piles should

be placed at a sufficient distance from the test pile so as to avoid any interaction of soil

resistances.

47

FIG. 3.23 Arrangement of Hydraulic Jack, Datum Bar & Dial Gauges

FIG. 3.24 Typical Pile Load Test Frame under Fabrication.

48

3.9.1 Types of Load Tests : Load test can be carried out in any of the following

loading procedure as given below

(a) Maintained Equilibrium Load Test

(b) Constant Rate of Penetration Test

(c) Cyclic Load Test.

In the first method of test, the loads are applied in increments to the pile head and

maintained for the certain time till further settlement with time is negligible. Settlement

of the pile head is recorded at each load level. After the pile has been loaded to the

desired level, the load is released in stages and rebound of settlement is recorded. In CRP

test, the load is applied continuously in such a way that the rate of settlement or

penetration of pile is uniform.

In the case of Cyclic load test, the load is raised by increments and settlements are

recorded with time. When the settlement practically ceases, te load is released to zero and

again raised to a higher level and settlements recorded with time. The procedure is

continued till desired level of loading is reached.

FIG. 3.25 Maintained Load Test : Load vs Time , Time vs Settlement

and load vs Settlement Plots

A number of criteria has been suggested by different researchers for determining the

allowable working load from the load-settlement curve plotted with test data. A few of

the criteria are given below :

49

(a) The design load shall not exceed one-half of the ultimate load capacity

indicated by single or double tangent method.

(b) The design or allowable load shall not exceed the 50% of the ultimate load at

which the total settlement amounts to one-tenth of the diameter of pile.

(c) The allowable load is some time taken as equal to two-third of the load which

causes a total settlement of 12 mm.

(d) The allowable load is some times taken as equal to two-third of the load which

causes a net settlement of 6 mm.

All settlement considerations must be compared with the settlement that may be

permissible for the particular structure under consideration

3.9.2 Separation Friction and End-bearing Resistance of Pile ; The cyclic load test

data can be utilized to provide some indication of the distribution of load between shaft

friction and end-bearing. A plot of the elastic recovery at each unloading cycle versus

load applied at that cycle is used to separate the two components. The curve usually

becomes a straight line soon after the early load increments. The distance between this

curve and a line drawn through the origin and parallel to the straight part of the curve,

represents the portion of the load carried by shaft friction Qf and the remaining portion is

the end-bearing or point resistance value Qp. After obtaining the value of Qf , elastic

compression of pile can be estimated using the formula given below. The plot can be

repeated by drawing curve between Se and Q, and obtain new value of Qf ,and so on. The

procedure is only approximate.

The general equation for the settlement of pile at any load level may be written as

S = ∆L + Sb

where S = total settlement of pile head at load Q, ∆L = compression of pile

Sb = compression of soil at base, which comprises of elastic and plastic

compression of soil. = Se + Sp.

Elastic compression of soil Se ( or elastic recovery of soil) can be estimated if elastic

compression of pile ∆L can be determined as given below

∆L = (Q -- Qf / 2) L

AE A = cross sectional area of pile, and E = Young’s modulus of pile material.

3.9.3 Concrete and Reinforcement in Pile

The strength of the concrete in the pile must be considered in all cases where a load test is

to be carried out, in order to ensure that the concrete is not over-stressed during testing.

This is particularly important with preliminary test piles where the stresses in the

concrete may be very high. Preliminary test piles are often loaded to between two and

50

three times their normal specified working load and this may call for higher grades of

concrete than those to be used in the works. Enhanced reinforcement may also be

required in preliminary piles to prevent structural failure under such loading conditions

FIG.3.26 Method of Separating Pile Shaft capacity and Point Resistance

For working pile tests, the test should not proceed until compressive tests on works cubes

have confirmed that the concrete strength is at least twice the concrete stress in the pile at

the maximum specified test load. It is also necessary to ensure that the trimmed head of

the pile is in intimate contact with the pile cap with a horizontal, clean and well formed

joint.

Common examples of factors contributing to unsuccessful static load tests are: -

• Four jacks are so placed applying eccentric load.

