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Research in Civil and Environmental Engineering 2013 1 (05) 287-299
REVIEWING PERFORMANCE OF PILED RAFT AND PILE GROUP
FOUNDATIONS UNDER THE EARTHQUAKE LOADS
*
Ali Akbari a, Mohammad Nikookar b , Mahdi Feizbahr c
a
Postgraduate of Geotechnical Engineering, Department of Civil Engineering, Amirkabir University of
Technology, Tehran, Iran
b
Master of Science Student of Geotechnical Engineering, Faculty of Engineering University of Guilan, Rasht,
Iran
C
School of Civil engineering, Engineering campus, University sains Malaysia,14300 Nibong tebal , Penang,
MALAYSIA
Keywords
piled raft
pile group
load bearing proportion
earthquake load
superstructure response
1
A B S T R A C T
The primary idea of using deep foundations was brought up, in order to
control overall and differential settlement of foundation under heavy
structural loads. However, gradually by increasing height of structures, using
deep foundations were considered for controlling inversion and deal with
shear loads resulting from the earthquake, more than was previously
considered. But the important matter about deep foundations is to focus on
shearing and bending forces in connection place of cap to piles, as well as
their growth in depth, which it must be reviewed accurately for planning
goals. There has been used from ABAQUS finite element software, to
investigate interaction of soil, foundation, and structure. The studied soil is
clay type with Mohr-coulomb behavioral model. There have been modeled
structural sections with dual system performance in three storey type,
including 10, 20, and 30 storey. The study aims to review transaction effect
of soil, foundation, and structure for transferring shearing resulted from
earthquake to superstructure, pile group loading share from horizontal
loading and shearing and bending forces in piles.
INTRODUCTION
In the past, using reinforced mat foundation was the best option for dealing with heavy structural loads.
Gradually there was considered idea of using pile, in order to control and decrease settlement, because of
heavier structures and necessity to consider settlement in final designation and settlement limitation to the
allowable settlement. There was designed a traditional deep foundation with relative high safety factor for
* Corresponding author (E-mail: nikookar2006@yahoo.com).
Ali Akbari et al - Research in Civil and Environmental Engineering 2013 1 (05) 287-299
the piles. In the past few decades, researchers concluded that it can be gained a more economical plan
than free pile group by taking into account the capacity of raft in the load and division safety factor
between raft and piles. Piled raft foundation is an economic and logical foundation in dealing with vertical
static loads because the load is transferred to the ground by both components of piles and raft; in addition,
it can be accessed to the same settlement by reducing number of piles in comparison with pile group
(Hemsly, 2000). With this idea, many attempts were performed to investigate piled raft behavior against
vertical static and dynamic loads. Behavior of piled raft foundation is relatively complicated during
earthquakes, because of dynamic interaction between raft, pile and soil. In areas with high activity of
seismicity such as Japan, the piles tolerate high load during earthquakes, especially when imposed
superstructure initial force will be high, which it is often the same. Stress is focused in head of pile, because
connection type between raft and piles is usually fixed. Dynamic behavior of piles depends on deformations
of foundation ground, in addition to the imposed inertia force from superstructure on the foundation. In
Huguken-Nambu earthquake, the piles were failed because of the ground deformations, even in absence of
superstructure (Horikoshi et al., 1996). Until 2003, there was ignored pile capacity in designing piles under
earthquake loads in Japan (Matsumoto et al., 2004).
Poulos and Davis (1980) offered a comprehensive relation for designing deep foundations under
static lateral load. In the past few decades, there were conducted significant researches to understand
behavior of deep foundations under dynamic loads. There were presented various methods to assess the
dynamic behavior of piles based on linear behavior of soil including subgrade reaction method (Tucker et
al., 1964), lumped mass idealization method (Prakash et al., 1973), Novak continuum approach (Novak,
1974). But results of dynamic and centrifuge laboratory experiments (Pak et al., 2003; Boominathan et al.,
2006; Puri et al., 1992; Anandarajah et al., 2001) show major difference between the observations and the
estimated parameters due to the resulted nonlinear behavior of soil and also due to separation between
pile shaft and soil under vibration. In recent year’s scholars such as Matsumoto et al., (2004), Nakai et al.,
(2004), Horikoshi et al., (2003) and Banerjee (2009) have studied performance of piled raft foundation by
using centrifuge laboratory tests and shaking table. Based on elasticity theory, Nakai et al., (2004) have
offered approximate analysis method for piled raft foundation under static lateral load, in order to
determine contribution of horizontal load in each component of raft and piles.
