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Proceedings ArticleDOI

Parametric Study on the Interaction Factor of Piled Raft

01 Jan 2013-pp 829-834
TL;DR: In this article, a qualitative and a quantitative assessment of the interaction behavior through a non-dimensional interaction factor, through the results of a series of 1g model studies, is presented, and the effect of various important parameters like pile length, pile raft area ratio and spacing on the interaction factor are discussed.
Abstract: The performance of the piled raft foundation system depends upon the interaction level between the raft-soil-and the pile group. Understanding of this process is more complicates than the conventional pile group as the raft in contact with the ground influences the confining pressure, making the interaction behaviour more complicated. This paper presents a qualitative and a quantitative assessment of the interaction behaviour through a non dimensional interaction factor, through the results of a series of 1g model studies. The effect of various important parameters like pile length, pile raft area ratio and spacing on the interaction factor are discussed.

Summary (2 min read)

INTRODUCTION

  • The design of foundation system to support structures that are either heavily loaded or sensitive for settlements was always tradition bound.
  • The presence of the raft and its contribution in sharing the load is ignored and the design does not weigh the transfer of the load directly below the raft or the pile cap as the case may be.
  • However it is interesting to note that every structure has got certain amount of permissible settlement and this fact has come to recognition recently.
  • The piled raft foundation system transfers the load by a three dimensional interaction among the constituent elements, namely the raft, pile and the soil.

THE STUDY

  • The study was carried out by conducting a series of 1g model tests on circular, square and rectangular shaped piled raft.
  • The tests included tests on un-piled raft, free standing pile group and the piled raft.
  • The tests were carried out in steel tank by preparing the bed adopting sand raining process in pre-calibrated manner so that the required densities could be achieved.
  • The 8mm thick raft diameter was kept as 200mm; the piles were 10mm dia 160mm long and the pile –raft area ratio was kept as 5.2% and the 21 piles were placed in radial grid pattern.
  • The bed material was poorly graded palar sand.

RESULTS ANALYSES AND DISCUSSIONS

  • Figure 1 presents the load settlement response of free standing pile group and pile group of piled raft.
  • In the case of free standing pile group the raft was not in contact with the soil and was kept 40mm above the bed level.
  • Load was applied in small increments till the settlement reached 20mm.
  • In the case of pile group of piled raft till the settlement level reached 2mm, the response was very stiff and the load at this level was 1.8kN which is nearly 50% more than the load taken by the free standing pile group.
  • The characterized load settlement response presented in Figure 2 clearly shows that the freestanding pile group has a two phase response with the initial stage showing a stiff response till the friction is overcome.

QUALITATIVE ASSESSMENT

  • The interaction between the raft pile and the soil can be explained based on the interesting hypothesis proposed by Chin [6].
  • It may be visualized that when the soil mass is under a compressive stress the load is transmitted by internal columnar grain structures and that as they reach the limiting loads more and more columns begin to support the load each having approximately same yield load.
  • It is seen that as the pressure applied from the raft increases, the assembly of columnar structure become denser creating higher confining pressure.
  • Once the applied stress increases beyond the limiting value , the grain structure tends to collapse or does not resist the applied stress but pile soil system stiffness remains at the enhanced value, helping the raft to take higher load.
  • The loss of stiffness is gradual till the settlement reaches 6mm and at the final settlement of 20mm the.

QUANTITATIVE ASSESSMENT

  • Quantification has to be done through a suitable parameter in non-dimensional form mainly because the interaction is a measure of varying relative stiffness.
  • Randolph [7,8] in his studies had expressed the interaction between the pile and the cap considering the average displacements beneath the cap due to the loading on the pile, and down the center line of the pile due to the cap.
  • Clancy [9] had later established the raft pile interaction factor pr and based on this, more rigorous analyses were conducted to show that the value is constant as the number of piles increases.
  • All measured at the same settlement level.

Effect of pile length

  • Figure 3a presents the bilinear variation of the interaction factor with the pile length at different settlement level.
  • As the settlement level increases the interaction factor reduces and at 20mm the value is the least.
  • When the pile raft area ratio is very high (9.25% with 37 piles) the interaction factor varies from 0.8 to 1.2 indicating that the piled raft has a tendency to behave as a fully piled system.
  • The interaction factor varies from 0.75 to 1.2 ; at 2mm settlement the pile group has very high stiffness and tends to function as fully piled.
  • However under certain optimum conditions the variation is similar to what has been obtained by Randolph [7].

