Contribution to long term performance of piled raft foundation in clayey soil

INTERNATIONAL JOURNAL OF CIVIL ENGINEERING
International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308
(Print), ISSN 0976 – 6316(Online), Volume 5, Issue 7, July (2014), pp. 130-148 © IAEME
AND TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 5, Issue 7, July (2014), pp. 130-148
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CONTRIBUTION TO LONG TERM PERFORMANCE OF PILED RAFT
FOUNDATION IN CLAYEY SOIL
Mohammed Y. Fattah(1) Mosa J. Al-Mosawi(2) Abbas A. O. Al-Zayadi(3)
1Professor, Building and Construction Eng. Dept., University of Technology, Iraq
2Professor, College of Engineering, Civil Eng. Dept., University of Baghdad, Iraq.
3Lecture, College of Engineering, Civil Eng. Dept, University of Al-Mustansiriyah, Baghdad, Iraq.
130
ABSTRACT
Raft and pile groups are the two alternative foundation options to support structures with
heavy column loads. Raft is normally designed as rigid in order to withstand high moment and
differential settlement, which is a function of intensity of load and relative stiffness of raft and soil.
In the case of pile groups more number of piles is provided than required to cater the column load
and to practically eliminate the settlement, which makes the foundation to be very expensive.
This study is devoted to carry out numerical analysis by the finite element method of the
consolidation settlement of piled rafts over clayey soils and detecting the dissipation of excess pore
water pressure and its effect on bearing capacity of piled raft foundations. The ABAQUS computer
program is used as a finite element tool and the soil is represented by the modified Drucker-
Prager/cap model. Five different configurations of pile groups are simulated in the finite element
analysis.
It was found that the distribution of load between piles becomes more uniform with the
increase of raft thickness. For an unpiled raft with high stiffness, the pile may share the same
amount of load. Increasing the pile diameter leads to decreasing the pore water pressure under the
raft, reducing the settlement of the raft and changing the shape of moment distribution in the raft as
well as increasing the piles share of load. Spacing between piles affects directly the pile-soil
interaction. Pile group with small spacing between piles may tend towards the block behavior,
therefore to perform the pile raft concept properly, the spacing between piles need to be wide enough
to allow the raft to participate in taking part of the load and using the pile strategically as settlement
reducers.
Keywords: Piled Raft, Foundation, Finite Elements, Time, Clay, Pile-Soil Stiffness.