Due to complexity of horizontal load sharing between raft and pile group in the piled raft foundation,
there is designed most piles for entire horizontal load which it very non-economic, because according to
Nakai et al., (2004) there can be decreased pile horizontal contribution 60 to 80 percent with regard to soil
type and recognizing foundation performance, which it requires more exact survey of horizontal load
mechanism in this type of foundation. Reaction of the two above introduced system will be certainly
different, under earthquake loads or any other dynamic load such as wind, with regard to their different
performance and interactions in dealing with vertical static loads. This study aims to describe some of these
differences. Figure 1 shows the apparent difference between piled raft and pile group foundation.
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(a)
(b)
Fig. 1 Schematic form of: (a) piles group, (b) piled raft
2
MODELING
Using and developing accurate numerical models for simulating any engineering structure is
inevitable because of high costs for constructing laboratory samples in a large scale. It can be studied role
of different parameters in different stages of loading by using a numerical model and laboratory samples
can be only used for modeling calibration. In this paper, it has been used from ABAQUS finite element
software to examine the interaction of soil – foundation – structure. The studied soil is clay type with Mohrcoulomb behavioral model, which it have been used from un-drained shear resistance (SU), in order to
consider un-drained conditions under dynamic load. Physical and mechanical parameters of used materials
in this study are shown in Table 1.
Table 1 Mechanical and physical parameters of the used materials
Clay
Bedrock
Concrete
Steel
Density:
γ, (kN/m3)
19
20
24
78
Young’s
modulus:
E, (MPa)
40
500
40,000
210,000
Poisson
ratio:
υ
0.45
0.3
0.25
0.2
friction
angle:
ϕ,
(deg) 0
45
-
shear
strength: SU,
(kPa)
60
0
-
In dynamic studies, model boundaries should not be considered fixed, but should provide conditions
that part of energy from the vibrations is exited from the boundaries, which they are called transient
boundaries. There have been used infinite elements around the soil model, in order to consider transient
boundary conditions and semi-infinite mass. Infinite elements are defined as which connected nodes to
original model is the closed level and nodes of infinite side are open which it provides support fixed
conditions in far field. In this review, there have been used three types of foundations with arrangement
3×3, 4×4, and 5×5 with different length and diameter of piles, based on increasing number of storey,
according to specifications in Table 2. Structural part has been modeled with dual system function
(composition of brace and bending frame on earthquake direction) in three floor types of 10, 20 and 30 as
superstructure, by considering to importance of structural part in dynamical analysis, especially analysis
related to inertia. Specifications of the studied structure and foundation are shown in Table 3. Each
structure contains beam, column, brace and roof in each floor and there has been avoided from details
modeling such as how to connect, walls, base plates and etc, all connections have assumed fixed.
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Table 2 Specifications of the studied structure and foundation
Storey
number
Total
weight
(Ton)
Storey
height
(m)
Number
of
openings
Raft
dimensions
(m2)
Raft
thickness
(m)
Number
of piles
Diameter
of piles
(m)
Length
of piles
(m)
Distance
of piles
(m)
10
20
30
700
3,000
7,200
3
3
3
2
3
4
10×10
15×15
20×20
0.8
1.4
1.8
9
16
25
0.6
0.9
1.2
10
14
18
4
4
4
Thickness of soil layer has been selected 2 times of pile length and its width is 4 times of raft width. In
order to manner separate in the simulation of pile group and pail raft, cap has been located with the
distance of one-fifth of the raft thickness from the soil surface in the pile group to avoid direct contact with
the soil and just get involve to support horizontal and vertical load while in the pile raft, raft has been
defined in the contact interaction with the soil to consider the simultaneous behavior of raft and piles
under the load.
