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 :
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(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