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INTRODUCTION
Piled raft foundations are composite structures unlike classical foundation where the building
load is either transferred by the raft or the piles alone. In a piled raft foundation, the contribution of
the piles as well as the raft is taken into account. The piles transfer a part of the building loads into
deeper and stiffer layers of soil and thereby allow the reduction of settlement and differential
settlement in a very economic way. Piles are used up to a load level which can be of the same order
of magnitude as the bearing capacity of a comparable single pile or even greater.
Appling load to saturated soft soil layers by structures such as buildings causes the
development of excess pore water pressure. Initially, the structure will undergo an immediate
settlement as the excess pore pressure develops. However, with time, the excess pore pressure will
dissipate as water flows from regions of high excess pore water pressure to regions of lower water
pressure. As the excess pore pressure decreases, the effective stresses in the soil increase
progressively, and this leads to further settlement of the foundation with time.
Analysis of structures on consolidating soils has been limited in the past because of the
complexity of the time dependent interaction between the soil and the structure. Some solutions are
available for the consolidation of a circular raft on a porous elastic soil and for a pile group treated as
a solid block, but these solutions are limited in their scope and application (Small and Liu, 2008).
Poulos (1993) extended the method to incorporate the effect of free-field soil movement, load
cutoffs for the pile-soil and raft-soil interfaces to examine the interaction mechanism between the
piled raft and a soil subjected to externally imposed vertical movement. The analysis was
implemented via a computer program BRAWN (Piled Raft With Negative Friction).
The reliability of the plane strain model was examined by comparing its results with the
results of other models. Poulos et al. (1997) compared the results of the elastic-plastic plane strain
model and other elastic- plastic models for the case history described by Franke et al. (1994). In
general, the plane strain finite element model is sufficient for modeling vertically loaded piled rafts,
suggesting satisfactory modeling and result interpretation procedures.
Katzenbach and Reul (1997) described a structural model which employed the finite element
method for the geometrical modeling of the continuum, an elastoplastic constitutive model to
describe the soil behavior and a step-by-step analysis for numerical simulation. The piles were
modeled by 3-dimensional isoparametric finite elements and the raft was modeled by shell elements.
A realistic stress-strain behavior of the soil was simulated by a constitutive model for the soil which
consisted of two main yield-surface segments: a pressure dependent perfectly plastic shear failure
surface and a compression cap yield surface.
Mendonca and de Paiva (2000) presented a boundary element method for the analysis of
piled rafts in which full interaction between the raft, piles and the soil is considered. Unlike the other
approaches, discretization of the foundation system was not required in this approach. The soil was
represented by a Mindlin elastic linear homogeneous half space. The raft was assumed to be a thin
plate and was represented by integral equations. The pile was represented by a single element and the
shear stresses along it were approximated by a second-degree polynomial. The interaction between
the raft and soil was analyzed by dividing the interface into triangular elements and the subgrade
reaction was assumed to vary linearly across each element.
Prakoso and Kulhawy (2001) analyzed the piled raft foundation by the use of linear elastic
and non linear plane strain finite element model which involved the analysis of three dimensional
piled rafts as two dimensional strip piled raft. This analysis was performed by the geotechnical code
PLAXIS (Vermeer and Brinkgreve, 1995). This model can be used to analyze a relatively large piled
raft without excessive modeling and computing time. As the rows of piled were simplified into
strips, the in plan row of piles has to be simplified into plane strain pile with an equivalent pile

Young’s modulus Eeq in terms of the number of piles in the row considered, the dimensions of the
pile and the dimensions of the raft:
E − = (1)
132
n A E
p row i p p
eq L D
r p
where,
p row i n − = number of piles in row i,
p A = area of pile cross section,
p E = pile Young’s modulus,
r L = length of the raft, and
p D = pile diameter.
Maharaj (2003) presented results based on a three-dimensional nonlinear finite element
analysis of piled raft foundation. The raft, pile and soil have been discretized by eight nodded brick
elements. The soil has been idealized as a Drucker-Prager elastoplastic continuum. It was found that
the ultimate load carrying capacity of flexible raft increases with increase in soil modulus and length
of pile. Piles of length even less than the width of a flexible raft have been found effective in
reducing differential settlement. It was found that although the increase in soil modulus reduces the
overall settlement, differential settlement increases with increase in soil modulus for the same overall
settlement.
Reul and Randolph (2003) presented a three-dimensional elasto-plastic finite element method
for the analyses of piled raft foundations in over consolidated soil (Frankurt clay). The analysis was
implemented by the program ABAQUS. In the finite element method, the soil was modeled by
hexahedron elements and the piles were modeled by triangular prism elements. The circular piles
were modeled by square piles with the same shaft circumference. The interfaces between the raft and
soil and between the pile and soil were modeled by thin solid continuum elements and were assumed
to be perfectly rough. The soil was modeled by a cap model to simulate the non-linear behavior. The
cap model consisted of three yield surface segments: the pressure-dependent perfectly plastic shear
failure surface, the compression cap yield surface, and the transition yield surface. The same model
was used by these authors to perform a parametric study of a piled raft with different pile
configurations and subjected to non-uniform vertical loading (Reul and Randolph, 2004).
Mendonca and de Paiva (2003) described a coupled boundary element and finite element
formulation in which full interaction of the structure has been incorporated into the analysis. The pile
was represented by a single element and the stresses along the shaft were approximated by a
quadratic function and the pressure at the pile tip was assumed to be constant over the cross-section
of the base. The soil was represented by a Mindlin’s linear-elastic homogeneous half space. The raft
was analyzed by the finite element method through the use of a combination of flat triangular
elements based on discrete Kirchhoff theory and the hybrid stress model. The plate-soil interface was
modeled by triangular elements to account for the plate-soil interaction. The analysis dealt with a raft
subjected to vertical load only.
Based on both experimental evidence and three-dimensional finite element analyses, de
Sanctis and Mandolini (2006) proposed a simple criterion to evaluate the ultimate vertical load of a
piled raft as a function of its component capacities, which was evaluated by the conventional bearing
capacity theories. The presented results provide a guide to assess the safety factor of a vertically
loaded piled raft. The numerical analyses were performed via the FE code ABAQUS version 6.2,