There has been considered mechanical contact between soil surface and foundation components
including raft bottom, piles shaft and end by introduction a surface for transferring shear tension between
two contact surfaces, by using penalty frictional formulation and normal behavior for superficial
introduction for transferring normal tensions. Due to symmetry in geometry and loading, there have been
modeled all models symmetry for time reduction, and displacement constraints have been used in
symmetry surface (displacement and rotation have been closed in symmetry and other both directions
respectively). Figure 2 represents modeling and meshing a foundation 3×3 piles array in the ABAQUS
software.
(a)
(b)
(d)
(c)
Fig. 2 Foundation model with 3×3 piles array in the ABAQUS software:
(a) Foundation meshing and the field soil, (b) array type of piles
(c) Model configuration, (d) three-dimensional model
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There have been applied loading in three steps contain geostatic step for considering in situ soil
tensions, static for gravity loading resulting superstructure weight, and dynamic time history loading as
horizontal acceleration in bed rock 2 meter thick to the model. There has been used from explicit approach
for analysis in the above three steps. There has been used from time history acceleration earthquake in
Kobe with PGA= 0.371 g and effective duration of 16 seconds (time 2 to 18 seconds) recorded in the NishiAkashi station for dynamic loading. Time history of this acceleration is shown in Figure 3. This record has
been selected because of suitability for start effective time (analysis time reduction), relatively slow end,
and average PGA.
Acceleration (g)
0.4
Kobe Earthquake PGA=0.371 g
0.2
0
0
2
4
6
8
10
12
14
16
-0.2
-0.4
Time (S)
Fig. 3 recorded time history acceleration of Kobe earthquake in Nishi-Akashi Station
3
VALIDATION
There has been used from results of experimental study of Matsumoto et al., (2004) for validation of
modeling in ABAQUS software. In this study, some shaking table experiments on piled raft foundation with
three different type of superstructure has been done, which there has been used from superstructure with
a maximum height in validation of finite element model. Used soil in the study is dry sand soil with Young’s
modulus 70 MPa and internal friction angle 45 degrees. Dynamic loading has been applied as harmonic
acceleration with frequency 5Hz and PGA=0.1 g in model bed. According to Table 3, cited in original article,
finite element model has been modeled by using laboratory conversion coefficient to prototype model.
Figure 4 represents modeling of laboratory sample in ABAQUS software.
Table 3 coefficients of conversion laboratory model to real model (Prototype model)
291
length
gravity
acceleration
tension
force
λ
1
1
λ
λ3
Ali Akbari et al - Research in Civil and Environmental Engineering 2013 1 (05) 287-299
(a)
(b)
(c)
Fig. 4 (a) meshing foundation plan, (b) meshing cross-section foundation, (c) the dimensions and configuration
of finite element model
30
30
20
20
10
0
-10
0
1
2
-20
-30
Time (S)
)a(
3
Horizontal Load (N)
Horizontal Load (N)
Figure 5(a) shows time history of total shear force in finite element model. In this figure, numeric
value of shear force has become to laboratory model by using force coefficients in Table 3. In laboratory
model, maximum shear force at foot structure has been reported almost equal to 30N and it is 28.5N in
finite element model. Figure 5(b) shows time history of shear force at piles head for finite element model.
Maximum shear force of piles is almost to 12N and it is 12.5N in finite element model. Results of modeling
finite element and laboratory test do not show significance difference.
10
0
-10
0
1
2
3
-20
-30
Time (S)
(b)
Fig. 5 Time history for: (a) total shear force, (b) piles head shear force in finite element model
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4
ANALYSIS RESULTS
Figure 6 shows time history of shear force at the base of structure for 10 storey structure. As figure
shows, Recorded maximum shear force in pile group is greater than compound raft. This reduction in
response of piled raft foundation system is due to raft surface to surface contact with soil which this
contact is more maintained and foundation sliding is less, so superstructure based on piled raft foundation
receives less response from earthquake. In pile group, shear force is transferred to soil by only piles with
less contact surface in relation with the piled raft, while in the piled raft; part of this force is transferred to
soil by raft and another part by piles. This performance difference leads to different response to these two
types foundation under earthquake loads. Maximum acceleration and shear force values for two types of
foundations and three types of structures are shown in Table 4. It can see that by increasing number of
storey, response of superstructure based on pile group to earthquake is more than the piled raft.
