Dhananjay
Dhananjay
Dhananjay
A DISSERTATION
submitted by
PATEL DHANANJAY A.
for the partial fulfillment of the award of the degree
MASTER OF TECHNOLOGY
(CIVILSTRUCTURAL ENGINEERING)
PROF. K. N. SHETH
I hereby declare that, this Report entitled Analysis of Piled Raft on Winklers
Linear Spring Bed with Piles modelled as Nonlinear Springs by t-z approch
using SAP2000 is a record of the work/activities carried out by me under the
guidance and supervision of Prof. K. N. Sheth for partial fulfillment of M. Tech.
Dissertation Part II.
I further declare to the best of my knowledge, this dissertation does not contain
any part of work, which has been submitted for the award of any degree either in
this University or any other University without proper citation.
Prof. K. N. Sheth
Guide
Department of Civil Engineering
Dharmsinh Desai University, Nadiad.
II
CERTIFICATE
This is to certify that the Report submitted herewith is record of the work carried
out for M. Tech. Dissertation Part II by
It embodies bonafide work carried out by him under my guidance and supervision
for the partial fulfillment for award of Master of Technology (Civil-Structural
Engineering) degree of Dharmsinh Desai University, Nadiad.
Prof. K. N. Sheth
Guide
Head, Department of Civil Engineering
Faculty of Technology
Dharmsinh Desai University, Nadiad.
Prof. D. G. Panchal
Dean
Faculty of Technology
Dharmsinh Desai University, Nadiad
III
ACKNOWLEDGEMENT
With great pleasure I express my sincere gratitude towards my guide Prof. K.N. Sheth
for his precious guidance and persistent support. I would like to give my thanks for his continual
help in technical and other problems throughout the dissertation Work. From the very
beginning of the work, his valuable guidance, inspiration, continuous reviews and suggestions
have helped me a lot during this study. I shall always be indebted to him for all that he has done
for me.
I express my deep sense of gratitude to Prof. R. K. Sheth and Prof. R. N. Prajapati for
providing support and guidance from time to time and for his incessant kind words of
encouragement and motivation throughout the dissertation.
A part from this, I would like to thank Prof. V. G. Virani (Mechanical Engg. Dept., RK
University, Rajkot) and Prof. D. S. Patel (Dept. of Mechanical Engg., DDU) for providing
continuous support and helping me in my dissertation work.
Patel Dhananjay A.
IV
ABSTRACT
In recent years, there has been a significant rise in use of high-rise structures in India.
Hence there arises a need for the foundations to carry heavy loads. The use of piled raft
foundation is an effective way of minimizing both, total and differential settlements, and of
improving the bearing capacity of shallow foundations. The behavior of piled raft is determined
by the complex interaction between raft, pile and the soil. Thus the understanding of these
effects is indispensable for the reliable design of the foundation. Hence there is a need of
appropriate procedure for the analysis and design of piled raft for achieving economic
The analysis of Piled raft is a complex problem, even more complex than that
of a soil supported raft, as too many parameters influence the behavior of the system.
There are various parameters which influence the sharing of load between piles and raft,
between piles themselves and between piles and soil. As such the exact behavior is
buildings and their corresponding foundations are analyzed. Here, raft is modelled as supported
on Winklers spring bed and piles are represented by nonlinear springs behave as per t-z
approach. This analysis is carried out only for vertical loads. Finite Element technique is used
The parameters varied for the study are: I) modulus of subgrade reaction of the soil,
II) number of storeys and III) size of raft. A comparison of load distribution between Piles and
Raft is done and load settlement curve for Piles Raft system is generated.
V
TABLE OF CONTENTS
TITLE PAGE NO
CANDIDATE S DECLARATI ON II
CERTIFI CATE III
ACKNOWLEDGEMENT IV
ABSTRACT V
TABLE OF CONTENTS VI
LIST OF FIGURES VIII
LIST OF T ABLE XVI
1. INTRODUCTION 1-4
1.1 GENERAL 1
1.2 PILED RAFT AS FOUNDATION SYS TEM 2
1.3 SOIL MODE L FOR RAFT SUPPORT 2
1.4 PILE WITH t -z APPROCH 3
1.5 OBJECTIVE OF STUDY 3
1.6 SCOPE OF WORK 4
VI
TITLE PAGE NO
REFERENCES 114
VII
LIST OF FIGURES
Figur e No T it le Page No
VIII
Figur e No T it le Page No
IX
Figur e No T it le Page No
X
Figur e No T it le Page No
XI
Figur e No T it le Page No
XII
LIST OF TABLE
Table No T it le Page No
XIII
Table No T it le Page No
4.13 Load Dist . in P ile and Raft ( Ks = 5000 kN/ m 3 , 30 67
st orey, 23x23 raft )
4.14 Load Dist . in P ile and Raft as % ( Ks = 5000 kN/ m 3 , 67
30 st orey, 23x23 raft )
4.15 Load Dist . in P ile and Raft ( Ks = 5000 kN/ m 3 , 30 69
st orey, 28x28 raft )
4.16 Load Dist . in P ile and Raft as % ( Ks = 5000 kN/ m 3 , 69
30 st orey, 28x28 raft )
4.17 Load Dist . in P ile and Raft ( Ks = 5000 kN/ m 3 , 30 71
st orey, 33x33 raft )
4.18 Load Dist . in P ile and Raft as % ( Ks = 5000 kN/ m 3 , 71
30 st orey, 33x33 raft )
4.19 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 50 73
st orey, 23x23 raft )
4.20 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 73
50 st orey, 23x23 raft )
4.21 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 50 75
st orey, 28x28 raft )
4.22 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 75
50 st orey, 28x28 raft )
4.23 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 50 77
st orey, 33x33 raft )
4.24 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 77
50 st orey, 33x33 raft )
4.25 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 40 79
st orey, 23x23 raft )
4.26 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 79
40 st orey, 23x23 raft )
4.27 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 40 81
st orey, 28x28 raft )
4.28 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 81
40 st orey, 28x28 raft )
4.29 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 40 83
st orey, 33x33 raft )
4.30 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 83
40 st orey, 33x33 raft )
4.31 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 30 85
st orey, 23x23 raft )
4.32 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 85
30 st orey, 23x23 raft )
XIV
Table No T it le Page No
4.33 Load Dist . in P ile and Raft ( Ks = 4000 kN/ m 3 , 30 87
st orey, 28x28 raft )
4.34 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 87
30 st orey, 28x28 raft )
4.35 Load Dist . in P ile and Raft ( Ks = 4 000 kN/ m 3 , 30 89
st orey, 33x33 raft )
4.36 Load Dist . in P ile and Raft as % ( Ks = 4000 kN/ m 3 , 89
30 st orey, 33x33 raft )
4.37 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 50 91
st orey, 23x23 raft )
4.38 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 91
50 st orey, 23x23 raft )
4.39 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 50 93
st orey, 28x28 raft )
4.40 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 93
50 st orey, 28x28 raft )
4.41 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 50 95
st orey, 33x33 raft )
4.42 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 95
50 st orey, 33x33 raft )
4.43 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 40 97
st orey, 23x23 raft )
4.44 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 97
40 st orey, 23x23 raft )
4.45 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 40 99
st orey, 28x28 raft )
4.46 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 99
40 st orey, 28x28 raft )
4.47 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 40 101
st orey, 33x33 raft )
4.48 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 101
40 st orey, 33x33 raft )
4.49 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 30 103
st orey, 23x23 raft )
4.50 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 103
30 st orey, 23x23 raft )
4.51 Load Dist . in P ile and Raft ( Ks = 3000 kN/ m 3 , 30 105
st orey, 28x28 raft )
4.52 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 105
30 st orey, 28x28 raft )
XV
Table No T it le Page No
4.53 Load Dist . in P ile and Raft ( Ks = 3 000 kN/ m 3 , 30 107
st orey, 33x33 raft )
4.54 Load Dist . in P ile and Raft as % ( Ks = 3000 kN/ m 3 , 107
30 st orey, 33x33 raft )
XVI
1
INTRODUCTION
1.1 GENERAL
1
1.2 PILED RAFT AS FOUNDATION SYSTEM
The common practice to design the foundation is to consider first the use of shallow
foundation such as isolated footing or raft to support structure and then if this is not adequate
than fully piled foundation in which entire loads are taken by piles through pile caps. Despite
of this, raft is provided at bottom because of need of basement below structure. Nevertheless,
in the past few decades, there has been an increasing recognition that the use of pile groups
in conjunction with the raft can lead to considerable economy without compromising the
safety and performance of the foundation. Such a foundation makes use of both the raft and
the piles, and is referred as piled raft.
For an optimum design of piled rafts it is necessary to understand the mechanism of
load transfer from raft to piles and to soil to predict i) the behaviour of raft which includes
settlements, bending moments and proportion of load carried by raft and ii) the behaviour of
piles which includes the displacements and load distribution along piles. For most of piled
raft foundations, the primary purpose of piles is to act as settlement reducers. The proportion
of load carried by the piles is the secondary issue in the design. Piled raft foundations have
been used successfully in many parts of the world in high rise buildings and industrial
structures.
Today the most well-known and used foundation model for SSI analysis, by structural
engineers, is the Winkler model. It is also the oldest and simplest method to model the
subgrade which consists of infinite number of springs on a rigid base. For a structural model
there will be a finite number of springs, see Fig.1.1.
2
In that assumed the foundation model to consist of closely spaced independent linear springs,
as shown in Fig. 1.1. The load deection equation for this case can be written as
q = kw
where k is the spring constant and is often referred to as the foundation modulus, and w is
the vertical deection of the contact surface. For the present task Winklers spring model
has been adopted for soil for raft support.
For modelling of Pile the t-z approach is used. The method uses to transfer the shear
along the shaft as show in Fig. 1.2. The t-z curve gives the nonlinear relationship between
t-pile resistance and z-pile movement at various point along the pile surface.
The objective of the thesis is to analyze Piled Raft foundation employing Non-
linear Finite Element Method in SAP2000 software.
Raft in considered to be resting on linear elastic springs and pile behaviour is
based on nonlinear load settlement curve.
To find load distribution of Piled Raft Foundation in Piles and Raft and generate
load settlement curve of Piled Raft Foundation.
3
1.6 SCOPE OF WORK
A non-linear load settlement curve for axially loaded pile has been prepared.