with reference either to a capped pile (circular footing and one pile) or different foundation systems
(unpiled rafts, pile groups, and piled rafts) resting on a clay soil in undrained conditions.
Zhao et al. (2008) discussed the mechanism of long-short composite piled raft foundation,
assuming the relationship between shear stress and shear strain of the surrounding soil to be elasto-plastic.
The shear displacement method was employed to establish the different explicit relational
equations between the load and the displacement at the top of pile in either elastic or elasto-plastic
period.
de Sanctis and Russo (2008) adopted an innovative criteria for the design and some aspects of
the observed behavior of the piled foundations of a cluster of circular steel tanks. The design
analyses and the back-analyses have been performed with relatively simple procedures as the one
suggested by Mandolini and Viggiani (1997) based on the availability of a pile load test and using
the code NAPRA (Numerical Analysis of Piled RAft) developed by Russo (1998). Among the
innovative concepts included in the code, there was the possibility of taking advantage of the load
sharing between piles and raft. It was stated that if the raft provided adequate bearing capacity, the
piles can be used as a means to reduce the settlement and no safety factors were prescribed to their
ultimate capacity. The general agreement between the analyses and the experimental results is rather
satisfactory confirming the validity of both the computer code and the procedure of analysis.
The objective of this study is to carry out numerical solution by the finite element method.
The problem takes the effect of time into consideration through the studying of consolidation of
clayey soils and detecting the dissipation of excess pore water pressure and its effect on bearing
capacity of the foundation. Settlement of the piled raft can be estimated even after years of
completing the construction of any structure over a piled raft foundation.
Although there has been a great deal of attention paid to the settlement of pile groups and
piled raft foundations, little attention has been paid to the time- dependent behavior. Therefore, this
study will be directed to the analysis of piled raft system over consolidating soils.
DESCRIPTION OF THE PILED RAFT PROBLEM
Using a raft alone as a foundation results in excessive settlement and the use of pile groups is
too costly, a piled raft is a feasible solution. The use of a piled raft as the foundation for building has
proven to be an effective and economical way to control the total settlement as well as bearing
capacity.
The performance of a piled raft can be influenced by several factors such as the condition of
the supporting soil, loading condition, size and length of the piles, pile arrangement, and other
factors. Regarding the soil situation, the permeability has significant effect on the rate of dissipation
of pore water pressure, and therefore affects the consolidation settlement.
In this study, the pile groups are constructed in clayey soil with the existence of pore water
pressure. The ABAQUS computer program is used as a finite element tool and the soil is
represented by the modified Drucker-Prager/cap model.
In the finite element solution, the circular cross sections of piles are replaced with square
ones with the same equivalent area, and both the soil and piled raft systems are meshed with the 8-
nodes brick elements, whereas the pore water pressure is allowed for soil and not allowed though the
piled raft. In the program ABAQUS, it is important to mesh the interacted surface with the same type
and size of the element, since the interaction is simulated through defining surfaces with contact
properties (as the case of pile-soil interaction and raft-soil interaction).
As shown in Figure 1, the five configurations of piles are arranged symmetrically, though in
the finite element solution, only quarter of the problem can be used to represent the whole problem
keeping the necessary computer time as minimum as possible. Boundary conditions are used to
simulate the axes of symmetry, where displacement normal to the axis of symmetry is set zero. The
133