Shear Force at
Structure Base (kN)
1200
Case PR SFmax = 1028 kN
800
400
0
-400
0
2
4
6
8
10
12
14
10
12
14
16
-800
-1200
Time (S)
Shear Force at
Structure Base (kN)
1200
Case PG SFmax = 1170 kN
800
400
0
-400
0
2
4
6
8
16
-800
-1200
Time (S)
Fig. 6 Time history of shear force at the base of structure in 10 storey structure
Table 4 Comparison of maximum acceleration and shear force in types of foundation and structure
Foundation
type
Number of
storey
10
20
30
Pile group
max
acceleration
m/s2
5.62
3.94
3.32
Piled raft
Max shear
force
kN
1,170
3,990
6,840
max
acceleration
m/s2
5.38
3.53
2.84
max
shear force
kN
1,028
3,810
5,970
Figure 7 represents the comparison of settlement in raft center during an earthquake for two types of
foundations. According to the figure, settlement in piled raft is less than pile group, because of compound
performance of raft and piles. Settlement difference for 10 storey superstructures is not considerable,
during and after earthquake, while this difference will be significantly increased by increasing number of
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storey, and piled raft shows suitable performance for settlement, especially when number of storey is
increased.
0
2
4
6
8
10
12
14
16
Settlement (mm)
0
-1
-2
Case PR
Case PG
(a)
-3
Time (S)
0
2
4
6
8
10
12
14
16
Settlement (mm)
0
-5
-10
-15
Case PR
Case PG
-20
(b)
-25
Time (S)
0
2
4
6
8
10
12
14
16
Settlement (mm)
0
-30
-60
-90
-120
Case PR
-150
Case PG
(c)
Time (S)
Fig. 7 Comparison of settlement in raft center during earthquake: (a) 10 storey structure, (b) 20 storey structure,
(c) 30 storey structure
Table 5 shows horizontal load bearing proportion of piles in pile group and piled raft. In pile group,
there is transferred horizontal loading to soil only through piles, because of lacking cap connection to soil
(default propose in pile group analysis), so 100% of horizontal load is transferred to soil by piles; while in
piled raft, part of this force is transferred to soil by raft and another part by piles. It is observed that
regardless of horizontal load bearing proportion of raft or cap in pile group foundation made increasing in
piles vertical and lateral (shear) designing forces. This Represents use of piled raft bearing concept is
economic method in vertical and horizontal loading.
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Table 5 Percent of piles horizontal load in pile group system and piled raft
Foundation type
pile group
piled raft
100
100
100
45.6
66
69.3
Number of storey
10
20
30
Figure 8 shows horizontal load bearing proportion of P1 and P2 piles in two types of foundations and
three types of structures. As it can be seen piles have less horizontal load bearing proportion because of
anticipating raft in horizontal loading. Also it can be seen that horizontal load bearing proportion of P1 pile,
which it is located the most distance from foundation center, is more than P2 central pile, which this
distance is increased by increasing number of storey. In the figure, decreasing horizontal load bearing
proportion of each pile is resulted by increasing number of storey, because as it is seen in table 5,
increasing number of piles and piles diameter will increase group role in horizontal loading.
9.1
8
6
5.8
4
4
2
)b(
9.4
10
8
6
4
6.4
5.1
3
2
Case PR
P1
Case PG
P2
)c(
10
8
5.4
6
4
2
3.6
2
1.3
0
0
0
Proportion of lateral load
carried by each pile (%)
10
12
12
11
)a(
Proportion of lateral load
carried by each pile (%)
Proportion of lateral load
carried by each pile (%)
12
Case PR
P1
Case PG
P2
Case PR
P1
Case PG
P2
Fig. 8 Horizontal load bearing proportion of P1 and P2 piles: (a) 10 storey structure, (b) 20 storey structure, (c) 30
storey structure
Figure 9 shows changes of vertical loading for P1 and P2 piles in a 10 storey structure during
earthquake. As it can be seen, vertical loading oscillations are very low for both two foundations types, in
central pile (P2), while this oscillation is very high in corner pile (P1). Despite large vertical load oscillation
of pile P1 in the group pile, because of high initial value of vertical load on P1, this pile will not under
tension during earthquake. Therefore piles in piled raft system are more critical than piles in pile groups in
terms of the tension. These results are also similar for increasing number of storey.