A 30 storey, 40 storey, 50 storey building has been analysed for gravity and lateral
loads. Piled Raft foundation analysis has been carried out for above mentioned
storeys using various soil subgrade modulus Ks = 3000kN/m3, Ks = 4000kN/m3,
Ks = 5000kN/m3 for raft sizes 23m 23m, 28m 28m, 33m 33m considering
Vertical load only.
Modelling of Piled and raft has been done SAP2000.
Chapter 1 gives introduction of Piled raft and its need. The objective of study and Scope of
work is mentioned.
Chapter 2 gives information about the literature collected for analysis of Piled raft
foundation. The literature covers Analysis and Design aspects.
Chapter 3 with the help of problem formulation and sample problem covers the information
regarding modelling of raft and pile is discussed.
Chapter 4 covers the all results for piled raft system for the dissertation work and
comparison of different parameters.
4
2
LITERATURE REVIEW
2.1 GENERAL
Literature survey is carried out to familiar with the amount of work done in this
area. The survey gives ideas about the extent of work to be carried out during project. It
helps in framing the scope of work. It also helps in deciding the line of action of work. It
generates the clear vision of the work and gives the overall scenario of it. During this survey
many new things, concepts, and ideas will emerge which improve the clarity of the topic.
A literature review for the present task is presented under the following headings:
1. Raft Foundation
2. Pile Foundation
3. Piled Raft Foundation
4. Software Information and few features of the selected software
5. Special Literature
Raft or Mat foundation is a combined footing that covers the entire area beneath a
structure and supports all walls and columns. Raft or mat normally rest directly on soil rock,
but can also be supported on piles as well.
Raft foundation is generally suggested for the following situations:
Whenever building loads are heavy or the allowable pressure on soil is small that
individual footing would cover more than floor area.
Whenever soil contains compressible lenses or the soil is sufficiently erratic and it
is difficult to define and assess the extent of each of the weak pockets or cavities and, thus,
estimate the overall and differential settlement.
When structure and equipment to be supported are very sensitive to differential settlement.
5
Where structures naturally lend themselves for the use of raft foundation such as
silos, chimneys, water towers, etc.
Floating foundation cases wherein soil is having very poor bearing capacity and the
weight of the superstructure is proposed to be balanced by the weight of the soil removed.
Buildings where basements are to be provided or pits located below ground water table.
Building where individual foundation if provided, will be subjected to large widely
varying bending moments which may result in differential rotation and differential
settlement of individual footing causing distress in the building.
Rigid Foundation
This is based on the assumptions of linear distribution of contact pressure. The basic
assumptions of this method are
a) The foundation is rigid relative to the supporting soil and the compressible soil layer is
relatively shallow.
6
Fig. 2.1 Types of Raft Foundation
b) The contact pressure variation is assumed as planar, such that the centroid of the contact
pressure coincides with the line of action of the resultant force of all loads acting on the
foundation.
This method may be used when either of the following conditions is satisfied.
a) The structure behaves as rigid (due to combined action of super-structure and
the plate) with a relative stiffness factor k > 0.5
b) The column spacing is less than 1.75/ where
kB
4
4EI
Where,
k = Modulus of subgrade reaction (N/m3)
B = width of plate
E= Modulus of elasticity of concrete (N/m2)
I = Moment of Inertia of plate (m4)
7
With these simplifications the problem is statically determinate and use of statics is
made for analysis.
The raft is analysed as a whole in each of the two perpendicular directions. Further
analysis is also based on statics. In cases of uniform conditions when the variations in adjacent
column loads and column spacing's do not exceed 20% of the higher value, the raft may be
divided into perpendicular strips of widths equal to the distance between mid-spans and each
strip may be analysed as an independent beam with known column loads and known contact
pressures. Such beams will not normally satisfy statics due to shear transfer between adjacent
strips and the design may be based on suitable moment coefficients or on moment distribution.
Flexible Foundation
1. Simplified method: - In this method, the plate is considered to be flexible. This method
may be used when following conditions are satisfied.
a) The structure (combined action of super structure and plate) may be
considered as flexible (relative stiffness factor k < 0.5)
b) Variation in adjacent column loads does not exceed 20 % of the higher value.
2. Soil Line Method: This method takes into account the relative flexibility of plate by making
use of solutions of beams on Wrinkler foundation (M. Hetenyi, 1946)
3. General Method: For the general case of flexible foundations, the method based on closed
form solutions of elastic plate theory may be used. This method is based on the theory of
plates on Winkler foundations, which takes into account the restraint on deflection of a
point, by continuity of the foundation in orthogonal direction. The distribution of deflection
and contact pressure on the plate due to column loads is determined by the plate theory.
Since the effect of a column load on an elastic foundation is damped out rapidly, it is
possible to determine the total effect at a point of all column loads within the zone of
influence by the method of super- position. The computation of the effects at any point may
be restricted to column of two adjoining bays in all directions. The procedure is outlined in
App. F of IS 2950:1981
8
4. Finite Difference Method: The differential equation for deflection of a plate on Winkler
spring bed is given in above equation To solve this differential equation by finite
differences the plate is divided into square areas (h x h). In this case of an interior point
a the deflection at a point can be expressed by finite difference equation in terms of
deflection at adjacent points to the right, left, top, bottom.
5. Finite Element Method: A great deal of work is done for analysis of plate bending using
finite element method. Triangular elements, rectangular elements, quadrilateral elements,
isoperimetric elements, etc. are developed for this purpose. Rectangular elements with
corner nodes have been used for the present work as these have proven record for good
accuracy and convergence of results.
9
2.2.3 Bearing Capacity of Raft Foundation
The gross ultimate bearing capacity of a raft foundation can be determined by the
same equation used for shallow foundations
qu = cNcScdcic+qNqSqdqiq+1/2BNSdi
The term B in above equation is the smallest dimension of the raft. The net ultimate
capacity of a mat foundation is
qnet = qu q
A suitable factor of safety should be used to calculate the net allowable bearing capacity.
The net allowable bearing capacity for raft constructed over granular soil deposits can
be adequately determined from the standard penetration resistance numbers. From below
equation for shallow foundations,
N D Se
q net 1 0.33 f
0.08 B 25
where,
qnet = net allowable capacity (kN/m2)
N = standard penetration resistance number
B = width (m)
Se= settlement (mm)
The maximum limiting values for settlement of raft foundation as per IS 1094-1986 are
in Table 2.1.
Sand and Hard Clay Plastic Clay
Type of Max. Differential Angular Max. Differential Angular
Structure Settlement Settlement Distortion Settlement Settlement Distortion
mm mm mm mm
Steel 75 0.0033L 1/300 100 0.003L 1/300
RCC 75 0.0021L 1/500 100 0.002L 1/500
Multi-
storey
75 0.0025L 1/400 125 0.003L 1/300
with Panel
walls
Table 2.1 Permissible settlement for Raft Foundation
10
Settlement Calculation of Raft
Tezarghi and Peck give the formulation for finding the settlement of raft on sand.
12q
S CwCd
N
where,
q = Bearing capacity of soil
N = Blowcount
= 15+0.5(N-15)
Cw = Water correction factor
for surface footing
W
Cw = 1 2.0
2B
W = depth of water for surface
for a fully submerged footing W D
D
Cw = 1 2.0
2B
Cd = Depth correction factor
D
Cd = 1 0.25
B
11
A number of factors affect the value of modulus of subgrade reaction. They areas used
below:
Terzaghi (1955)
1. Effect of Size: The value of k decreases with increasing width of footing.
2
B 0.3
k k1 (for granular soil)
2B
2. Effect of Shape: For footings with the same width b under the same pressure q and supported
on the same soil, k decreases with increasing length L of the footing.
2
0.5B
1 L
k ks
1.5
Where,
ks = value for a square footing (B x B)
k = value for a footing of size (L x B)
This indicates that k value for an infinitely long footing is equal to two-thirds of that
for a square footing.
3. Effect of depth: The elastic modulus E, of sand increases with depth and it may be expressed
by:
E=c z
Where,
c = constant, depending on the properties of sand,
= unit weight of sand, and
z = depth.
So, k = q/S = c (1 + 2Df /b) (k < 2k)
This indicates that the settlement of a footing is reduced to one-half, if it is lowered
from the ground surface to a depth equal to one-half of the width of the footing.
12
The modulus of elasticity for a purely cohesive soil with uniform properties from,
ground to a great depth is practically constant throughout. However, for stiff and hard clays
the above correction is applied, as soil is partly frictional and partly cohesive. The modulus of
subgrade reaction (k) as applicable to the case of load through 30 cm x 30 cm or beams 30 cm
wide on the soil is given in table 2.2 for cohesion less soil and in table 2.3 for cohesive soils.(IS:
2950 Part 1 - 1981)
Soil Characteristics
Unconfined Compressive Modulus of Subgrade
Consistency
Strength (qu) in kg/cm2 Reaction (k) in kg/cm3
Stiff 1 to 2 2.7
Very Stiff 2 to 4 2.7 to 5.4
Hard 4 & above 5.4 to 10.8
Table 2.3 Modulus of Subgrade Reaction (k) of Cohesive Soils
The above values apply to a square plate 30 cm x 30 cm. above values are based on the
assumption that the average loading intensity does not exceed half the bearing capacity.
Vesics equation given for laboratory determination of k. The modulus of elasticity, BS is
first found out using triaxial compression test. In carrying out the test, continuous cell pressure
may be so selected as to be representative of the depth of average stress influence zone (about
0.5B to B). The value of k shall be determined from the following relationship:
4
E sB Es 1
k 0.6512 2
EI 1 B
Where,
ES = Modulus of Elasticity of soil - triaxial compression test
E = Youngs Modulus of foundation material (concrete)
= Poissons ratio of soil
I = Moment of Inertia of the structure.
13
2.4 PILE FOUNDATION
Piles are structural members of made of either timber, concrete or steel and are used to
transmit surface loads to lower levels in soil mass. Piles are commonly used for the following
purposes:
To carry the superstructure loads into or through a soil stratum. Both vertical and lateral
loads may be involved.
To resist uplift, or overturning, forces, such as for basements mats below the water table
or to support tower legs subjected to overturning from lateral loads such as wind.
To compact loose, cohesion less deposits through a combination of pile volume
displacement and driving vibration.
To control settlements when spread footings or a mat is on a marginal soil or is underlain
by a high compressible stratum.
To stiffen the soil beneath foundations to control both amplitudes of vibration and
natural frequency of the system.
As an additional safety factor beneath bridge abutment's and/or piers, particularly if
scour is potential problem.
