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
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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