295
Vertical Load on Pile
head (kN)
Ali Akbari et al - Research in Civil and Environmental Engineering 2013 1 (05) 287-299
1500
Case PR
P2
P1
1000
500
0
0
2
4
6
8
10
12
14
16
10
12
14
16
-500
Vertical Load on Pile
(kN)
Time (S)
1500
P2
Case PG
P1
1000
500
0
0
2
4
6
8
Time (S)
Fig. 9 Changes of vertical loading for P1 and P2 piles in 10 storey structure during earthquake
Figure 10 shows maximum bending moment and shear force diagram of P1 pile in maximum shearing
and bending moment. Time of maximum shear force and maximum bending moment is different, because
effect of initial forces due to superstructure weight in static step. As it can be seen, amount of bending
moment and shear force will be decreased significantly, by increasing depth in both two foundation types
and three structures types, and the force is transferred to soil through the first several meters, which it
shows domination of inertia-based transaction on kinematic transaction. Maximum bending moment and
shear force in piles head that connected to pile group system are bigger than piled raft, because of the cap
not participating in vertical and horizontal loading. The general forms of the bending moment and shear
force variations along the pile in both types of foundation are roughly similar. Variations and suddenly
distance not observed.
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Max Shear Force (kN)
0
200
-200
400
0
0
0
-2
-2
-4
-4
Depth (m)
Depth (m)
-200
Max Bending Moment (kN.m)
Case PR
Case PG
-6
200
Case PR
Case PG
-6
-8
400
-8
-10
-10
(a)
Max Shear Force (kN)
0
200
400
-400
600
0
0
0
-2
-2
-4
-4
Case PR
Case PG
-6
Depth (m)
Depth (m)
-200
-8
400
800
1200
-6
Case PR
Case PG
-8
-10
-10
-12
-12
Max Bending
Moment (kN.m)
-14
-14
(b)
Max Shear Force (kN)
0
200
400
600
800
-400
0
0
0
-2
-2
-4
-4
-6
-6
-8
Case PR
-10
Case PG
Depth (m)
Depth (m)
-200
Max Bending Moment (kN.m)
-8
-10
-12
-12
-14
-14
-16
-16
-18
400
800
1200
1600
Case PR
Case PG
-18
(c)
Fig. 10 maximum bending moment and shear force diagram of P1 pile: (a) 10 storey structure, (b) 20 storey
structure, (c) 30 storey structure
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5
CONCLUSION
Compound raft foundation is the most complete of foundation system which it provides an economic
plan to deal with structural heavy loads and lateral loads such as earthquake, by considering capacity of raft
bearing and pile group components. In recent decades, this type of foundation has been increasingly used
for transferring heavy structural loads to the soil in different types, including connected and disconnected
pile, compound pile raft, and floating piled raft. In this paper, performance of piled raft and pile group
under earthquake loading have been studied by ABAQUS finite element software, and its results are:






The imposed shear force on foundation by superstructure under earthquake loading in is less than pile
group. In other words, made structure on piled raft foundation receives less response from earthquake.
After earthquake, foundation settlement in pile raft foundation is less than the pile group, and
settlement difference in two systems will be significantly increased by increasing number of storey.
In pile group, entire shear force is transferred to the soil by piles, while in the piled raft foundation; part
of this force is transferred to soil by raft and another part by piles.
The horizontal load bearing proportion of pile is increased by distance pile from foundation center.
Oscillation of vertical loading in central piles is very low and in corner piles is very high, which it is less
in the used piles in pile group, in comparison with piled raft.
With increasing depth of pile, bending moment and shear force are significantly reduced and force
transmission from pile to soil is done in a few meters of pile.
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