In offshore construction to transmit loads above water surface through the water and in
to underlying soil. This case is one in which partially embedded piling is subjected to
vertical (and buckling) as well as lateral loads.
14
Based on Load Transfer mechanism piles are of mainly 3 types,
a) End Bearing Piles: - They transfer the load through their bottom tips in the weak material
to a firm stratum below acting as a column. These types of piles are also known as point-
bearing piles.
b) Friction Piles: - They transfer the load through skin friction developed between the
embedded surface of the pile and the surrounding soil.
c) Combined End Bearing and Friction Piles: - Load is transferred by the combination of
end bearing at the bottom of the pile and friction along the surface of the pile shaft.
A pile subjected to load parallel to its axis will carry the load partly by shear generated
along shaft, and partly by normal stresses generated at base of pile. The ultimate load capacity
Qu, of pile under axial load is equal to sum of base capacity Qu, and shaft capacity Qf.
Q = Qu + Qf = Abqb + Asfs
The relative magnitude of the shaft and base capacities will depend on the geometry of
the pile and the soil profile. In cohesive soil, the shaft capacity is generally paramount, while
in non-cohesive soil the overall capacity will be more evenly divided between shaft and base.
The shaft capacity of a pile is mobilised at much smaller displacements of the pile (typically 0.5
to 2% of the pile diameter) than is the base capacity. The latter may require displacements as
large as 5 to 10% of the pile base diameter (even larger for low-displacement piles in granular
soil) in order to be fully mobilised. This difference between the load deformation characteristics
of the pile shaft and that of the pile base is important in determining the settlement response of
a pile, and the sharing of load between shaft and base, under working conditions.
Various empirical and theoretical methods are available for developing curves for axial
load transfer and pile displacement, (t-z) curves. As per API(2002) the analytical expression to
find the skin friction capacity of piles is given by
t z z
2
t max zc zc
Where,
t skin friction at nodal point (kN/m2)
t max maximum skin friction (kN/m2)
z = local pile displacement (m)
zc = relative pile displacement (m)
In the absence of more denitive criteria the following curve is recommended for sands.
16
As per API(2002) the analytical expression to find pile tip settlement curve is given by
1/ 3
Q z
Q b z cb
Where,
Q b total end bearing capacity (kN/m2)
z = local pile displacement (m)
zcb = maximum displacement for Qb (m)
In the absence of more denitive criteria the following curve is recommended for sands.
17
2.6 DEVELOPMENT OF p-y CURVES
A typical family of curves is show in fig. 2.6 The curves consist of three straight lines
and a parabola. The initial portion of the curve represents linear elastic behaviour of the sand
and the horizontal portion of the curve represents plastic behaviour.
The step by step procedure is as follows:
1. Obtain the values for significant soil properties phi and mu, pile diameter D and the
depth X at which p-y is to be constituted.
2. The following parameter are computed:
/ 2 to / 3 for loose sand and for dense sand.
45 / 2 ;
Ko=0.4 for loose sand and Ko=0.5 for dense sand
Ka=tan2( 45 / 2 )
3. The following expression are used for computing ultimate soil resistance p1 near
ground surface and p2 at great depth:
K X tan sin tan
p 1 X o (D X tan tan ) K o X tan (tan sin tan ) K a D
tan( ) cos tan( )
p 2 K a D X tan 8 ( 1) K o D X tan tan 4
18
3D
4. Establish yu = . Compute pu = A pc by choosing A from fig for particular X/D.
60
D
5. Establish ym = . Compute pm = B pc by choosing B from fig for particular X/D.
60
Fig. 2.7 Non Dimensional Coefficient A Fig. 2.8 Non Dimensional Coefficient B
6. Establish the slope of the initial portion of the p-y curve by selecting appropriate
value of K from the table 2.4. The initial slope is given by Es=K X
Relative
Loose Medium Dense
Density
25 30 33 36 > 36
K (T/m3) 463 544.5 1089 1633.5 3403
p = c y(1/n)
pu pm
m
yu ym
pm
my m
19
pm
c
y m (1 / n )
determine point K as
/ n 1
c
yk
KX
compute the appropriate number of points on the parabola using equation p = c y(1/n).
Desired number of curves can be developed by repeating the steps above for each depth.
Where,
S = vertical settlement of the top of a single pile
Ss = settlement due to the axial deformation of the pile shaft
L
Ss (Qb Qf )
AE
Qb = tip resistance of the pile
Qf = shaft resistance of the pile
= number that depends on the skin friction distribution of pile from fig. 3.5
L = length of the pile
A = cross-sectional area of the pile
E = modulus of elasticity of the pile
Sf = settlement of the pile tip due to the load transferred at the tip
Qf (0.93 0.16 D / B)
Sf
Dq
B = pile diameter or width
q = unit ultimate tip bearing capacity
D = embedded pile length
Sp = settlement of the pile tip caused by load transmitted along the pile shaft
QpCp
Sp
Bq
20
Cp = empirical coefficient given in Table 2.5
As an aid for understanding the manner in which a pile transfers load to the soil under
working conditions, an approximate solution has been suggested by Randolph[22] for the load
settlement response of pile in elastic soil. The solution leads to an expression for pile stiffness
in closed form. In developing this solution, the manner in which load is transferred to the soil
from pile shaft and pile base are examined separately before combining both to give response
of complete pile.
Pile Shaft
The mechanism of Load transfer from pile shaft is through the shear stresses generated
in soil on vertical and horizontal planes, with little change in vertical normal stress (except near
the base of pile). Following are some of its assumptions
21
1) Pile is considered as surrounded by concentric cylinders of soil with shear stresses on
each cylinder.
2) Shear stress decreases with distance such that r = 0 r0 where = shear stress at distance
r, 0 = shear stress on the pile soil interface, r0 = pile radius. Thus only soil very close to
pile is ever highly stressed.
3) Shear stress are negligible beyond a radial distance rm (distance beyond which soil does
not deform)
4) Radial soil displacements due to pile loads are assumed negligible when compared to
vertical deformations. Therefore, simple shear condition failure in the soil. i.e. shear
strain = / r where = vertical deflection.
A maximum radius rm has been introduced at which deflections in the soil are assumed
to become vanishingly small. The deflection of pile shaft is thus given by
0r 0
w=
G
Where
= ln (rm / r) = ln (2rm / d)
Overall load taken by Pile shaft = Ps= dL 0
Hence substituting value of 0 and we get,
Ps 2 LG
ws
Where,
G = average shear modulus of the soil over embedment depth of the pile.
22
Pile Base
At the pile base, it is sufficient to ignore the pile shaft and surrounding soil, and treat
base as rigid punch acting at surface of soil medium. The base stiffness can be obtained from
the standard solution as below
Pb 2dbGb
wb (1 v)
Combining Shaft and Base
For a rigid pile, the base settlement and shaft settlement will be similar to settlement of
the pile head, wt. The total load Pt, may be written as
Pb Ps
Pt = Pb + Ps = w t ( )
wb ws
Where,
Pt = Load on Piled raft and
wt = settlement of Piled raft
In developing a general solution for the axial response of a pile, it is convenient to
introduce a dimensionless load settlement ratio for the pile. The pile stiffness is Pt / wt and this
may be made dimensionless by dividing diameter of pile and an appropriate soil modulus at the
level of the pile base. Hence by making use of above equations we get a generalised equation
as
Pt 2 d bG b 2 LG
G L dw t (1 v ) dG L G L d
Pile compression
Most piles exhibit some amount of shaft compression under working loads and this
should be allowed for estimating pile deflection. The axial strain at any level down the pile is
given by
z = / z = -4P / (d2Ep) = -4P / (d2GL)
2 / 2z = -(P / z) x (4 / d2GL)
Where,
Ep = Young's modulus of pile.
= Ep/GL
The load P will vary down the pile as load is shed in surrounding soil such that
P / z = -do
Solving above and we get
23
2w 8G
2
w
z E pd 2
Considering Vertical non-homogeneity of the soil as increase in stiffness with increase
of depth of soil i.e. Gibson's type of soil as shown in figure 3.6. A non- homogeneity factor is
introduced which is ratio of average shear modulus to that of base i.e. = G / GL. For
consideration of End bearing Piles, a factor is introduced which is ratio of shear modulus at
pile tip to the shear modulus just below pile tip i.e. = GL/Gb. For consideration of under-
reamed piles a constant = db / d is introduced which is ratio of diameter below pile base to that
average diameter.
The differential equation now may be solved to yield w in terms of hyperbolic cosine
function and sine function of z. Substituting in the appropriate boundary conditions at the pile
base yields an expression of the load settlement ratio of the pile head as
2 2 tanh( L ) L
Pt (1 v ) L d
G L dw t 1 8 tanh( L ) L
(1 v ) L d
24
2.9 PILED RAFT FOUNDATION
Foundations for heavy structures on weak soils are usually on piled raft systems. These
are also called basement raft supported piles Pile-raft systems are shown in Fig. 2.11. The piles
also stiffen the raft and reduce the differential settlements and tilting. When piles are founded
on a compressible soil like clays, the pile-raft system settles gradually resulting in a gradual
build-up of pressure at the bottom of the raft. Thus, there is a gradual transfer of the total load
to the raft and a reduction in the load carried by the piles. The soil beneath the raft (located at a
relatively shallow depth) is then compressed, causing a partial load transfer back to the piles.
This is a continuous interaction process with the total load being shared by the raft and the piles.
The piled raft foundation consists of three load bearing elements: piles, raft and subsoil.
According to its stiffness, the raft distributes total load of structure Stot as contact pressure
represented by Rraft, as well as over the n piles, generally represented by sum of pile resistance
Rpile, in the ground (i=1,2,3. . . n).Hence Total Resistance of piled raft Rtot is given by
25
Rtot = Rraft + Rpile Stot
It means that total resistance of the foundation must not be less than total load of
superstructure. In cases where raft is founded below water table, Stot in equation is replaced by
the total effective load of structure S'tot, given by Stot less the ground water buoyancy force.