lateral boundaries of the soil are chosen to be far enough from the zone of influence under the
vertical load, the lower boundary simulating the depth of the soil layer is also kept far enough from
the pile bottom as the piles are assumed to be floated piles (i.e. the piles are not driven to a rigid
stratum). Even though the vertical displacement at the lateral and vertical boundary is zero, a
boundary condition prescribing the vertical displacement with a value of zero is considered. The
pore water pressure is assumed to remain zero at the upper boundary, therefore one way drainage is
alloed.
The piled raft foundation material is assumed to be linear elastic having the following
properties: Young’s modulus of elasticity Er = 20 x 106 kN/m2 and Poisson’s ratio vr = 0.3.
The clayey soil is modeled as homogeneous isotropic elasto-plastic soil following the
modified Drucker-Prager/cap constitutive relation. The properties of the soil considered in this study
represent part of Baghdad soil, according to Al-Saady (1989) who tested compacted clay samples.
The results of Al-Saady tests are used to calculate the effective strength parameters of Drucker-
Prager constitutive model, the parameters are listed in Table 1.
Table 1: Properties of clayey soil (Al-Saady, 1989)
Young’s modulus of elasticity, Es 1.22 x 104 kN/m2
Effective Poisson’s ratio, vs 0.20
Effective cohesion, c’ 0
Effective angle of internal friction, f’ 38 o
Permeability, k 1 x 10-7 m /sec (0.259 m/month)
For expressing the load settlement behavior below piled and unpiled rafts under vertical
concentrated load, dimensionless factors are used as follows:
134
PD
u E BW
I
z s
z =
(2)
where:
P = Applied vertical load,
uz = Displacement at the center of the raft,
Es = Soil modulus of elasticity,
B = Length of the unpiled raft,
W = Width of the unpiled raft, and
D = Pile diameter.
Unpiled raft is simulated in the finite element program ABAQUS in its full size, with
dimensions of (B x W). The behavior of the clay with time is expressed in terms of pore water
pressure which is generated after applying the load.
A block of modeled soil with dimensions of (6W * 6B * 8B) is used in the finite element
solution, these dimensions are chosen to be far enough from the zone of influence, biased meshing
techniques is used to discretize the soil, where finer meshes are used near the raft as the
concentration of vertical stress and displacement is expected to occur.
A vertical load of 10000 kN is applied in a short time simulating the gradual loading to the
unpiled rafts of the five different dimensions. The load is concentrated in the middle of the raft. The
settlement under the raft is calculated at different times, where time is introduced through the

