International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
NUMERICAL ANALYSIS OF PILE RAFT WITH CUSHION
AND WITHOUT CUSHION
Sreechithra P.1, Niranjana K.2,
1
PG student, Department of Civil Engineering,
Thejus Engineering College,
Thrissur, Kerala
2
Assistant Professor, Department of Civil Engineering,
Thejus Engineering College,
Thrissur, Kerala
Abstract: Due to invent of high rise building there was need to study and improve deep foundation system to
reduce cost of foundation. Piled raft foundation proved to be economical in case of high rise buildings and can
provide safe bearing capacity and serviceability requirements. A combination between the use of piles and raft
foundation is known as piled raft foundation. The use of strategically located piles improves the load capacity of
raft and reduces the differential settlement. In pile raft foundation, piles act as settlement reducers. Pile is
disconnected from the raft and to treat these piles as reinforcement to subsoil rather than as structural member.
Favorable conditions for unconnected pile raft are similar to that of connected pile raft. But unconnected pile
raft foundation results less effective than pile raft foundation system. For to improve the efficiency provide
geotextile casing around the cushion. This study reveals the performance of piled raft foundation in sandy soil
through numerical analysis using Plaxis 3D.
Keywords: Pile raft system; Failure load; Settlement; Plaxis 3D.
1. Introduction
Foundation is a structural part of a building on which a building stands. Foundation transmits and
distributes its own load and imposed loads to the soil in such a way that the load bearing capacity of the
foundation bed is not exceeded. Mainly, Three well known foundation option to transfer heavy structural
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loads (1) raft foundations, where loads are transferred to the ground via a foundation raft, (2) pile
foundations, where loads are transferred to deeper bearing layers, and (3) piled raft foundations, at which
the loads are partially transferred by piles and partially by raft. In traditional foundation design, consider first the
use of shallow foundation such as a raft. If it is not adequate, deep foundation such as a fully piled foundation
is used instead. In the former, it is assumed that load of superstructure is transmitted to the underlying ground
directly by the raft. In the latter, the entire design loads are assumed to be carried by the piles.
In recent decades, another alternative intermediate between shallow and deep foundation, what is called piled
raft foundation or settlement reducing piles foundation, has been recognized by civil engineers. The concept of
piled raft foundation was firstly proposed by Davis and Poulos. Soil strata consisting of relatively stiff clay or
medium, dense sand is the favorable condition for pile raft foundation. Conversely, the unfavorable situations
for piled raft include soil profiles containing soft clays near the surface, soft compressible layers at relatively
shallow depths and some others. In the unfavorable cases, the raft might not be able to provide
significant loading capacity, or long-term settlement of the compressible underlying layers might reduce the
contribution of raft to the long-term stiffness of foundation. Many theories concerning the analysis of piled raft
foundation have been proposed by various researchers.
Figure 1 : Pile raft
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International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
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. But it is well known that the finite element
method is very versatile for studying complex problems. In order to clarify behavior of the new type of
foundation, especially the influences of cushion, resort to three-dimensional finite element method. Pile is
disconnected from the raft and to treat these piles as reinforcement to subsoil rather than as structural member.
Unconnected pile raft foundation is also a through researched method and it was found generally to be
economical than connected pile raft foundation. Cushion, which is composed of sand-gravel mixture, gravel
itself compacted in the layers between raft and top of piles, place an important role in mobilizing the bearing
capacity of the subsoil and modify the load transfer mechanism of piles. Favorable conditions for unconnected
pile raft are similar to that of connected pile raft. But unconnected pile raft foundation was found relatively less
effective foundation system. Thus we provide geotextile casing around the cushion to increase the effiency. In
an unconnected pile raft foundation system, the location of maximum axial load is shifted downwards to a
certain length below the pile head and their important role in mobilizing the bearing capacity of the subsoil and
modifying the load transfer mechanism of piles. Cushion is placed between raft and pile head. This study reveals
the performance of piled raft foundation in sandy soil with and without cushion carried through experimental
and numerical analysis using plaxis 3D.This study reveals the performance of piled raft foundation in sandy
soil with and without cushion carried through experimental and numerical analysis using PLAXIS 3D. PLAXIS
3D is a finite element package intended for three-dimensional analysis of deformation and stability in
geotechnical engineering. It is equipped with features to deal with various aspects of complex geotechnical
structures and construction processes using robust and theoretically sound computational procedures
2. Previous Study
Numerous researchers were conducted study on pile raft. From the literature survey important points
are follows. From the literature survey important points are follows. Piled raft foundation is a combination of
the piles and the raft, which has every qualification of good integrity, stiffness and high capacity. Soil
conditions, load distribution, quantity and location of piles were important factors in raft foundation design.
