Literature Review:

1. Balakumar, V & Illamparuthi, K. 2004. Laboratory study on the behaviour of Piled Raft on
Granular Soils, 15th Southeast Asian Geotechnical Society Conference, Bangkok, Thailand: 293-
298.

Year 2004
Type Experimental
Abstract In order to understand the settlement behaviour of piled raft on granular
soil, a series of small scale 1g tests were conducted in the geotechnical
engineering laboratory of Anna University. The constituent elements of the
model were determined based on the typical dimensions of tank foundations
in around this place. The pile length was so chosen that the length is not
more than the raft dimension. The tests were carried out on plain raft,
combined piled raft and free standing pile group embedded in sand beds of
three different densities. The performance of square piled raft and circular
piled raft were compared. The factors influencing the load settlement
behaviour of piled raft were identified. Further this paper focuses on the
results of the parametric study and establishes the governing factor on the
settlement reduction of the raft due to the presence of piles.
Type of soil Sand
Testing Small scale model test
Test Box size 1.0 m x 0.6m x 0.6m deep
Raft Steel Plate (6, 8,10 and 12mm thk)
Size – 200sq and 200dia
Pile Solid rods of dia 6, 8 and 10mm
Lengths 200, 160, 120 and 100mm
Soil density Sand densities – 14,5, 15.5 and 16.5 kN/cum
Condition of Plain raft, Combiled piled raft and free standing pile group
testing
Maximum test 20kN
load
Settlement Sensitive dial gauges
measured
Lab tests Load – Settlement behaviour studied for circular and square raft and for
conducted different pile diameter, pile spacing (4D and 6D) and different pile lengths
for the following conditions
a. Free standing group and piled raft group (for three densities)
b. Raft, Pile and Piled Raft (for three densities)

Parameter A. Settlement ratio (Sr)

studied
 - Pile lengths vs Sr
- Pile dia vs Sr
- Soil Density vs Sr
B. Load sharing between raft and pile
pr = Ratio between total loads taken by piles total load on the piled
raft
- pr vs Settlements (three soil densities)
- pr vs Settlements (shape of raft)
- pr vs Length of piles
- pr vs Pile diameters
Conclusions 1. The load on piles of piled raft system is higher than the free standing pile
groups irrespective of length, diameter, spacing of piles and density of sand
bed. The contribution by the piles against load beyond the settlements
corresponding to limiting friction is stable and function of intensity of load on
the piled raft.

2. The settlement of square piled raft is lesser than the circular piled raft for
a given set of condition of the piled raft.

3. A term called settlement reduction ratio (SR) is used to understand the

influence of various parameters of piled raft on settlement variation. SR
increases with increase in length of pile and pile diameter. However SR
showed decreasing trend with increasing in density of sand bed.

4. The load sharing between the piles and raft of a piled raft system is mainly
depends on the settlement. At settlements lesser than the deformation
required for limiting friction, Most of the applied load is taken by the piles.
Beyond this settlement, raft shares most part of the load irrespective of the
thickness and shape of the raft, length and the spacing of the piles.

5. The load distribution factor αpr decreased rapidly at the initial stages of
loading, and almost reached a constant value with increase in settlement.
The αpr value is higher in loose sand than denser states of sand bed.
However this effect is more pronounced in square raft than the circular raft.

6. The αpr value is higher for longer length of pile and higher diameter
irrespective of order of settlement of piled raft.

7. The thickness of the raft investigated in this study does not show
significant effect on load sharing irrespective of sand bed and various
parameters of pile.
2. Soumya Roy, Bikash Chandra Chattopadhyay,Ramendu Bikash Sahu. 2011. Piled-raft
foundation on Consolidating Soft Soil, Proceedings of Indian Geotechnical Conference, Kochi:
879-882.

Year 2011
Type Analytical - Established simpler design method for calculation of load carrying
capacity of piled raft system in a very soft consolidating underlying soil stratum.
Abstract In traditional design of piled foundations with large caps, the superstructure
loads are assumed to be carried by piles only and contact between the pile
cap and ground surface is purposefully avoided. On the other hand, a raft if
used, may provide high load carrying capacity without the possibilities of
shear failure of the supporting soil but the resulting settlement will be
beyond the permissible limit. Allowable pressure on supporting soil is
governed by loads corresponding to the tolerable settlements. As a result, on
soft soils, for heavily loaded structures, piles are generally adopted
foundations, however at a high cost. In such cases, increasing application of
piled raft foundations are being made for economic reasons. In this paper,
based on nature of the subsoil profile and field tests on pile, a simplified
design approach considering the load sharing mechanism of piles and raft is
proposed. Due considerations are given to the site conditions, raft-soil, pile-
soil and piled raft interaction.
Type of soil Clay
Basis Analyzed by utilizing the field performance of pile from routine pile load test
conducted at the construction site in Eastern part of Kolkata city in a very unique soft
clay deposit extending from 2 to 14 m (more or less) below the ground level.