In same way Rtot in equation is replaced by the total effective resistance of piled raft R'tot, given
by Rtot less the ground water buoyancy force. By conventional foundation design, it has to be
proved that building load is transferred either by raft or by the piles in the ground. In each case
it has to be proved that either the raft or piles will support the working load of the building
with adequate safety against overall loss of stability and against bearing resistance failure. The
Design of piled raft foundation requires a new understanding of soil-structure interaction as
shown in fig. 2.12 because the contribution of both raft and piles is taken in to consideration
to verify the ultimate bearing capacity and serviceability of overall system. Moreover the
interaction between raft and piles makes it possible to use piles up to a load level which can be
significantly higher than permissible design value for bearing capacity of a comparable single-
isolated pile. The behaviour of piled raft can be characterised by coefficient cp defined as
cp = (Rpile / Rtot)
This describes the load sharing between the piles and raft. In cases where raft is founded below
water table, Rtot in equation is replaced by the total effective resistance of piled raft R'tot. In
same way Rraft is replaced by the R'raft, it represents the effective contact pressure between raft
and subsoil. A piled Raft coefficient of cp=0 represents the case of shallow foundation and a
coefficient of unity represents the case of fully piled foundation without contact pressure
beneath raft. Piled raft foundation covers range of 0<cp<1, whereby some extent every piled
foundation really acts as like a piled raft, except where contact pressure between raft and soil
is impossible as for example in pile foundations of shore structure. The influence of piles to
reduce the settlements of raft depends upon piled raft coefficient which in turn depends upon
subsoil conditions and the geometric proportions of piled raft.
For the same subsoil condition and same area of raft, the piled raft coefficient is a
function of number and length of piles as shown in the Fig. 2.13.
26
Fig. 2.12 The interactions in a piled raft foundation system
27
Advantages of Piled Raft Foundation
The foundations system of the combined piled raft can lead to the following
advantages in comparison to a raft or pile foundation
a) Reduction of settlements and differential settlements of structure.
b) Reduction of tilt in consideration of eccentric loading on inhomogeneous soil
conditions.
c) Improvement in bearing capacity of raft foundation using load share between pile and
raft.
d) Reduction of number of piles and pile length in comparison to a pile foundation.
e) In an economic way reduction of forces, stresses within the raft by an optimal
arrangement of pile beneath raft.
The way of load transfer from pile to soil in case of piled raft is different from that in
case of regular piles. This is because the raft forces the soil immediately below it to settle by
same amount as settlement of piles. Hence there is no relative movement between pile and
soil up to certain depth. Hence, friction mobilized is almost negligible in upper portion. Fig.
2.14, Fig. 2.15 and Fig. 2.16 shows the development of load transfer in case of piled raft as
compared with that case of regular pile. Fig. 2.14 shows axial load distribution curves, when
single pile is loaded, the load transfer begins from top portion of pile and as load increases,
more load is transferred to deeper levels. In case of piled raft, for same load, the
Fig. 2.14 Axial Load distribution in single pile and Piled raft
load is transmitted up to bottom of pile and skin friction mobilises only after the soil between
the piles gets compressed. Fig. 2.15 shows the development of shaft resistance with pile top
28
settlement. In single pile the development of shaft resistance is predominant even at small
magnitude of settlement. In piled raft the development of shaft resistance with
Fig. 2.15 Development of Shaft resistance in single pile and Piled raft
settlement is small for initial value of settlement and then after the shaft resistance increases
with increase in settlement. This is because of the top portion of soil in between the piles in
contact with raft and move with pile. As there is no relative moment between pile and soil,
there is no development of shaft resistance. The shaft resistance in single pile develops from
top and progresses to bottom as load on pile increases. While in piled raft it develops from
bottom of pile. Relative displacement is maximum at tip and progresses to top as soil gets
compressed under higher loads. Single piles show no development of end bearing resistance
for initial load. Increase in settlement shows development of end bearing resistance as in
Fig. 2.15 Development of End bearing resistance in single pile and Piled raft
Fig. 2.16. Piled raft shows development of end bearing resistance even at initial settlement
and increases with increase in settlement. A single pile first takes load through skin friction
and when there is significant mobilization of skin friction it transfers the load through end
bearing.
29
Design Concepts
As with any foundation system, a design of a piled raft foundation requires the consideration of
a number of issues, including
a) Ultimate load capacity for vertical, lateral and moment loadings
b) Maximum settlement
c) Differential settlement
d) Raft moments and shears for the structural design of the raft
e) Pile loads and moments, for the structural design of the piles
In much of the available literature, emphasis has been placed on the bearing capacity
and settlement under vertical loads. While this is a critical aspect, the other issues must also be
addressed. In some cases, the pile requirements may be governed by the overturning moments
applied by wind loading, rather than the vertical dead and live loads.
Three different design philosophies with respect to piled rafts have been defined
a) The conventional approach, in which the piles are designed as a group to carry the
major part of the load, while making some allowance for the contribution of the raft,
primarily to ultimate load capacity.
b) Creep Piling in which the piles are designed to operate at a working load at which
significant creep starts to occur, typically 70-80% of the ultimate load capacity.
Sufficient piles are included to reduce the net contact pressure between the raft and the
soil to below the preconsolidation pressure of the soil.
c) Differential settlement control, in which the piles are located strategically in order to
reduce the differential settlements, rather than to substantially reduce the overall
average settlement.
In addition, there is a more extreme version of creep piling, in which the full load
capacity of the piles is utilized, i.e. some or all of the piles operate at 100% of their ultimate
load capacity. This gives rise to the concept of using piles primarily as settlement reducers,
while recognizing that they also contribute to increasing the ultimate load capacity of the entire
foundation system.
Clearly, the latter three approaches are most conducive to economical foundation
design, and will be given special attention herein. However, it should be emphasized that the
30
analysis and design methods to be discussed allow any of the above design philosophies to be
implemented.
Fig. 2.17 illustrates, conceptually, the load-settlement behaviour of piled rafts designed
according to the first two strategies. Curve "0" shows the behaviour of the raft alone, which in
this case settles excessively at the design load. Curve "1" represents the conventional design
philosophy, for which the behaviour of the pile-raft system is governed by the pile group
behaviour, and which may be largely linear at the design load. In this case, the piles take the
great majority of the load. Curve "2" represents the case of creep piling where the piles operate
at a lower factor of safety, but because there are fewer piles, the raft carries more load than for
Curve "1". Curve "3" illustrates the strategy of using the piles as settlement reducers, and
utilising the full capacity of the piles at the design load. Consequently, the load-settlement may
be non-linear at the design load, but nevertheless, the overall foundation system has an adequate
margin of safety, and the settlement criterion is satisfied. Therefore, the design depicted by
Curve "3" is acceptable and is likely to be considerably more economical than the designs
depicted by Curves "1" and "2".
Figure 2.17 Load settlement curves for piled rafts according to various design philosophies
Design Strategies for piled rafts may be divided into two main categories
a) Small pile groups, where the ratio of overall width B of the group to the pile length Lp
is less than unity. Piles are needed to ensure adequate bearing capacity, and the pile cap
31
(or raft) can easily be made sufficiently stiff to eliminate differential settlements. Even
where the pile cap bears directly on the ground it will not contribute significantly to the
overall performance of the foundation.
b) Large pile groups, with B/Lp >1, where the pile cap alone will usually provide sufficient
margin against bearing failure, and will contribute significantly in terms of transferring
load directly to the ground. The design of such foundations hinges more on limiting the
average and differential settlements to an acceptable level. Since for large rafts the
flexural stiffness will be low, the location and length of any pile support should be
chosen in order to minimise differential settlements.
The most effective application of Piled raft occurs when the raft can provide adequate
load capacity, but the settlement and the differential settlement of the raft alone exceed the
allowable value. Poulos [12] has examined a number of idealized soil profiles, and has found
that the following situations may be favourable for Piled raft foundation.
Soil profile consisting of relatively stiff clays.
Soil profiles consisting of relatively dense sand.
In both circumstances, the raft can provide a significant proportion of the required load capacity
and stiffness, with the piles, acting to boost the performance of the foundation, rather than
providing the major means of support. The unfavourable situations for Piled raft are as follows
a) Soil profiles containing soft clays near the surface
b) Soil profile containing loose sand at the surface
c) Soil profiles that contain soft compressible layers at relatively shallow depths
d) Soil profiles that are likely to undergo consolidation settlement
e) Soil profiles that are likely to undergo swelling movement due to external causes.
In the first two cases, the raft may not be able to provide significant load capacity and
stiffness, while in the third case long term settlement of layers may reduce the contribution of
the raft to the long term stiffness of the foundation. The latter two cases should be treated with
considerable caution. Consolidation settlements (such as those due to dewatering and shrinking
of an active clay soil) may result in a loss of contact between raft and soil, thus increasing load
on the piles, and increase in settlement of the foundation system. Additional tensile forces may
be induced in the piles because of the action of the swelling soil on the raft and as such Piled
raft foundation is not suitable for swelling type of soil.
32
2.9.2 Methods of Analysis
Several methods of analyzing piled rafts have been developed, and some of these have
been summarized by Poulos [14]. Three broad classes of analysis method have been identified
1. Simplified calculation methods
2. Approximate computer-based methods
3. More rigorous computer-based methods
A simple approach to evaluating the overall stiffness of a piled raft, and assessing the
load sharing between pile group and cap, has been suggested by Randolph [22].The overall
foundation stiffness, kpr, is obtained from following equation
K pg K r (1 2 rp )
K pr
1 2rp K r / K p
33
where,
Kpr = Stiffness of piled raft
Kpg = Stiffness of pile group
Kr = Stiffness of raft alone
rp = Raft-Pile interaction factor
The raft stiffness Kr can be estimated via elastic theory from equation whereas pile group
stiffness can be estimated from equation, The proportion of the total applied load carried by the
raft is given by
Pr K r (1 rp )
Pt K pg K r (1 rp )
where,
Pr = Load carried by the raft
Pt = Total applied load
The raft-pile interaction factor can be obtained from following equation
ln(d r / d p )
rp 1
where dr is the effective diameter of the element of raft associated with each pile. This may be
calculated such that, for a group of n piles, n d2/4 equals the actual area of the raft.
FEM analysis can be carried out for specic problems whose applicability has to be
looked into judiciously in view of the complex nature of input values of parameters and large
scale computations. Clancy and Randolph (1996) presented selectively simple methods to carry
out the preliminary design of the pile-raft systems. However, they recommend that a detailed
FEM analysis with proper inputs of the parameters of pile, soil and raft is essential to arrive at
a nal design.
34
There are several parameters that affect the responses of pile-raft systems to applied
loads are given below in Table 2.6.
Dimensionless parameters Expression Practical range of values
Pile slenderness ratio Lp / Dp 10100
Pile spacing ratio sp / Dp 2.58
Pilesoil stiffness Kps = Ep / Es 10010 000
Aspect ratio of raft
Lr / Br 110
dimensions
4E r Br t 3r (1 s2 )
Raftsoil stiffness Krr = 0.00110
3Es L4r (1 2r )
Table 2.6 Signicant dimensionless parameters for the analysis of pile-raft systems
The raft is modelled with the plate element resting on winklers linear spring and pile is
modelled as nonlinear spring using t-z approach. Discretization of piled raft system is show in
fig. 2.18.