Use of long piles underneath the heavily loaded area can help to minimize the risk of tilting as well as to reduce
the overall and differential settlements. Pile diameter, length and raft thickness increases results decrease
in total and differential settlement up to a optimum point. Piled raft is very beneficial to be used as settlement
reducer. For an economic design, it is necessary to consider the optimum number of piles in piled raft
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foundation system based on the allowable settlements.Recently, the significant computing and memory
resources available to the geotechnical engineer, combined with low cost, have made the Finite element
Method (FEM) a powerful, viable alternative. The modeling of piles in a 2D finite element as a plate has
limitation to model the pile-soil interaction, which is strongly 3D phenomenon.
3. Materials and Methodology
The Index and Engineering properties of sand used for the study was determined as per IS
specifications. The sand was collected from Pavaratty, Thrissur. The Tests were conducted on a model tank of
mild steel with dimension 0.75 m x 0.75 m x 0.65m. The raft was modeled by a square Perspex plate of side
dimensions 0.150m and 0.006m in thickness. Circular piles of 10 mm diameter at three different lengths of
120mm, 150mm and 187.5mm were inserted in the holes. The arrangement of pile-raft system is shown in figure
2.
Figure 2 : Model of pile raft
Cushion, which is composed of sand-gravel mixture, gravel itself compacted in the layers between raft
and top of piles. The arrangement of pile-raft system is shown in figure 3.
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International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
Figure 3 : Pile raft with cushion
Table 1: Properties of sand
PROPERTIES VALUE
Specific gravity 2.66
Particle size distribution
Percentage of gravel (%) 0
Percentage of sand (%) 96.5
Percentage of fines (%) 3.5
Angle of internal 36
friction(degree)
Relative density(%) 40
Minimum dry IJRERD
14.84
density(kN/m3)
Maximum dry 17.66
density(kN/m3)
4. Finite Element Anlaysis
PLAXIS 3D was used in the study to model the pile raft system. To simulate the behavior of
soil, different constitutive models are available in the software. In the present study the soil behavior is
simulated by Mohr- Coulomb model. Medium meshing is adopted in all the simulations.The input parameters
of soil used in the test and the input parameter for model pile raft footing are shown in TABLE 2.
Table 2: Input parameters of soil
PROPERTIES VALUE
SAND
2
30000
Modulus of elasticity ,E( kN/m )
Poisson’s ratio(µ) 0.3
Cohesion, C (kN/m²) 1
Angle of internal friction,ϕ (degree) 36
Dilatancy angle,ψ (degree) 6
FOOTING
2
Modulus of elasticity ,E (kN/m ) 2.5x106
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International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
Poisson’s ratio(µ) 0.38
Pile length (m) 0.120,
0.150,
0.1875
Raft Thickness .006
Table 3: Input parameters of geotextile
PROPERTIES VALUES
Axial stiffness, EA (kN/m) 1300
The piles are arranged in 3X3 grid pattern with spacing 5 times the diameter of the pile.
The various combination are named as combination 1,2,3,4,5,6 and 7 respectively.
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Figure 4: Different combination of pile raft
5. Results Of Numerical Analysis
To study the behavior of the pile group with varying pile length seven combinations of pile
configuration have been studied.
Figure 5: Load settlement graph of pile raft without cushion - combination 1,2,3
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International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
Figure 6: Load settlement graph of pile raft without cushion -combination 4,5
Figure 7: Load settlement graph of pile raft without cushion -combination 6,7
Comparing combination 1,2and 3, it is found that length increases the capacity of pile raft also
increases. From the results, combination 6 showed better results. As a part of cost of construction
combination 3 not become a suitable one. Also when compared to combination 5 and 6, number of shorter
pile is more in combination 6 thus reduces the cost of construction because there is no greater difference in
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failure loads.
Figure 8: Load settlement graph of pile raft with cushion - combination 1
Figure 9: Load settlement graph of pile raft with cushion – combination 2
5|Page www.ijrerd.com
International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
Figure 10: Load settlement graph of pile raft with cushion - combination 3
Figure 11: Load settlement graph of pile raft with cushion - combination 4
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Figure 12: Load settlement graph of pile raft with cushion - combination 5
Figure 13: Load settlement graph of pile raft with cushion - combination 6
Figure 14: Load settlement graph of pile raft with cushion - combination 7
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International Journal of Recent Engineering Research and Development (IJRERD)
Volume No. 02 – Issue No. 05, ISSN: 2455-8761
www.ijrerd.com, PP. 01-07
From the lab results, pile raft with cushion shows tremendous decrease in settlement. Important role in
mobilizing the bearing capacity of the subsoil and mobilizing the load transfer mechanism of piles. Long piles
underneath the heavily loaded area helps to minimize the risk of tilting as well as to reduce the overall and
differential settlements. Combination 6 shows better result
6. Conclusion
From the study, it was observed that as the length of the pile increases failure load also increases.
And load sharing behavior is better for pile raft when compared to plain raft. Connecting pile with different
arrangement shows better results. Pile raft with encased geotextile cushion shows better results when compared
to pile raft without cushion. Thus we can reduce the cost of construction by limiting number of long piles.
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