Arrived The procedure is simple the piled raft capacity is equal to the arithmetic sum
of pile capacity and the raft capacity for the settlement value of pile
Conclusions The following conclusions can be drawn from the proposed simplified approach for
the piled raft foundation on soft consolidating subsoil condition.

􀁸 In order to economize the design of a piled raft system, raft must be allowed to
share some part of the superstructure load.

􀁸 To minimize the differential settlement in piled raft foundation due consideration

must be given to both initial as well as consolidation settlement of the raft.

􀁸 The present method includes the subsoil interaction with pile, raft and composite
piled raft foundation system.

􀁸 In this proposed method the pile dimensions, raft dimensions, different suitable
methods of pile group arrangement could be incorporated in tentative designs to make
a most cost effective and efficient foundation system for a prototype foundation
system.

􀁸 The proposed method can help the designer in the first design stage to check the
rationality of a piled raft foundation and to investigate both the serviceability
requirements as well as the ultimate limit states of the foundation required for a
specific project and site condition.
3. Dharmarajan, N & Illamparthi, K. 2011. Piled-raft analysis and Design Methodology,
Proceedings of Indian Geotechnical Conference, Kochi: 859-862.

Year 2011
Type Proposed to investigate the load sharing mechanism between raft and
pile through model test and numerical simulation.
Abstract In the present investigation, model tests and numerical simulation of piled raft were
carried out on medium dense sand bed to understand the influence of area ratio of
piled raft on load- settlement behavior and load sharing mechanism. The pile in 1%
and 3% area ratio piled rafts take load predominantly upto settlement of 0.004D but
for 5% area ratio piled raft it is 0.01D. From the parametric study, reduction in
settlement takes place due to increase in pile length as well as with increase in area
ratio of piled raft system. It is sufficient to reduce the differential settlement in piled
raft foundation when the ratio of diameter of raft to length of pile is four.
Type of soil Sand
Testing Small scale model test
Test Box size 0.74 m x 0.74m x 0.6m deep
Raft Steel Plate (8mm thk)
Size –200dia
Pile Single pile dia 20/35/45mm and Group piles 9/15/20mm
Length 200mm
Soil density Sand densities – 15.5N/cum
Condition of 200dia plate with single big dia pile for area ratio 1%, 3% and 5%
testing 200dia plate with five pile group for area ratio 1%, 3% and 5%
Maximum test 20kN
load
Settlement Sensitive dial gauges 2nos LC 0.01mm Travel 50mm
measured
Lab tests Load – Settlement behaviour studied for circular and square raft and for
conducted different pile diameter, pile spacing (4D and 6D) and different pile lengths
for the following conditions
c. Free standing group and piled raft group (for three densities)
d. Raft, Pile and Piled Raft (for three densities)

Parameter Settlement behaviour with single pile and pile group for same % area
studied contribution
Load sharing behaviour between raft and pile for dfferent % area
contribution
Piled raft stiffness for different area ratios and settlements
Compared the experimental results with the Analytical results (FEM-Plaxis-
2D)
Conclusions From the 1g model tests, the settlement reduction increases with increase in area ratio
of the piled raft system. The load share of pile increases with increase in area ratio of
thepiled raft foundation.

The stiffness of the piled raft system increases with increase in area ratio of the piled
raft foundation and decreases with increase in settlement.
 In view of load share, the pile in 1% and 3% area ratio piled rafts shares major part of
applied load up to settlement of 0.004D but for 5% area ratio, the pile is active in
sharing the load upto the settlement of 0.01D.

Reduction in settlement takes place due to increase in pile length as well as with
increase in area ratio. The ratio of diameter of raft to length of pile = 4 is sufficient to
reduce the differential settlement in piled raft foundation.

