Available online at www.sciencedirect.com
Procedia Engineering 00 (2012) 000–000
Non-Circuit Branches of the 3rd Nirma University International Conference on Engineering
(NUiCONE 2012)
Analysis of Piled Raft Foundation by Finite Element Method
Dr.G.Srilakshmia, Chethan Gowda R Kb
a
Associate professor Department of civil engineering, R.V.C.E, Bangalore, 560059, India
b
M.tech Student, Department of civil engineering R.V.C.E, Bangalore, 560059, India
Abstract
The piled raft is a geotechnical composite structure, consisting of the three elements piles, raft and soil, which is very much
adapted for the tall buildings in an increasing number. Ideally speaking, a three-dimensional analysis is the best for rafts and
piled raft foundations, but two-dimensional plane strain analysis can yield good results for piled raft analysis without
excessive computing and modelling time. In this parametric study, different piled raft configurations have been analyzed by
two-dimensional plane-strain finite element analyses using ANSYS. In the study, raft dimensions, length and diameter of
pile have been varied to find ultimate capacity of foundation under compression in sand. From the study it is observed that,
settlement of foundation is decreasing with increase in diameter of piles where as ultimate capacity is increasing. Piled raft
foundations having longer piles takes more load at higher values of settlements. Here piled raft is treated as linear materiel
and soil as non linear, where Drucker-Prager constitutive model is used for soil. The non linear incremental iterative method
of analysis is used which yields good accuracy.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Institute of Technology Nirma
University, Ahmedabad.
Keywords:Foundation; Piled raft; sand;FEM; ANSYS
Nomenclature
H
Horizontal extension of soil [three times the width of the raft] (m)
V
Vertical extension of soil [two times of pile length] (m)
E
Modulus of elasticity (kPa)
C
Cohesion (kPa)
Greek symbols
µ
Poisson’s ratio
ρ
Density (Kg/m³)
ψ
Flow angle
1. Introduction
Foundation is the interface between the superstructure and the ground. Its task is to transfer the building loads safely into the
ground and to keep settlement within the permissible limit. It supports superstructure weight, resists horizontal forces from
the adjacent ground and forces imposed on foundation due to wind and earthquake. Piled raft foundation is one of the most
effective types of foundation and it has the both advantages of shallow and deep foundation. Piled raft foundation is a
composite structure consists of pile and raft. It carries the load to the soil with out differential settlement in the soil. Piled
raft is used where the base soil has low bearing capacity and where the bearing capacity of raft is satisfactory but settlement
is not within permissible limits, a combination of raft and piles will provide the advantages of both raft and pile. There is no
Dr.G.Srilakshmi, Chethan gowda R K./ Structural Engineering 00 (2012) 000–000
proper procedure for the design of piled raft foundation, while designing load is assumed to be carried either by the raft or
by the piles. There are two approaches for the design of piled raft foundation, in first approach piles are called upon only to
take a small percentage of loads and rest is designed to be carried by raft and here piles are designed as settlement reducers.
In second approach, in case of high raised buildings or when subsoil conditions such as thick clay deposits even with a high
water table and the clay shear strength is very low, long load-bearing piles are introduced to transfer the entire load to
deeper and stiffer soil layers and here majority of load is carried by piles.
In this work, variation of load carrying capacity of piled raft foundation in sand with size and thickness of raft, length and
diameter of pile is considered. Finite element software ANSYS is used to understand the behavior of piled raft foundation
under different parameters. Also as a part of this paper the effectiveness of the ANSYS is validated with static formula of
bearing capacity of raft alone and also with existing analytical data on piled raft foundation from available literature by
means of two-dimensional plane-strain finite element analyses.
H.chow et al (2011) have studied piled raft foundation on soft soil and they come to conclusion that piled raft on soft ground
is an economical foundation system. Use of friction piles in piled raft enhances the bearing capacity and controls settlement.
Piled raft is more economical compared to conventional piled foundation. They also made study on piled raft foundation
having piles of varying lengths. They conclude that longest piles in the middle and progressively shorter piles towards the
edge is the economical way of providing piles in case of piled raft having piles of varying pile length.
Balasubramanium et al (2008) have carried out analysis on unpiled and piled raft foundation in sandy soil. Effect of
parameters such as raft thickness and raft sizes was studied using finite element software PLAXIS. They concluded that raft
thickness affects the differential settlements and bending moments of foundation.
2. Modeling and Methodology
2.1 Finite element modeling
ANSYS is finite element software which is designed to analyze engineering problems with the help of advanced computer
facilities. The material of Piled raft is considered as a linear, soil as an elasto plastic constitutive Drucker-Prager model and
the interference is considered as non linear.
In this analysis the horizontal and vertical extensions of soil are considered from trial and error. It has been observed that the
value of V=2L and H=3B, where B is the width of the raft and L is the length of the pile is giving reliable results. These
values are used in this study.
2.2 Methodology
Two dimensional plane strain non linear analysis (ANSYS) has been carried out to find the ultimate capacity and settlement
of foundation.
Steps involved:
Pre processing
1) Define element type, Plane-82 for pile, raft, soil and the element behaviour is specified as plane strain. Target -169 and
contact-172 elements are used for interface between pile-soil and raft-soil.
2) Assign material properties like linear and nonlinear.