dimensionless time factor defined in equation (3), this factor can be written in the following term
(Small and Liu, 2008):
= (3)
135
(1 )
u
s s
−
v g u u
(1 2 ) (1 ) ( H
)2
k E t
T
− +
w s s
where, t is the time,
gw is the unit weight of water,
k is the coefficient of permeability, and
H is the thickness of the clay layer which represents the drainage path, the raft is considered
as impermeable footing.
The behavior of piled raft foundation can be influenced by several factors such as the
thickness of the raft, arrangement of piles, size and length of pile and relative stiffness between piles
and soil. In the design of piled raft, it is important to take into account these factors to achieve the
objective of economic construction with satisfactory performance.
In this paper, the effect of these factors on the time dependent performance of piled rafts of
(3x3) group of piles will be examined. The results will be presented in terms of the (i) overall
settlement of the raft, (ii) bending moment in the raft, and (iii) pore water pressure the center of the
raft, (iv) percent of load carried by piles. All these parameters are presented at time t =50 months (i.e.
at in the end of consolidation steps).
Since only vertical load is applied to the 9-pile group, the behavior of piles can be
categorized in three sets: center pile, corner pile and edge pile. The load distribution between piles
with time will be observed. Figure 1 shows the (3x3) piled raft configuration adopted in the
parametric study.
The results of the parametric study are presented in a non-dimensional form except the pore
pressures shown in its true value. The case of (3x3) piled raft with pile diameter of 1.0 m, pile length
of 20 m, spacing to diameter ration of 5, and raft thickness of 1.0 is considered as a reference to the
other cases for the purpose of comparison only. In the parametric study; the spacing to diameter ratio
is kept constant with a value of (5), and to keep the size of the raft the same in this study, when
studying the effect of pile diameters, it is necessary to use spacing to diameter ratio of (3) to keep a
minimum spacing between pile of (3D) as required in the design of pile groups and piled rafts.
Effect of raft thickness
The effect of raft thickness can be studied by considering four thickness of (0.75 m, 1.00 m,
1.25 m, and 1.5 m) for the case of (3x3) piled raft with pile diameter of 1.00 m, length of piles of 20
m, spacing to diameter ratio of 5. Figure 2 shows the variation of excess pore water pressure under
the raft for different thicknesses. For the raft thickness of 0.75 m, the pore water pressure increased
by about 40% and reduced by 20% for the case of raft thickness of 1.25 m as compared to the
reference case, this can be attributed to the change of raft stiffness with changing of its thickness.
The dimensionless factor used to express the settlement under the raft is presented in equation
(2). As expected, the settlement and the moment in the raft decreases with the increase of the raft
stiffness, Figures 3 and 4 indicate the settlement under the center line of the raft, and the moment
generalized in the raft, respectively. The case of 0.75 m raft thickness recorded the highest settlement
with 20% more than the reference case ((3x3) piled raft with tr =1.00 m, D 1.00 m, L = 20 m) and
the highest moment of 30% less than the reference case, this case also showed a noticeable
differential settlement between the center of the raft and the edge.

The distribution of load between piles becomes more justice with the increase of raft
thickness as shown in Figure 5. For the case of 0.75 m raft thickness, the center pile seems to take
the maximum share as compared to the other piles individually, but at the case of the raft thickness
of 1.25 all piles share the same amount of load, then after for 1.75 m raft thickness the corner pile
carry load slightly more than the center or the edge pile.
Fig. 1: Piled raft of (3x3) group adopted in the parametric study.
Fig. 2: Variation of pore pressure under the center of the raft for (3x3) group of piles, L = 20 m, D =
1.0 m, S/D = 5, and different raft thicknesses.
136

Fig. 3: Normalized vertical displacement under piled raft of (3x3) group, with L = 20 m, D = 1.0 m,
S/D = 5, and different raft thicknesses
Fig. 4: Normalized moment in the raft of (3x3) group of piles, with L = 20 m, D = 1.0 m, S/D = 5,
and different raft thicknesses
Fig. 5: Effect of raft thickness on the percent of load carried by piles of (3x3) piled raft with L = 20
m, S/D = 5, D = 1.0 m
137