Where,
1 = Winklers linear spring
2 = Nonlinear spring equivalent to pile behaviour
3 = Nonlinear t-z spring
4 = Nonlinear Q-z spring
5 = Applied load
35
2.10 SOFTWARE INFORMATION AND SELECTED SOFTWARE
For the present work various software like ANSYS, Plaxis, FLAC, SAP2000 etc. had
been evaluated and finally SAP2000 has been selected for performing the analysis of Piled
Raft Foundation.
The main purpose of selecting this software for the present task is some of its features
such as:
Availability of Multi Linear Plastic link equivalent to nonlinear spring
Ability to do Nonlinear analysis
H.G. Poulos [15]: This paper sets out some principles of design for such foundations,
including design for the geotechnical ultimate limit state, the structural ultimate limit state and
the serviceability limit state. The advantages of using a piled raft will then be described with
respect to two cases: a small pile group subjected to lateral loading, and then the design of the
Incheon Tower in South Korea. Attention will be focussed on the improvement in the
foundation performance due to the raft being in contact with, and embedded within, the soil.
H.G. Poulos [16]: This paper sets out the principles of design for a pile or piled raft
foundation system for tall buildings via a limit state design approach. This approach involves
three sets of analyses: 1. An overall stability analysis in which the resistances of the
foundation components are reduced by the appropriate geotechnical reduction factor and
the ultimate limit state (ULS) load combinations are applied. 2. A separate ULS analysis is
carried out in which the ULS load combinations are applied but in which the unfactored
resistances of the foundation components are employed. The consequent computed
foundation actions (i.e. pile forces and, if appropriate, raft moments and shears) are then
multiplied by a structural action factor to obtain the values for structural design. 3. A
serviceability analysis, in which the best-estimate (unfactored) values foundation resistances
and stiffnesses are employed and the serviceability limit state (SLS) loads are applied.
Randolph M. F. [27]: This paper reviews analytical approaches for pile groups and piled raft
foundation and discusses appropriate choice of soil modulus for pile problems. The manner in
which the nonlinear stress strain response of soil affects single pile response and the interaction
36
between pile is explored and illustrated though and example which shows the relative non
linearity of pile group response by compression with single pile and the average shaft and base
response. An equivalent pier analogue of pile groups and piled rafts is proposed as the most
direct method of estimating the stiffness of complete pile foundations and new design
principles are introduced for piled raft with the aim of minimising differential settlements by
optimal location of the pile supports beneath the raft. The various approaches are illustrated
through case histories and example application.
Balakumar V. et al [2]: This paper describes the design of piled raft foundation involves two
stages namely a preliminary stage and the final stage. The preliminary design stage involves
the identification of the essential parameters namely the number of piles, their diameter and
the length along with Es value of the subsoil strata for an optimum design which can produce
the required settlement reduction. The existing design methods, whose accuracy depends upon
the accuracy of the evaluation of in-situ parameters like Es although can produce satisfactory
results, the computational efforts involved and the time does not justify the use of them for the
preliminary analyses which will involve repetitions.
S.R. Gandhi, D.K. Maharaj [28] : In this paper Piled raft Foundation has been analysed using
FEM. Soil is assumed to be isotropic, homogeneous and elastic. Raft, Pile and Soil have been
idealised as 8-noded brick element. Load Transfer Mechanism in Piled Raft is also described.
Parametric study has been carried out varying pile length, soil modulus, raft thickness and pile
spacing. Based on parametric study design charts were developed to estimate optimum pile
spacing or load sharing for Frankfurt clay. Data available in literature compares well with
design charts results. Also parametric study indicates thickness of raft does not have significant
influence on load sharing between pile and raft and the increase of length of pile reduces loads
on raft.
S.R. Gandhi, D.K. Maharaj [29]: In the present work a strip of infinite piled raft foundation
has been analysed by finite element method. The effect of raft rigidity on the differential
settlement and effect of size of raft on settlement has been studied. The load sharing between
pile and raft have also been studied. It is found that by increasing the rigidity of the raft the
differential settlement of piled raft can be reduced but the load shared by middle pile reduces.
It has been found that piles longer than the width of raft are effective reducing settlement.
37
H.G. Poulos [14]: This report summarises the philosophy of using piles as a settlement
reducers and outlines key requirements of design methods of piled raft foundation. Many
existing methods of design for piled raft were reviewed and their capabilities and limitations
are discussed. A simple example has been taken and has been solved with different approaches
and their results were compared. Some of methods were found to be useful only for preliminary
design and for checking purposes, while others were capable of giving detailed performance
predictions and can be used for detailed design. Conclusions were reached regarding the utility
of some of the current methods used for design and the limitations of 2D numerical analysis.
C. H. Solanki et al. [4]: This review paper on piled raft foundation postulates that considerable
research, either experimentally or theoretically has been conducted on the behaviour of piled
raft foundation. The plate on spring, 2D finite element analyses and hybrid approach are
incompetent of analysing the torsional behaviour and material alteration in third axis.
Therefore 3D finite element method is the most competent to replicate the complex
behaviour of piled raft foundation. A number of 3D numerical models have been developed
but no effort is found to evolve analytical method based on numerical methods. Analytical
methods were stated only to access the settlement of the piled raft foundation but the
forecasting of differential settlement and ultimate bearing capacity is yet to be done.
Meisam Rabiei [23]: In this paper a parametric study on pile configuration, pile number, pile
length and raft thickness on piled raft foundation behaviour are considered. It has been found
that the maximum bending moment in raft increases with increase raft thickness, decrease pile
number and decrease in pile length. Central and differential settlement decreases with increase
raft thickness and uniform increase in pile length. It has also been found that pile configuration
is very important in pile raft design.
G. Srilakshmi and Darshan Moudgalya N. [10 ]: In this paper, analysis of piled raft
foundation has been carried out by using finite element software ANSYS. For understanding
the behaviour of piled raft foundation, parametric studies has been carried out in medium sand
by varying pile diameters and pile lengths in different combinations. It has been found out that
Pile diameter has significant influence on the ultimate capacity of piled raft foundation whereas
the pile length has not of much significance.
38
H.G. Poulos & J. C. Small [13]: In this paper analysis is presented for piled raft foundations
where the piles exhibit nonlinear load-deflection behaviour. The raft is analysed through the
use of finite element methods, while the piles are treated as springs having a variable stiffness,
so as to model any non-linear behaviour. The soil is treated as an elastic medium that may
consist of layers of soil having different stiffness. Interaction between the piles in the group
is assumed to remain constant even though the stiffness of the piles may change with
load level.
D. K. Maharaj [6]: This paper presents the three dimensional nonlinear finite element analysis
of piled raft foundation which is under the application of uniformly distributed load. Load
settlement curves of raft and piled raft foundation have been provided for different raft and
pile stiffness. The increase in stiffness of pile has been found to increase the load carrying
capacity of piled raft foundation and reducing overall settlement up to a limiting value of pile
stiffness. There is a specific combination of stiffness of raft and pile in a piled raft foundation
beyond which further increase in stiffness of raft and pile neither increases the load carrying
capacity nor reduces settlement. It has also been found that the pile of known stiffness is more
effective in reducing the settlement of flexible raft than the stiff raft. The results obtained from
the present 3D finite element model compares well with the result reported in the literature.
Books
Randolph M.F et al. [26]: The book covers detailed analysis and design of pile foundation. It
gives the equation for calculating pile stiffness at shaft and end bearing. A simple method for
estimating group stiffness has also been suggested. An approach for combining the separate
stiffness of the raft and pile group in Piled raft stiffness has been given.
H.G. Poulos and E.H. Davis [12]: This book covers the elastic based analysis of piled
foundation and their use in predicting load settlement response. Settlement analysis of is done
for an individual pile as well as pile group under vertical and lateral loads. The interaction
factor theory for computing settlements in pile group is explained in detail.
P.C. Varghese [25]: It covers various geotechnical aspects for the design foundation. It also
gives various correlations of modulus of elasticity with SPT of soil. Concepts of bearing
capacity and settlement of raft and pile are explained concisely.
39
Joseph E. Bowles [22]: The book covers analysis and design of both rectangular and circular
Mat foundation. It also includes mat settlement effect. The analysis of the raft /mat foundation
includes three methods of analysis namely,
i) Approximate Method,
ii) Approximate Flexible Method
iii) Discrete Element Method.
40
3
3.1 GENERAL
In the present work an attempt has been made to carry out a parametric study Piled Raft
system. The raft is modelled as resting on winklers linear spring bed and pile modelled as
nonlinear spring using tz approach. For the parametric study the stiffness of the soil is varied
with different number of storey on different raft sizes. For evaluating load distribution in
piles and raft and to show its behaviour generate load settlement curve of Piled Raft System.
This chapter consists of analysis of piled raft for a building with 50 storey. The
procedure adopted for the present task consists of the following
1. Analysing building using software Staad Pro and obtaining maximum loads at base
below column for the load combination 1.5(DL+LL) only.
2. Determination of soil parameters base on taken Ks values.
3. Work out structural dimension of piles modelling of pile and generate load settlement
curve for pile in SAP 2000.
4. Work out structural dimension of Raft and Modelling of raft in SAP 2000.
The building was analysed using software Staad.Pro. Figure 6.1 plan. All the
dimensions given are in meter. The data for building is as follows,
41
Floor Finish : 1 kN/m2
Lateral Load: Earthquake load as per IS 1893-2002
Floor height : 3.2 m
Wall load : For Floors - 230 mm wall on periphery and 115 mm on inner beam
Slab thickness : 120mm
Beam size : 380mm600mm
Column size : 1.1m1.1m (GL to 10), 1m1m (11 to 20), 0.85m0.85m (21 to 30),
0.7m0.7m (31to 40),0.5m0.5m (41 to 50)
The load combinations 1.5(DL+LL) used for getting maximum reactions as shown in
Table 3.1.