• Pile cap not concentric with pile shaft

• Poorly formed joint between pile head and pile cap

• Poorly designed/insufficient reinforcement in pile head or pile cap to withstand

bursting stresses

• Pile cap concrete of inadequate strength or poor quality

3.9.4 Quality Control

The defects that are likely to be developed in pile making processes have already been

discussed. All cares must be taken during construction stages to minimise all possible

defects. The satisfactory performance of piles to meet the design requirements is

extremely important. The purpose of pile testing is to determine the following :

51

(a) Whether the pile tip has reached firm stratum or it is resting on loose soil at the

bottom of hole in case of bored cast in situ piles.

(b) Whether the concreting of the pile shaft has been done properly and without any

discontinuity.

(c) Whether the load settlement characteristics are satisfactory.

There is no readily available method of checking the condition of the soil at the pile tip

prior to concreting. Normally at the end of boring, reverse circulation is done to remove

all loose soils from the bottom of the hole, and the length is finally obtained from the

length of the tremie pipe and depth of boring done. For large diameter piles in stiff clay,

the condition of the hole may be checked by lowering an instrument eye, but in most

cases such facilities are not possible to organize in large scale. Attempts have to be

made to test the pile for load-settlement characteristics and the integrity of the pile after

installation and casting.

The defects in pile shaft normally occur in the form of unfilled voids which cause

discontinuity in the pile shaft. The sides of bore holes may collapse if adequate

precautions are not taken by using full casing or bentonite mud slurry. The discontinuities

in a cast in situ pile may occur as a result of

(a) Encrustation of hardened concrete on the inside may cause the concrete to be

lifted as the casing is withdrawn.

(b) The falling concrete may arch across the casing or between the casing and

reinforcement.

(c) Falling concrete may get jammed between the reinforcing bars and not towards

the bore hole wall.

(d) Clay lumps may fall into the hole as the concrete is placed.

(e) Soft or loose soil may squeeze into the pile shaft from the bottom

Most of the above defects can be minimized by having the inside of the casing properly

cleaned, using high slump concrete and having sufficient gap between reinforcing bars.

Also proper care should be taken in lifting the casing while concreting particularly in

unstable soil.

3.9.5 Integrity Testing

Integrity testing may be conducted to check the soundness of the pile shaft after

installation. The following methods of integrity testing of piles are generally available :

(i ) Excavation surrounding the pile shaft.

(ii) Exploratory boring through the pile shaft.

(iii) Accoustic and radiometric tests.

(iv) Seismic and dynamic response tests., (v) Load tests.

52

Excavation around the pile shaft is only possible up to shallow depth. It is hardly feasible

that a deep pile can be fully exposed for visual inspection, except pulling out the same.

Drilling through the pile shaft is possible through large diameter piles. Cores of concrete

can be examined for soundness and they can be tested to determine their comprehensive

strength, and for cavities or honeycomb. .

Various radiometric and acoustic tests may also be done in drilled holes. Seismic and

dynamic response tests are gaining popularity in recent years, because of their simplicity

and adaptability. In the seismic method, a weight is dropped on the pile head and the time

for return of the seismic wave after reflection from the toe is measured.

In the dynamic response method, an electro-dynamic vibrator is mounted on the pile head

to apply a constant amplitude stress wave at the pile top and the response of the pile is

seen through an oscilloscope or digital indicator. Various types of pile diagnostic systems

and pile driving analyser are now available to facilitate such testing.

In sonic integrity testing method, an impact is caused at the pile head with the help of a

hammer. Response of the pile is measured with the help of a pick up connected to an

oscilloscope. The continuity of the pile can be verified, knowing the wave velocity in

concrete.

Load test is the most direct method of determining the capacity of the pile. But it is also

having its own limitations. While other methods of integrity testing determine primarily

the soundness or quality of concrete, the load test gives an integrated method of

determining both the soundness of concrete and the response of the soil under load, and

permits an evaluation of the load capacity of pile based on load-settlement characteristics.

Prof.(Dr) Sudhendu Saha

Chartered Professional Engineer