138
EFFECT OF PILE DIAMETER
Pile diameter affects directly its bearing capacity. In this study, five different diameters are
considered (0.5 m, 0.75 m, 1.0 m, 1.25 m, and 1.5 m). During this analysis, only the pile diameter is
changed while other parameters are kept unchanged. The effect of pile diameter is clear in decreasing
the pore water pressure under that raft, reducing the settlement of the raft and changing the shape of
moment distribution in the raft as well as increasing the piles share of load. Figures 6 to 9 are
devoted for expressing the effect of pile diameter on pore pressure under the raft, settlement of the
raft, moment in the raft and load sharing between piles, respectively. The cases of 0.5 m and 0.75 m
pile diameters show approximately the same maximum value of pore pressure but the latter case
show faster dissipation of pore pressure, whereas the case of 1.75 m pile diameter recorded 40%
decrease in the pore pressure under the raft in comparison to the reference case. The settlement of the
raft is of maximum value for the case 0.75 m which is 16% greater than the reference case, and it is
of minimum value for the case of 1.75 m raft thickness with 22% reduction relative to the reference
case. The increase in pile diameter also works to decrease the differential settlement by 55% less
than the reference case for a raft with piles of 1.75 m diameter, but the reduction in pile diameter
causes an increase of 60% in the differential settlement relative to the reference case for piled raft
with 0.75 m piles diameter.
The moment distribution shape in the raft is changed with each case of different diameter, the
case of 0.75 m diameter showed the maximum moment where the piles were ineffective in reducing
the moment as the case of piled raft of 1.75 m piles diameter. The reference case adopted in studying
the pile diameter has spacing to diameter ratio of 3 and pile diameter of 1.0 m. In this case, the
percent of load taken by piles is about 83%, this value continues to increase until reaching a value of
92% for the case of 1.75 m pile diameter, 92% is the maximum value of load taken by piles relative
to the total applied load recorded in the parametric study.
EFFECT OF PILE LENGTH
As it is known, the total carrying capacity of piled raft depends on the length of piles. To
study the effect of piles length in the case of (3x3) piled raft, piles with length varied from 15 m to
30 m with increment of 5 m are studied. Spacing, diameter of piles, and thickness of the raft are
holed constant with the values 5D, 1.00 m, and 1.00 m, respectively. Figures 10 to 13 show the effect
of pile length on the variation of pore water pressure with time under the center of the raft, the
normalized vertical settlement under the center of the raft, the normalized moment generalized in the
raft, and the load sharing between the piles relative to the total applied load. The spacing to diameter
ratio adopted herein is (5), and the pile lengths studied are ranged from 15-30 m with increment of 5
m each time. It can be noticed that the studied parameters are changing uniformly as the length
increases except for the case of 30 m, where the change is greater. Settlement under the raft
decreases by 4% as compared to the reference case, while the differential settlement for the case of
30 m pile length is about 75% less than that of the reference case. The moment in the raft is reduced
uniformly with increasing the pile length, while the load taken by pile increases with the increase of
pile length. The case of 30 m pile length recorded the best result if a statistical evaluation is made,
but from an engineering point of view, the enhancement in the result relative to the reference case is
not much appreciated relative to the additional cost resulted from additional pile length.

Fig. 6: Variation of pore pressure under the center of the raft for (3x3) group of piles, L=20 m, S/D =
3, and different pile diameters
Fig. 7: Normalized vertical displacement under piled raft of (3x3) group, with L=20 m, S/D = 3, and
different pile diameters
Fig. 8: Normalized moment in the raft of (3x3) group of piles, with L=20 m, S/D = 3, and different
pile diameters
139

Fig. 9: Effect of pile diameter on the percent of load carried by piles of (3x3) piled raft with L=20 m,
S/D = 5
140
EFFECT OF SPACING BETWEEN PILES
The arrangement of piles in the 9-pile group can influence the vertical settlement of the piled
raft and the bending moment generalized due to the applied load. The interaction between piles
inside the group is influenced by the spacing between piles. Therefore, spacing to diameter ratio is
considered in the parametric study. The square shape of the group is maintained unchanged, while
four values of the spacing to diameter ratio are considered as follows (S/D = 3, 4, 5, and 6). The
diameter of piles is chosen to be 1.00 m, length of piles is kept constant and equal to 20 m, and the
raft thickness has a value of 1.00 m. Spacing between piles affect directly the interaction between
piles, pile group with small spacing may tend towards the block behavior, therefore to perform the
piled raft concept properly, the spacing between piles need to be wide enough to allow the raft to
participate taking part of the load and using the pile strategically as settlement reducers.
Fig. 10: Variation of pore pressure under the center of the raft for (3x3) group of piles, D=1.0 m, S/D
= 5, and different pile lengths.