42
Node L/C Fy (kN)
1 1.5(DL+LL) 19474.107
2 1.5(DL+LL) 21599.869
3 1.5(DL+LL) 22018.477
4 1.5(DL+LL) 21599.869
5 1.5(DL+LL) 19474.107
6 1.5(DL+LL) 21599.869
7 1.5(DL+LL) 23890.502
8 1.5(DL+LL) 24328.418
9 1.5(DL+LL) 23890.502
10 1.5(DL+LL) 21599.869
11 1.5(DL+LL) 22018.477
12 1.5(DL+LL) 24328.418
13 1.5(DL+LL) 24771.727
14 1.5(DL+LL) 24328.418
15 1.5(DL+LL) 22018.477
16 1.5(DL+LL) 21599.869
17 1.5(DL+LL) 23890.502
18 1.5(DL+LL) 24328.418
19 1.5(DL+LL) 23890.504
20 1.5(DL+LL) 21599.869
21 1.5(DL+LL) 19474.107
22 1.5(DL+LL) 21599.869
23 1.5(DL+LL) 22018.477
24 1.5(DL+LL) 21599.869
25 1.5(DL+LL) 19474.107
Total Load (kN) 556416.679
Soil parameter
The assumed soil subgrade modulus of soil is 5000 kN/m3, Gross Bearing capacity of
sand is 375 kN/m2, Unit weight of soil is 18 kN/m3, foundation provide at 1.5m depth from
ground level and pile resting on hard rock ( = 37).
43
The standard penetration resistance number determined form the net allowable capacity as
per given as under,
N D S
q net 1 0.33 f e
0.08 B 25
where,
qnet = net allowable capacity (kN/m2)
N = standard penetration resistance number
B = width (m)
Se= settlement (mm)
So,
qu
qgross =
FOS
qu = 375 x 2.5 = 937.5 kN/m2
q u Df
q net
FOS
Df = Depth of foundation = 1.5 m
= 18 kN/m2
qnet = (937.5 18 x 1.5) / 2.5 = 364.2 kN/m2
From IS 1940:2015 fig 1 get the value of angle of internal friction(), = 30 that show in
fig. 3.2
Structural Dimension of Pile
Total Load of building = 556416.68 kN (factored)
= 370944.45 kN (unfactored)
Load taken by raft = 225 23 23 = 198375 kN
Load taken by piles = 370944.45 198375 = 172569.45 kN
Location of pile is exact below of column and provide 25 piles,
So capacity of each pile = 172569.45 / 25 = 6902.77 kN
44
Pile Capacity Calculation:
Diameter of pile = 1.5 m
Length of pile = 24 m
Self weight of pile = 25 1.5 24 = 1060.28 kN
Total Load on pile = 6902.77 + 1060.28 = 7963.05 kN
Total Ultimate Capacity of pile
Qu = Qs + Qb
= [(1 0.5 18 24 tan(22.5) ( 1.5 24)]+
[(18 24 42.92+0.5 18 1.5 66.19) (/4 1.52)]
= 10118.83 + 34344.48
Qu = 44463.31 kN
Allowable Capacity of pile = 17785.32 kN
Now pile split in to small 2m elements and find t-z curve for at different 2m depth.
At tip of pile find Q-z curve.
t-z curves
t - z Curves 2m
100 4m
90 6m
Skin Friction (kN/m2)
80 8m
70 10m
60
12m
50
14m
40
30 16m
20 18m
10 20m
0 22m
0 10 20 30 40 50 24m
z (mm)
45
Q-z curves
Q - z Curve
40000
35000
30000
Tip Load (kN)
25000
20000
15000 24 m
10000
5000
0
0 25 50 75 100 125 150
z (mm)
After obtaining soil spring data create model of signal pile and obtain load settlement curve
of pile. Some steps are as follow,
Define > Section Properties > Link/Support Properties > Add New Property
Fig. 3.5 Define Link Properties for t-z and Q-z Curves
46
Link/Support Type > MultiLinear Plastic
Now apply load and note down settlement of pile and obtain load settlement curve of Single
Pile.
Load (kN)
0 5000 10000 15000 20000
0
0.2
0.4
Settlement (m)
0.6
0.8
1.2
48
Fig. 3.10 Typical Layout plan of Piles
= 24771.737 617.69
(1.76 1.76 / 4 1.5 1.5) 16500
= 7449.92 kN
Shear strength of concrete
c 0.25 f ck
Now, load settlement curve of Piled Raft apply 20%, 40%, 60%, 80%, 100% loading
intensity and for that maximum settlement and plot.
50
3.4 SOLUTION
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 90.65% 85.59% 80.78% 75.62% 71.12%
Raft 9.35% 14.41% 19.22% 24.38% 28.88%
500000
400000
Load (kN)
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
52
4
4.1 GENERAL
In this thesis a parametric study is carried out of Piled Raft system. The raft is modelled
as resting on winklers linear spring and pile modelled as non-linear spring using t-z approach.
This Piled raft system is analysed by Finite Element technique.
The methodology of modelling and analysis has been discussed in the previous chapter.
Results obtained from the analysis are presented in tabular and chart form as follows:
53
4.2 Ks = 5000 kN/m3, 50 storey, qg = 375 kN/m2, = 30
20 x 20 23 23 0.7 25 1.5 24 37
54
Load S et t lemen t Cu rve
600000
500000
400000
Load (kN)
200000 Raft
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.3 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 120538.11 231821.48 343104.80 454388.13 565670.78
Piles (kN) 109262.13 198406.05 277176.16 343597.94 402348.56
Raft (kN) 11275.98 33415.43 65928.64 110790.19 163322.22
Settlement (mm) 4.73 14.49 28.7 46.27 67.28
Table 4.1 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 90.65% 85.59% 80.78% 75.62% 71.12%
Raft 9.35% 14.41% 19.22% 24.38% 28.88%
Table 4.2 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 50 storey, 23x23 raft)
55
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.75 36 1.5 24 37
56
Load S et t lemen t Cu rve
1000000
900000
800000
700000
600000
Load (kN)
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.6 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 183645.90 352588.65 521535.16 690481.69 859428.19
Piles (kN) 165958.34 298855.20 413505.79 512937.38 596104.81
Raft (kN) 17687.56 53733.45 108029.37 177544.31 263323.38
Settlement (mm) 5.03 15.64 31.74 50.30 74.02
Table 4.3 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 90.37% 84.76% 79.29% 74.29% 69.36%
Raft 9.63% 15.24% 20.71% 25.71% 30.64%
Table 4.4 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
57
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.75 49 1.5 24 37
58
LO AD S ETTLEM ENT CURVE
1400000
1200000
1000000
LOAD (KN)
800000
Piled Raft
600000 Pile
Raft
400000
200000
0
0 10 20 30 40 50 60 70 80 90
SETTLEMENT (MM)
Fig. 4.9 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 292107.89 527900.10 763680.61 999452.92 1199315.19
Piles (kN) 261391.48 442305.05 592834.76 729553.56 821568.75
Raft (kN) 30716.40 85595.05 170845.85 269899.36 377746.44
Settlement (mm) 6.56 17.51 34.47 53.41 79.23
Table 4.5 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 89.48% 83.79% 77.63% 73.00% 68.50%
Raft 10.52% 16.21% 22.37% 27.00% 31.50%
Table 4.6 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
59
4.3 Ks = 5000 kN/m3, 40 storey, qg = 375 kN/m2, = 30
20 x 20 23 23 0.7 25 1.4 22 35
450000
400000
350000
300000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.12 Load Settlement Curve (Ks = 5000 kN/m3, 40 storey, 23x23 raft)
Load Distribution in Pile & Raft and Settlement
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 96228.20 183206.18 270181.83 357157.54 444133.18
Piles (kN) 86295.08 150793.55 204567.63 249559.96 288060.44
Raft (kN) 9933.12 32412.62 65614.20 107597.57 156072.74
Settlement (mm) 4.40 14.26 28.69 45.27 64.44
Table 4.7 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 40 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 89.68% 82.31% 75.71% 69.87% 64.86%
Raft 10.32% 17.69% 24.29% 30.13% 35.14%
Table 4.8 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 40 storey, 23x23 raft)
61
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.7 36 1.4 22 35
62
Load S et t lemen t Cu rve
800000
700000
600000
500000
Load (kN)
200000
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.15 Load Settlement Curve (Ks = 5000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 144829.99 275958.43 407079.70 538201.06 669322.28
Piles (kN) 129258.54 224854.76 302386.10 368548.35 423035.96
Raft (kN) 15571.44 51103.67 104693.60 169652.72 246286.33
Settlement (mm) 4.66 15.05 30.93 48.57 69.75
Table 4.9 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 89.25% 81.48% 74.28% 68.48% 63.20%
Raft 10.75% 18.52% 25.72% 31.52% 36.80%
Table 4.10 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 40 storey, 28x28 raft)
63
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.75 49 1.4 22 35
64
Load S et t lemen t Cu rve
1000000
900000
800000
700000
600000
Load (kN)
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.18 Load Settlement Curve (Ks = 5000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 203543.66 386674.77 569805.79 752936.96 936067.83
Piles (kN) 180897.25 312844.11 417936.22 508674.48 580298.46
Raft (kN) 22646.41 73830.66 151869.56 244262.47 355769.38
Settlement (mm) 4.91 15.72 32.71 51.19 75.06
Table 4.11 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 88.87% 80.91% 73.35% 67.56% 61.99%
Raft 11.13% 19.09% 26.65% 32.44% 38.01%
Table 4.12 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 40 storey, 33x33 raft)
65
4.4 Ks = 5000 kN/m3, 30 storey, qg = 375 kN/m2, = 30
20 x 20 23 23 0.75 25 1.2 14 34
66
L o a d S e t t l e me nt C ur v e
400000
350000
300000
250000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.21 Load Settlement Curve (Ks = 5000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 78554.33 142971.32 207388.32 271805.43 336222.46
Piles (kN) 63499.32 101648.24 127614.64 151714.53 169264.27
Raft (kN) 15055.01 41323.08 79773.68 120090.90 166958.20
Settlement (mm) 7.05 18.27 33.37 49.13 66.73
Table 4.13 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 80.83% 71.10% 61.53% 55.82% 50.34%
Raft 19.17% 28.90% 38.47% 44.18% 49.66%
Table 4.14 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 30 storey, 23x23 raft)
67
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.75 36 1.2 14 34
68
Load S et t lemen t Cu rve
600000
500000
400000
Load (kN)
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.24 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 107282.47 199871.41 292459.28 385047.11 477634.97
Piles (kN) 87427.59 142652.90 180094.82 214794.03 240020.84
Raft (kN) 19854.88 57218.51 112364.46 170253.08 237614.13