3) Create solid model of horizontal length (H) about three times of raft size and vertical length (V) about two times of pile
length as shown in fig.1, by joining key points through lines and create areas for entire dimension of model.
4) Mesh the entire model by assigning material properties.
5) Contact elements have been modelled at the interface of piled raft and soil to induce the surface friction at the interface.
6) Apply boundary conditions to incorporate field conditions, i.e. Vertical lines are restricted against horizontal movement
and bottom lines are restricted in both horizontal and vertical direction.
.
Dr.G.Srilakshmi, Chethan gowda R K./ Structural Engineering 00 (2012) 000–000
.
Fig.1. Plane strain consideration of geometrical modelling of area of piled raft foundation
Solution and post processing
1) A non linear plane strain analysis is carried out using incremental iteration method.
2) Ultimate load carrying capacity of foundation and its corresponding settlements were observed from the analysis and
presented as shown in fig.2.
Fig.2. Displacement contours in Y-direction at the ultimate load
3. Parametric Studies
ANSYS software has given reliable results when compared with static formula. Then the authors have proceeded with
parametric studies after completing the validation. The properties of materials are considered as mentioned in Table 1. Here,
the length and diameter of pile, number of pile and size of raft are varied to measure the ultimate load bearing capacity and
settlement of piled raft foundation in sand. Number of piles used corresponding to size of raft of piled raft is given in Table
2.
Dr.G.Srilakshmi, Chethan gowda R K./ Structural Engineering 00 (2012) 000–000
Table.1. Material properties [1, 8, 9]
Pile, raft
Medium Sand
Material properties
Modulus of elasticity ,E
2.5x107 kPa
4x104 kPa
Poisson’s ratio,µ
0.15
0.3
Density ,ρ
2500 Kg/m³
1900Kg/m³
Cohesion ,c
-
10 kPa
Friction angle ,Φ
-
35°
Flow angle ,ψ
-
11°
Table.2. Raft size and number of piles
Raft size(m)
6x6
8x8
10x10
Number of piles
9
16
25
Raft thickness(m)
1.0
1.0
1.0
3.1 Results and discussion
Size of raft, length of pile and diameter of pile of piled raft is varied to measure the ultimate load carrying capacity of piled
raft foundation and is presented in table 3.
Table.3.values of ultimate load carrying capacity
Raft size (m)
6x6
8X8
Pile length(m)
Pile diameter (m)
0.3
3
0.4
0.5
0.3
4
0.4
0.5
0.3
5
0.4
0.5
10x10
Ultimate load(MN)
49.7
77.4
102.8
50.8
78.4
104
51.7
79.1
105.8
50.9
78.7
103.5
51.8
79.8
104.8
52.2
81.1
107
51.4
80.2
105.2
51.9
81.1
106.5
52.6
81.4
109.2
Dr.G.Srilakshmi, Chethan gowda R K./ Structural Engineering 00 (2012) 000–000
From the table 3 it is observed that for all sizes of raft of foundation, there is an increase in ultimate capacity of foundation
with the increase in the diameter of pile, this is because of increase in contact area of pile, which leads to increase in surface
friction and end bearing of the pile. It is also observed that there is 35% increase in ultimate load carrying capacity of
foundation with increase in raft size from 6m to 8m and 25% increase in ultimate load carrying capacity with increase in raft
size from 8m to 10m.
Table.4. Values of settlement
Raft size (m)
Pile length(m)
6x6
Pile diameter (m)
0.3
3
0.4
0.5
0.3
4
0.4
0.5
0.3
5
0.4
0.5
8X8
10x10
Settlement (mm)
97.135
88.81
77.75
93.95
84.53
72.97
91.12
82.22
72.03
120.85
114.95
98.57
116.06
106.45
92.41
111.39
102.95
91.18
139.46
133.87
118.8
132.91
125.67
110.09
128.35
120.47
100.5
It has been observed form the table 4, the settlement of foundation is decreasing linearly with the increase in pile diameter,
i.e., resistance of piles against settlement increases with the increase in diameter, which results in decrease of settlement of
foundation. Increase in length of pile for same raft size leads to increase in settlement because piled raft having longer piles
carries more load at maximum settlement. It is due to the piles of piled raft exhibits more surface friction at maximum
settlement values.
In case of piled raft having longer piles, piles experience less skin friction at less settlement values especially in the upper
third portion of pile shaft, this is due to prevention of movement between pile shaft and soil close to raft.
Following figure is obtained from ANSYS which gives displacement contours of piled raft of raft size 8mx8m, 4m pile
length and 0.4m pile diameter
Fig.3. Displacement contours in Y-direction at ultimate load
Dr.G.Srilakshmi, Chethan gowda R K./ Structural Engineering 00 (2012) 000–000
4. CONCLUSIONS
Behaviour of piled raft was analyzed for different sizes of raft, pile lengths, pile diameter by finite element method, by
assuming foundation system as a 2-D plane strain system using ANSYS. Effectiveness of the ANSYS and methodology
adapted for the parametric study is validated with static formula of unpiled raft, it has been observed that there is
compatibility between results. Ultimate load carrying capacity of foundation increases by 35% with increase in raft size
from 6m to 8m and 25% increase for with increase in raft size from 8m to 10m. Ultimate load carrying capacity of
foundation increases by 2.1% with increase in the length of piles. Piled raft foundations having longer piles takes more load
at higher value of settlements.
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