Fig. 11: Normalized vertical displacement under piled raft of (3x3) group, with D=1.0 m, S/D = 5,
and different pile lengths.
Fig. 12: Normalized moment in the raft of (3x3) group of piles, with D=1.0 m, S/D = 5, and different
pile lengths.
Fig. 13: Effect of pile length on the percent of load carried by piles of (3x3) piled raft with L=20 m,
S/D = 5
141

Figures 14 to 17 represent the effect of spacing to diameter ratio (S/D) on the studied
parameters, the pore pressure under the raft increased when the spacing ratio changed from 4 to 6,
where the settlement at the center increases, but the case of spacing to diameter ratio of 3 showed the
maximum pore pressure, and this may be due to the close distance between piles which prevent pore
pressure to dissipate where the raft and piles are considered as impermeable elements. The same
three cases of spacing ratio ranged from 4 to 6 have a central and differential settlement of 12 %
greater, 5% less than the reference case for S/D = 4 and 6, respectively, where the case of S/D =5 is
considered as the reference case. The maximum moment is noticed to be at the center of the raft
under the applied vertical load for the four studied spacing to diameter ratios with values
approximately the same, the moment distribution along the raft section is different for each case,
where the moment is vanishing at the edge and at the location of the edge pile (where the section of
the raft passes) there is an increase in the moment to a value of about 15% of the maximum moment.
The pile load relative to the total load is decreasing with the increase of the spacing to
diameter ratio, the percent of load taken by all pile drops from 90% to 76% as the S/D changes from
3 to 5, then changes slightly for S/D of 6. The center pile carries the minimum percent of load for the
case of S/D = 3, but the same pile takes the maximum value of the load relative to other piles for
S/D = 6.
Fig. 14: Variation of pore pressure under the center of the raft for (3x3) group of piles, D=1.0 m, L =
20 m, and different spacing to diameter ratios
Fig. 15: Normalized vertical displacement under piled raft of (3x3) group, with D=1.0 m, L = 20 m,
and different spacing to diameter ratios
142

Fig. 16: Normalized moment in the raft of (3x3) group of piles, with D=1.0 m, L = 20 m, and
different spacing to diameter ratios
Fig. 17: Effect of spacing to diameter ratio on the percent of load carried by piles of (3x3) piled raft
with L=20 m, D = 1.0 m
EFFECT OF RELATIVE PILE-SOIL STIFFNESS
The relative stiffness between piles and the supporting soil is studied through considering
different values for the relative pile soil stiffness ratio which defined as:
K = (4)
143
p
s
E
ps E
where, EP and Es are the modulus of elasticity for piled raft foundation and soil, respectively.
The other parameters (pile diameter, pile length, spacing between piles, and raft thickness) are not
changed and have the values of 1.00 m, 20 m, 5D, and 1.00 m, respectively.
Figures 18 to 21 are devoted for studying the effect of pile-soil relative stiffness. The pore
water pressure, normalized settlement of the raft, normalized moment, and the percent of load taken
by piles are presented. The pore pressure is significantly affected by the pile- soil stiffness ratio

(denoted by KPS) as shown in Figure 20, for values of KPS more than 1000, the soil becomes very
stiff and the generalized pore pressure reduces rapidly from KPS = 100 to KPS = 1000. The
settlement of the raft decreased with the increase of pile- soil stiffness ratio, where the maximum
settlement was for the case of KPS = 100, 58% reduction of the maximum settlement occurs when
using soil with KPS = 1000, and for higher values of KPS, the settlement seems to change lightly
and then remains constant. The differential settlement recorded the maximum value at small values
of soil-pile stiffness ratio, but this settlement become negligible for very high values of KPS.
The moment distribution in the raft seems not too much affected by changing the pile-soil
stiffness ratio, while the load taken by pile increased with increasing KPS from 100 to 1000, after
that the total load carried by pile remains unchanged for higher values of KPS as shown in Figure 23
where the percent of load taken by pile is plotted against the log of KPS. The center pile carries
higher percent of load relative to the other piles at the small value of KPS, then for high values of
KPS, all piles approximately share the same percent of load.
Fig. 18: Variation of pore pressure under the center of the raft for (3x3) group of piles, D=1.0 m, L =
20 m, and pile-soil stiffness ratios
Fig. 19: Normalized vertical displacement under piled raft of (3x3) group, with D=1.0 m, L = 20 m,
and different pile-soil stiffness ratios
144