4. An Experimental Study on the Behaviour of Vertically Raft on Soft Clay

S.P.Bajad
Research Scholar, Civil Engineering Department, Jadavpur University, Kolkata –
R. B. Sahu
Professor Civil Engineering Department, Jadavpur University, Kolkata – 700032

The 12th International Conference of International Association for Computer Methods and Advances in
Geomechanics (IACMAG) 1-6 October, 2008 Goa, India

Year 2008
Type Proposed to investigate the load sharing mechanism between raft and
pile through model test in soft clay
Abstract In order to understand the load sharing and settlement reduction behaviour
of piled raft resting on soft clay, 1g model tests were conducted on small-
scale models. The parameters studied were the effect of pile length and
number of piles. The load settlement behaviour obtained from the tests has
been validated using simplified method (Polous & Davis). Equivalent pier
concept of Poulos and Davis (1980) was adopted to understand the load
sharing response of piled raft and settlement reduction. The analytical and
1g model test results are found to be in reasonable agreement. The
equivalent pier concept has proved to be a very useful method in
representing the behaviour of piled raft.

Type of soil Clay (LL= 57% and PL =27%)

Testing Small scale model test
Test Box size Circular Dia 0.57 m Height 0.50m
Raft Steel Plate (10mm thk)
Size –Sq 100x100mm
Pile Pile dia 10mm and Group piles 4/9/16 numbers
Length 100mm and 200mm
Soil density Soft clay
Condition of Raft/Piled Raft/Free standing piles – with different pile groups (4/9/16) and
testing pile lengths (100mm and 200mm) -
Maximum test 400N
load
Settlement Sensitive dial gauges 2nos LC 0.01mm Travel 50mm
measured
Lab tests a. Raft
 conducted b. Piled Raft – 4-100mm/9-100mm/16-100mm
c. Piled Raft – 4-200mm/9-200mm/16-200mm

d. Free standing pile group - – 4-100mm/9-100mm/16-100mm

e. Free standing pile group – 4-200mm/9-200mm/16-200mm

Parameter Studied the settlement behaviour for above conditions

studied Load sharing behaviour vs settlements - between raft and pile for dfferent
conditions established
Studied time vs settlement behaviour (48 hrs)
Conclusions Through 1g model tests on piled raft in soft clay the influence of pile parameters such
as length and number of piles on load sharing and settlement reduction are brought
out through equivalent pier concept of Poulos and Davis (1980). Among the
parameters investigated, the load sharing and control on settlement are more
influenced by the length of the pile. Simplified calculation method suggested by
Poulos & Davis (1980) helps to compute the relationship between the number of piles
and the average settlement of the foundation. Such calculations provide a rapid means
of assessing whether the design philosophies for creep piling or full pile capacity
utilization are likely to be feasible with experimental load settlement response.

5. EXPERIMENTAL STUDY OF PILED RAFT FOUNDATION

A. K. Singh, Assoc. Prof., Deptt. of Civil Engg., N. I. T. Jamshedpur – 831014, India, email-
aksnitjsr@rediffmail.com
A. N. Singh, Consultant and PTL Women’s Polytechnic Gamhariya, Jamshedpur – 831014,
Jharkhand, India.

Proceedings of Indian Geotechnical Conference

December 15-17,2011, Kochi (Paper No D-378)

Year 2011
Type Experimental – Study on piled raft performance on Sand
Abstract In situations, where a raft foundation alone does not satisfy the design requirement, it
may be possible to enhance the performance of the raft by the addition of piles. The
use of a limited number of piles, may improve both the ultimate load carrying
capacity and the settlement and differential settlement. The analysis of piled raft is a
complex problem even more than that of a soil supported raft. The present study is
focused on the experimental investigation onperformance of piled raft foundation on
sand. Wooden rafts of different sizes with wooden piles (single or double) of three
different diameters and lengths have been used for experimental study. In particular,
the experimental results have shown that number of piles below the rafts and its
locations play vital role in improving the load carrying capacity of the piled rafts and
the elastic settlement response of the soil.