Settlement (mm) 6.28 17.17 32.25 47.85 65.51
Table 4.15 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 81.49% 71.37% 61.58% 55.78% 50.25%
Raft 18.51% 28.63% 38.42% 44.22% 49.75%
Table 4.16 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 50 storey, 28x28 raft)
69
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.75 49 1.2 14 34
70
Load S et t lememt n Cu rve
800000
700000
600000
500000
Load (kN)
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.27 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 154152.65 287892.95 421632.94 555373.10 689113.13
Piles (kN) 123742.25 200659.87 252783.16 299945.51 334920.46
Raft (kN) 30410.40 87233.08 168849.78 255427.59 354192.67
Settlement (mm) 6.89 19.23 35.52 53.06 72.22
Table 4.17 Load Dist. in Pile and Raft (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 80.27% 69.70% 59.95% 54.01% 48.60%
Raft 19.73% 30.30% 40.05% 45.99% 51.40%
Table 4.18 Load Dist. in Pile and Raft as % (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
71
4.5 Ks = 4000 kN/m3, 50 storey, qg = 300 kN/m2, = 29.5
20 x 20 23 23 0.55 25 1.6 30 37
72
Load S et t lemen t Cu rve
600000
500000
400000
Load (kN)
100000
0
0 10 20 30 40 50 60
Settlement (mm)
Fig. 4.30 Load Settlement Curve (Ks = 4000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 118554.92 229838.30 341118.76 452404.97 563688.28
Piles (kN) 109335.46 206892.93 298321.58 378191.12 454192.94
Raft (kN) 9219.46 22945.37 42797.18 74213.85 109495.34
Settlement (mm) 4.84 12.87 23.49 41.12 59.32
Table 4.19 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 92.22% 90.02% 87.45% 83.60% 80.58%
Raft 7.78% 9.98% 12.55% 16.40% 19.42%
Table 4.20 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 50 storey, 23x23 raft)
73
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.6 36 1.6 30 37
74
Load S et t lemen t Cu rve
900000
800000
700000
600000
Load (kN)
500000
Piled Raft
400000 Pile
Raft
300000
200000
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.33 Load Settlement Curve (Ks = 4000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 180702.07 349649.56 518596.07 687542.58 856489.03
Piles (kN) 166247.54 312317.82 449310.60 566529.83 678685.62
Raft (kN) 14454.53 37331.75 69285.47 121012.75 177803.41
Settlement (mm) 5.09 13.90 25.93 44.76 65.55
Table 4.21 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 92.00% 89.32% 86.64% 82.40% 79.24%
Raft 8.00% 10.68% 13.36% 17.60% 20.76%
Table 4.22 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 50 storey, 28x28 raft)
75
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.6 49 1.6 30 37
76
Load S et t lemen t Cu rve
1400000
1200000
1000000
Load (kN)
800000
Piled Raft
600000 Pile
Raft
400000
200000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.36 Load Settlement Curve (Ks = 5000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 252105.65 487891.15 723671.59 959452.15 1195232.60
Piles (kN) 231693.56 434356.13 623903.00 785651.50 939399.07
Raft (kN) 20412.09 53535.01 99768.59 173800.65 255833.52
Settlement (mm) 5.22 14.46 27.61 47.01 69.69
Table 4.23 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 91.90% 89.03% 86.21% 81.89% 78.60%
Raft 8.10% 10.97% 13.79% 18.11% 21.40%
Table 4.24 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 50 storey, 33x33 raft)
77
4.6 Ks = 4000 kN/m3, 40 storey, qg = 300 kN/m2, = 29.5
20 x 20 23 23 0.6 25 1.4 26 35
450000
400000
350000
300000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.39 Load Settlement Curve (Ks = 4000 kN/m3, 40 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 94907.43 181883.98 268859.78 355835.43 442811.08
Piles (kN) 87040.09 158830.23 223632.60 277051.51 326700.82
Raft (kN) 7867.34 23053.75 45227.18 78783.92 116110.26
Settlement (mm) 4.20 13.11 26.03 43.32 63.16
Table 4.25 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 40 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 91.71% 87.33% 83.18% 77.86% 73.78%
Raft 8.29% 12.67% 16.82% 22.14% 26.22%
Table 4.26 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 40 storey, 23x23 raft)
79
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.6 36 1.4 26 35
80
Load S et t lemen t Cu rve
800000
700000
600000
500000
Load (kN)
200000
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.42 Load Settlement Curve (Ks = 4000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 142875.60 273999.07 405120.36 536241.54 667363.67
Piles (kN) 130704.00 237348.98 332152.24 411168.57 482256.62
Raft (kN) 12171.60 36650.09 72968.12 125072.97 185107.04
Settlement (mm) 4.36 13.83 28.21 46.27 68.15
Table 4.27 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 91.48% 86.62% 81.99% 76.68% 72.26%
Raft 8.52% 13.38% 18.01% 23.32% 27.74%
Table 4.28 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 40 storey, 28x28 raft)
81
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.65 49 1.3 18 34
82
L o ad S et t leme nt C urv e
1000000
900000
800000
700000
600000
Load (kN)
400000 Pile
Raft
300000
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.45 Load Settlement Curve (Ks = 4000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 199457.51 382592.22 565723.27 748852.21 931985.27
Piles (kN) 182135.96 329753.51 459634.45 568718.15 665038.76
Raft (kN) 17321.55 52838.72 106088.82 180134.06 266946.51
Settlement (mm) 4.44 14.36 29.83 48.78 72.15
Table 4.29 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 91.32% 86.19% 81.25% 75.95% 71.36%
Raft 8.68% 13.81% 18.75% 24.05% 28.64%
Table 4.30 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 40 storey, 33x33 raft)
83
4.7 Ks = 4000 kN/m3, 30 storey, qg = 300 kN/m2, = 29.5
20 x 20 23 23 0.6 25 1.3 18 34
84
Load S et t lemen t Cu rve
350000
300000
250000
Load (kN)
200000
Piled Raft
150000 Pile
Raft
100000
50000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.48 Load Settlement Curve (Ks = 4000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 72346.74 136766.83 201183.76 265600.82 330017.88
Piles (kN) 64480.60 111151.08 148891.19 180675.91 208160.29
Raft (kN) 7866.14 25615.75 52292.57 84924.91 121857.59
Settlement (mm) 4.64 14.52 29.49 46.17 65.07
Table 4.31 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 89.13% 81.27% 74.01% 68.03% 63.08%
Raft 10.87% 18.73% 25.99% 31.97% 36.92%
Table 4.32 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 30 storey, 23x23 raft)
85
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.6 36 1.3 18 34
86
Load S et t lemen t Cu rve
600000
500000
400000
Load (kN)
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.51 Load Settlement Curve (Ks = 4000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 126960.00 217078.03 307196.03 397310.17 487432.05
Piles (kN) 92711.40 159305.18 213121.30 258279.96 297466.66
Raft (kN) 11627.93 37627.12 76396.49 123828.01 177229.14
Settlement (mm) 4.56 14.25 29.11 45.74 64.64
Table 4.33 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 30 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 73.02% 73.39% 69.38% 65.01% 61.03%
Raft 26.98% 26.61% 30.62% 34.99% 38.97%
Table 4.34 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 30 storey, 28x28 raft)
87
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.65 49 1.3 18 34
88
Load S et t lmen t Cu rve
700000
600000
500000
Load (kN)
400000
Piled Raft
300000 Pile
Raft
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.54 Load Settlement Curve (Ks = 5000 kN/m3, 30 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 151431.21 285171.19 418911.36 552651.40 686391.47
Piles (kN) 133459.74 227796.81 301305.23 364446.78 417214.05
Raft (kN) 17971.46 57374.37 117606.12 188204.62 269177.42
Settlement (mm) 5.04 15.81 32.47 50.55 71.65
Table 4.35 Load Dist. in Pile and Raft (Ks = 4000 kN/m3, 30 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 88.13 79.88 71.93 65.95 60.78%
Raft 11.87 20.12 28.07 34.05 39.22%
Table 4.36 Load Dist. in Pile and Raft as % (Ks = 4000 kN/m3, 30 storey, 33x33 raft)
89
4.8 Ks = 3000 kN/m3, 50 storey, qg = 300 kN/m2, = 29
20 x 20 23 23 0.5 25 1.7 30 37
90
Load S et t lemen t Cu rve
600000
500000
400000
Load (kN)
100000
0
0 10 20 30 40 50 60 70
Settlement (mm)
Fig. 4.57 Load Settlement Curve (Ks = 3000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 117895.13 229177.24 340460.57 451743.93 563030.39
Piles (kN) 112234.11 213968.11 310407.55 395932.74 477631.24
Raft (kN) 5661.03 15209.13 30053.02 55811.19 85399.15
Settlement (mm) 3.99 11.59 22.97 42.15 64.52
Table 4.37 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 50 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 95.20% 93.36% 91.17% 87.65% 84.83%
Raft 4.80% 6.64% 8.83% 12.35% 15.17%
Table 4.38 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 50 storey, 23x23 raft)
91
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.55 36 1.7 30 37
92
Load S et t lemen t Cu rve
900000
800000
700000
600000
Load (kN)
500000
Piled Raft
400000 Pile
Raft
300000
200000
100000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.60 Load Settlement Curve (Ks = 3000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 179724.83 348669.75 517616.32 686562.83 855508.83
Piles (kN) 170810.02 323695.53 467695.80 594947.24 714019.06
Raft (kN) 8914.81 24974.23 49920.52 91615.59 141489.77
Settlement (mm) 4.21 12.60 26.02 45.91 71.43
Table 4.39 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 50 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 95.04% 92.84% 90.36% 86.66% 83.46%
Raft 4.96% 7.16% 9.64% 13.34% 16.54%