Fig. 20: Normalized moment in the raft of (3x3) group of piles, with D=1.0 m, L = 20 m, and
different pile-soil stiffness ratios
Fig. 21: Effect of pile-soil stiffness ratio on the percent of load carried by piles of (3x3) piled raft
with L=20 m, D = 1.0 m.
EFFECT OF ADDING CUSHION UNDER THE RAFT
In the construction of the raft foundation, usually a layer of stiff material is placed and
compacted under the raft to enhance the bearing capacity of the soil. For piled raft foundation the
addition of cushion under the raft may enhance the performance of the raft and increase the percent
of load taken by it.
The case (3x3) piled raft is considered for studying the effect of adding a cushion of
modulus of elasticity and coefficient of permeability assumed to be equal 100 time those of the
underlying soil under the raft with thickness varying from zero to 3 meters. The cushion is assumed
to extend in the lateral direction for a distance of 1.0 m and has different thicknesses of (tc = 0, 1.0,
2.0, 3.0 m), as shown in Figure 1.
Adding granular cushion with high permeability under the raft is participated to reduce the
generated pore water pressure under that raft as shown in Figure 22. The cushion also works to
reduce the settlement of the raft and moment in the raft section, where 30% decrease in the
settlement of the center of the raft occurs and about 40% decrease in the moment occurs if a cushion
of 3.0 m thickness is added as shown in Figures 23 and 24, respectively. The cushion is also
participated in enhancing the bearing capacity of the raft and increasing the load taken by the raft
relative to the case of no cushion.
145

Figure 25 shows the percent of load shared by pile to the total load, the percent of load taken
by pile reduces from 76% to 60% after adding a cushion of granular material with thickness of 3.0 m,
and the distribution of load between piles becomes more uniform as the thickness of the cushion
increases.
Fig. 22: Variation of pore pressure under the center of the raft for (3x3) group of piles, and different
thicknesses of the cushion under the raft
Fig. 23: Normalized vertical displacement under piled raft of (3x3) group, and different thicknesses
of cushion under the raft
Fig. 24: Normalized moment in the raft of (3x3) group of piles with different thicknesses of cushion
under the raft
146

Fig. 25: Effect of thickness of the cushion under the raft on the percent of load carried by piles of
(3x3) piled raft with L=20 m, D = 1.0 m
147
CONCLUSIONS
1. The distribution of load between piles becomes more uniform with the increase of raft
thickness. For an unpiled raft with high stiffness, the pile may share the same amount of load.
2. Increasing the pile diameter leads to decreasing the pore water pressure under the raft, reducing
the settlement of the raft and changing the shape of moment distribution in the raft as well as
increasing the piles share of load.
3. Spacing between piles affects directly the pile-soil interaction. Pile group with small spacing
between piles may tend towards the block behavior, therefore to perform the pile raft concept
properly, the spacing between piles need to be wide enough to allow the raft to participate in
taking part of the load and using the pile strategically as settlement reducers.
The pile load relative to the total load is decreasing with the increase of the spacing to diameter
ratio. The percent of load taken by all piles drops from 90% to 76% as the S/D changes from 3
to 5. The center pile takes the minimum percent of load for the case of S/D = 3, but the same
pile takes the maximum value of the load relative to other piles for S/D = 6.
4. The moment distribution in the raft seems to be not too much affected by changing the pile-soil
stiffness ratio, while the load taken by pile increased with increasing KPS (relative pile soil
stiffness) from 100 to 1000. After that, the total load carried by a pile remains unchanged for
higher values of KPS.
The center pile carries higher percent of load relative to the other piles at small value of KPS,
then for high values of KPS, all piles approximately share the same percent of load.
5. The addition of a granular cushion under the raft notably enhances the bearing capacity of the
raft and reduces the expected settlement and differential settlement of the raft. The raft can
carry an additional 15% of the total load if a cushion of 3.0 m thickness is added under the raft.
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