Type of soil Sand

Testing Small scale model test
Test Box size Wooden Box – 860x710
Raft Different size wood raft - (217x270) mm, (105x108) mm, (166x171) mm,
 (328x121)
mm, (380x93) mm, (320x96) mm, (380x121) mm
Pile Wood Pile dia 30mm/40mm/50mm
Length 300mm/450mm/600mm
Soil density Uniformly graded Sand 1.67gm/cc – dropped from 400mm
Condition of Single pile –
testing Multiple Pile raft –
Maximum test Screw-Jack loading maximum applied load 600kg
load
Settlement Sensitive dial gauges 2nos LC 0.01mm
measured
Lab tests Single pile – Dia 30/40/50mm – Length – 300/450/600mm
conducted Multiple Pile raft – L=300/450/600mm and pile spacing with above three
diameter – 2d/3d/4d
Parameter Studied the settlement behaviour for above conditions
studied
Conclusions On the basis of the experimental findings on the behaviour of soil under piled raft
foundation, the following conclusions are drawn:
(i) When the load is taken by piles only under the raft, the settlement is quite faster
with little load on the piled raft.
(ii) The load carrying capacity increases considerably when the load is transferred to
the soil through raft and pile combined while the settlement per unit load is quite less.
(iii) In case of piled raft with single pile, the settlement per unit load is more with
decrease in the l/d ratio, (higher is the diameter compared to the length of the pile)
more effective is the piled raft foundation.
(iv) Greater is the size of the raft less is the settlement per unit load.
(v) In case of two piles supporting the piled raft, rate of settlement per unit load is
quite low, implies that the performance of the foundation improves
considerably.

In future, simple method of design of piled-raft foundations may be developed based

on graphical charts generated through laboratory and field experiments. More
exhaustive study can be made on various size of rafts, number of piles, shape and size
of piles and different types of soils.
6. BEHAVIOUR OF PILED RAFT FOUNDATION IN HOMOGENEOUS AND LAYERED SAND
Sona Vasudev. Post Graduate Student, Civil Engineering, College of Engineering Thiruvananthapuram, Kerala
Unnikrishnan, N., Lecturer in Civil Engineering, College of Engineering Thiruvananthapuram, Kerala

10th National Conference on Technological Trends (NCTT09) 6-7 Nov 2009

Year 2009
Type Experimental as well as FEM analysis
Abstract Abstract-Piled raft foundation is a foundation in which the total load coming
from the super structure is transferred to the soil by a load sharing
mechanism between raft and pile. Piled raft foundation is provided in the
circumstances where the raft alone does not satisfy the design requirements.
The addition of piles in the raft foundation improves the performance of raft.
The present study attempts to do a three dimensional finite element analysis
of piled raft foundation subjected to vertical load. Small scale laboratory
model test was conducted on model piled raft foundation for comparison of
results from the finite element analysis. The load settlement behaviour of
raft foundation obtained from the laboratory model test and finite element
analysis matches well. A parametric study was conducted to study the
influence of pile length on the behaviour of piled raft foundation in
homogeneous sand and layered sand. Considerable reduction in the
settlement as well as differential settlement of raft was noticed by increasing
the pile length in homogeneous and layered sand. For piled raft installed in
layered soil the reduction in settlement is more significant with smaller pile
length compared to homogeneous soil if the pile tip is resting on dense
sand.
Type of soil Sand
Testing Small scale model test
Test Box size Wooden Box – 860x710
Raft Different size wood raft - (217x270) mm, (105x108) mm, (166x171) mm,
(328x121)
mm, (380x93) mm, (320x96) mm, (380x121) mm
Pile Wood Pile dia 30mm/40mm/50mm
Length 300mm/450mm/600mm
Soil density Uniformly graded Sand 1.67gm/cc – dropped from 400mm
Condition of Single pile –
testing Multiple Pile raft –
Maximum test Screw-Jack loading maximum applied load 600kg
load
Settlement Sensitive dial gauges 2nos LC 0.01mm
measured
Lab tests Single pile – Dia 30/40/50mm – Length – 300/450/600mm
conducted Multiple Pile raft – L=300/450/600mm and pile spacing with above three
diameter – 2d/3d/4d
Parameter Studied the settlement behaviour for above conditions
studied
Conclusions On the basis of the experimental findings on the behaviour of soil under piled raft
foundation, the following conclusions are drawn:
(i) When the load is taken by piles only under the raft, the settlement is quite faster
with little load on the piled raft.
(ii) The load carrying capacity increases considerably when the load is transferred to
the soil through raft and pile combined while the settlement per unit load is quite less.
(iii) In case of piled raft with single pile, the settlement per unit load is more with
decrease in the l/d ratio, (higher is the diameter compared to the length of the pile)
more effective is the piled raft foundation.
(iv) Greater is the size of the raft less is the settlement per unit load.
(v) In case of two piles supporting the piled raft, rate of settlement per unit load is
quite low, implies that the performance of the foundation improves
considerably.

In future, simple method of design of piled-raft foundations may be developed based

on graphical charts generated through laboratory and field experiments. More
exhaustive study can be made on various size of rafts, number of piles, shape and size
of piles and different types of soils.