Table 4.40 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 50 storey, 28x28 raft)
93
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.55 49 1.7 30 37
94
Load S et t lemen t Cu rve
1400000
1200000
1000000
Load (kN)
800000
Piled Raft
600000 Pile
Raft
400000
200000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.63 Load Settlement Curve (Ks = 3000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 292117.17 527900.16 763680.60 999461.07 1192483.30
Piles (kN) 276908.90 485085.87 679399.49 852422.24 984215.86
Raft (kN) 15208.26 42814.29 84281.11 147038.83 208267.44
Settlement (mm) 5.09 14.67 29.00 48.70 75.66
Table 4.41 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 50 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 94.79% 91.89% 88.96% 85.29% 82.53%
Raft 5.21% 8.11% 11.04% 14.71% 17.47%
Table 4.42 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 50 storey, 33x33 raft)
95
4.9 Ks = 3000 kN/m3, 40 storey, qg = 300 kN/m2, = 29
20 x 20 23 23 0.55 25 1.5 26 35
450000
400000
350000
300000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.66 Load Settlement Curve (Ks = 3000 kN/m3, 40 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 94247.30 181222.92 268198.63 355174.32 442148.11
Piles (kN) 88864.51 164156.73 232347.08 291324.85 344087.93
Raft (kN) 5382.79 17066.19 35851.56 63849.47 98060.18
Settlement (mm) 3.83 13.11 28.34 47.90 72.80
Table 4.43 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 40 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 94.29% 90.58% 86.63% 82.02% 77.82%
Raft 5.71% 9.42% 13.37% 17.98% 22.18%
Table 4.44 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 40 storey, 23x23 raft)
97
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.55 36 1.5 26 35
98
Load S et t lemen t Cu rve
700000
600000
500000
Load (kN)
400000
Piled Raft
300000 Pile
Raft
200000
100000
0
0 10 20 30 40 50 60 70 80 90
Settlement (mm)
Fig. 4.69 Load Settlement Curve (Ks = 3000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 141898.06 273019.29 404140.70 535261.97 666383.11
Piles (kN) 133566.57 245792.52 345774.50 433224.57 509310.72
Raft (kN) 8331.49 27226.76 58366.20 102037.40 157072.39
Settlement (mm) 3.97 13.84 30.63 54.42 78.34
Table 4.45 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 40 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 94.13% 90.03% 85.56% 80.94% 76.43%
Raft 5.87% 9.97% 14.44% 19.06% 23.57%
Table 4.46 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 40 storey, 28x28 raft)
99
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.6 49 1.5 26 35
100
Load S et t lemen t Cu rve
1000000
900000
800000
700000
600000
Load kN)
200000
100000
0
0 10 20 30 40 50 60 70 80 90
Settlement (mm)
Fig. 4.72 Load Settlement Curve (Ks = 3000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 199461.26 382592.20 565723.26 748854.23 931985.31
Piles (kN) 187517.65 342933.18 479959.90 600435.71 702885.26
Raft (kN) 11943.61 39659.02 85763.36 148418.52 229100.06
Settlement (mm) 4.07 14.45 32.45 54.46 84.76
Table 4.47 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 40 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 94.01% 89.63% 84.84% 80.18% 75.42%
Raft 5.99% 10.37% 15.16% 19.82% 24.58%
Table 4.48 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 40 storey, 33x33 raft)
101
4.10 Ks = 3000 kN/m3, 30 storey, qg = 300 kN/m2, = 29
20 x 20 23 23 0.6 25 1.4 20 33
102
Load S et t lemen t Cu rve
400000
350000
300000
250000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70 80 90
Settlement (mm)
Fig. 4.75 Load Settlement Curve (Ks = 3000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 79489.15 143906.11 208323.24 272740.24 337157.35
Piles (kN) 69983.55 119324.22 159858.25 193338.52 221129.44
Raft (kN) 6571.72 21648.06 45531.12 76467.92 113093.95
Settlement (mm) 5.36 16.84 34.77 56.10 82.18
Table 4.49 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 30 storey, 23x23 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 88.04% 82.92% 76.74% 70.89% 65.59%
Raft 11.96% 17.08% 23.26% 29.11% 34.41%
Table 4.50 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 30 storey, 23x23 raft)
103
Building Detail Raft Detail Pile Detail
25 x 25 28 28 0.6 36 1.4 20 33
104
Load S et t lemen t Cu rve
500000
450000
400000
350000
300000
Load (kN)
100000
50000
0
0 10 20 30 40 50 60 70 80
Settlement (mm)
Fig. 4.78 Load Settlement Curve (Ks = 3000 kN/m3, 30 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 102276.04 189303.99 273087.59 353458.92 474695.92
Piles (kN) 95793.02 166885.46 225418.91 273590.70 314264.34
Raft (kN) 6483.02 22418.53 47668.67 79868.21 160431.58
Settlement (mm) 4.56 15.50 32.90 54.14 79.52
Table 4.51 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 30 storey, 28x28 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 93.66% 88.16% 82.54% 77.40% 66.20%
Raft 6.34% 11.84% 17.46% 22.60% 33.80%
Table 4.52 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 30 storey, 28x28 raft)
105
Building Detail Raft Detail Pile Detail
30 x 30 33 33 0.65 49 1.4 20 33
106
Load S et t lemen t Cu rve
800000
700000
600000
500000
Load (kN)
200000
100000
0
0 20 40 60 80 100
Settlement (mm)
Fig. 4.81 Load Settlement Curve (Ks = 3000 kN/m3, 30 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piled Raft (kN) 151431.13 285171.21 418911.18 552651.40 686391.34
Piles (kN) 137977.97 238678.51 319622.24 386270.62 435236.75
Raft (kN) 13453.16 46492.70 99288.94 166380.78 251154.59
Settlement (mm) 5.21 17.60 37.51 60.77 93.29
Table 4.53 Load Dist. in Pile and Raft (Ks = 3000 kN/m3, 30 storey, 33x33 raft)
Loading Intensity
Load taken by
20% 40% 60% 80% 100%
Piles 91.12% 83.70% 76.30% 69.89% 63.55%
Raft 8.88% 16.30% 23.70% 30.11% 36.45%
Table 4.54 Load Dist. in Pile and Raft as % (Ks = 3000 kN/m3, 30 storey, 33x33 raft)
107
Load Distribution as % Load Distribution as %
0
20
40
60
80
100
0
20
40
60
80
100
Ks=5000(kN/m3) Ks=5000(kN/m3)
69.36 30.64 71.12 28.88
4.11 Comparisons
Ks=4000(kN/m3) Ks=4000(kN/m3)
79.24 20.76
50 str.
80.57 19.43
50 str.
Ks=3000(kN/m3) Ks=3000(kN/m3)
83.46 16.54 84.83 15.17
Ks=5000(kN/m3) Ks=5000(kN/m3)
63.20 36.80 64.85 35.15
Ks=4000(kN/m3) Ks=4000(kN/m3)
72.26 27.74
40 str.
73.77 26.23
40 str.
Pile Raft
Pile Raft
Ks=3000(kN/m3) Ks=3000(kN/m3)
76.42 23.58 77.82 22.18
Ks=5000(kN/m3) Ks=5000(kN/m3)
50.25 49.75 50.34 49.66
Fig. 4.82 Chart for 23m x 23m Raft and num. of storey
Fig. 4.83 Chart for 28m x 28m Raft and num. of storey
Ks=4000(kN/m3) Ks=4000(kN/m3)
62.66 37.34 63.08 36.92
30 str.
30 str.
Ks=3000(kN/m3) Ks=3000(kN/m3)
66.20 33.80 66.16 33.84
for same raft size and number of storey with increases Soil modulus.
In the below chart comparison of load distribution of pile and raft is shown
108
33m x 33m Raft
17.24
21.41
24.59
100
28.65
31.50
36.45
38.01
39.22
51.40
80
60
82.76
78.59
75.41
71.35
68.50
Load Distribution as %
63.55
61.99
60.78
40
48.60
20
0
Ks=5000(kN/m3)
Ks=4000(kN/m3)
Ks=3000(kN/m3)
Ks=5000(kN/m3)
Ks=4000(kN/m3)
Ks=3000(kN/m3)
Ks=5000(kN/m3)
Ks=4000(kN/m3)
Ks=3000(kN/m3)
50 str. 40 str. 30 str.
Pile Raft
Fig. 4.84 Chart for 33m x 33m and num. of storey
In the below chart comparison of load distribution of pile and raft is shown
for same raft size and Soil modulus with increases number of storey.
100
20.76
23.58
27.74
30.64
33.80
36.78
90
37.34
49.75
80
70
Load Distribution as %
60
83.46
50
79.24
76.42
72.26
69.36
66.20
63.22
62.66
40
50.25
30
20
10
0
50 str. 40 str. 30 str. 50 str. 40 str. 30 str. 50 str. 40 str. 30 str.
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Pile Raft
Fig. 4.85 Chart for 23m x 23m and Soil Modulus
109
28m x 28m Raft
100
16.54
20.76
23.58
27.74
30.64
33.80
90
36.78
37.34
49.75
80
70
Load Distribution as %
60
50
83.46
79.24
76.42
72.26
69.36
66.20
40
63.22
62.66
50.25
30
20
10
0
50 str. 40 str. 30 str. 50 str. 40 str. 30 str. 50 str. 40 str. 30 str.
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Fig. 4.85 Chart for 23m x 23m and Soil Modulus
Pile Raft
100
17.24
21.41
24.59
28.65
31.50
90 36.45
38.01
39.22
51.40
80
70
Load Distribution as %
60
50
82.76
78.59
75.41
71.35
68.50
40
63.55
61.99
60.78
48.60
30
20
10
0
50 str. 40 str. 30 str. 50 str. 40 str. 30 str. 50 str. 40 str. 30 str.
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Pile Raft
Fig. 4.87 Chart for 33m x 33m and Soil Modulus
110
In the below chart comparison of load distribution of pile and raft is shown
for same number of storey and Soil modulus with increases raft size.
50 Storey
15.17
16.54
17.24
100
19.43
20.76
21.41
28.79
30.64
31.50
90
80
70
Load Distribution as %
60
84.83
83.46
82.76
50
80.57
79.24
78.59
71.21
69.36
68.50
40
30
20
10
0
23x23 28x28 33x33 23x23 28x28 33x33 23x23 28x28 33x33
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Pile Raft
40 Storey
100
22.18
23.58
24.59
26.23
27.74
28.65
35.15
36.80
38.01
90
80
70
Load Distribution as %
60
50
77.82
76.42
75.41
73.77
72.26
71.35
64.85
63.20
61.99
40
30
20
10
0
23x23 28x28 33x33 23x23 28x28 33x33 23x23 28x28 33x33
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Pile Raft
Fig. 4.89 Chart for 40 storey and Soil Modulus
111
30 Storey
100
33.84
33.80
36.45
36.92
37.34
90
39.22
49.66
49.75
51.40
80
70
Load Distribution as %
60
50
66.16
66.20
63.55
63.08
62.66
40
60.78
50.34
50.25
30 48.60
20
10
0
23x23 28x28 33x33 23x23 28x28 33x33 23x23 28x28 33x33
Ks=5000(kN/m3) Ks=4000(kN/m3) Ks=3000(kN/m3)
Pile Raft
112
5
5.1 Conclusion
113
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114
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115