CONCLUSIONS

  • The interaction behaviour of piled raft was studied through a series of 1g model tests and the nondimensional interaction factor which is a function of the pile group, raft and the piled raft stiffness was evaluated under various parametric conditions.
  • It was established that that the columnar grains contribute to the enhancement of the confining stress leading to the higher pile group stiffness and to the increased capacity of the whole system.
  • The interaction factor was computed from the model tests and were compared with the values predicted by Randolph.
  • The variation was found to be somewhat wider but perhaps within an acceptable level as the method of study was different.
  • Which is in close agreement with predicted value with numerical analyses by Randolph.

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Parametric Study on the Interaction Factor of Piled Raft
Author
Balakumar, V., Huang, Min, Gratchev, Ivan, Qin, Hongyu, Bolton, Mark, Balasubramaniam,
Bala
Published
2013
Conference Title
Proceedings of 18th Southeast Asian Geotechnical cumInaugural AGSSEA Conference:
Geotechnical Infrastructure
DOI
https://doi.org/10.3850/978-981-07-4948-4_259
Copyright Statement
© 2013 Research Publishing Services. The attached file is reproduced here in accordance with
the copyright policy of the publisher. Please refer to the conference's website for access to the
definitive, published version.
Downloaded from
http://hdl.handle.net/10072/56649
Link to published version
Http://rpsonline.com.sg/proceedings/9789810749491/index.html
Griffith Research Online
https://research-repository.griffith.edu.au

18
th
Southeast Asian Geotechnical & Inaugural AGSSEA Conference
29 - 31 May 2013, Singapore
Leung, Goh & Shen (eds)
PARAMETRIC STUDY ON THE INTERACTION FACTOR OF PILED
RAFT
V.BALAKUMAR
Senior consultant, Simplex Infrastructures Limited, Chennai, Tamil Nadu, India,
vb_kumar2002@yahoo.com
MIN HUANG
Arup Geotechnics, Brisbane, Australia.
IVAN GARTCHEV, HANG YU QIN, MARK BOLTON
Lecturer of Civil Engineering GriffithUniversity Gold Coast Campus, Brisbane,
Australia
A.S.BALASUBRAMANIAM
Professor of Civil Engineering GriffithUniversity Gold Coast Campus, Brisbane,
Australia, bala.balasubramaniam@griffith.edu.au
ABSTRACT
The performance of the piled raft foundation system depends upon the interaction level between
the raft-soil-and the pile group. Understanding of this process is more complicates than the
conventional pile group as the raft in contact with the ground influences the confining pressure,
making the interaction behaviour more complicated. This paper presents a qualitative and a
quantitative assessment of the interaction behaviour through a non dimensional interaction factor,
through the results of a series of 1g model studies. The effect of various important parameters like
pile length, pile raft area ratio and spacing on the interaction factor are discussed.
INTRODUCTION
The design of foundation system to support structures that are either heavily loaded or sensitive for
settlements was always tradition bound. Till recently deep piles had been the choice of the
designers to support structures sensitive for settlement even when the bearing capacity of the
ground was not a problem. Although the raft or the large pile caps were placed on favorable
ground, the presence of the raft and its contribution in sharing the load is ignored and the design
does not weigh the transfer of the load directly below the raft or the pile cap as the case may be.
Many of the codes also recommend ignoring the contribution of the raft in the load transfer.
Although this approach provides a safe design, the economics of the foundation design becomes
questionable. However it is interesting to note that every structure has got certain amount of
permissible settlement and this fact has come to recognition recently. This recognition has led to
the advent of the combined piled raft foundation system as a viable alternative to deep piles when
the ground conditions are favorable from bearing capacity point of view. The piled raft foundation
system transfers the load by a three dimensional interaction among the constituent elements,
namely the raft, pile and the soil. The interesting concept in the piled raft is that the pile group is
intended to control the settlements; in the initial stages of serviceability, the pile group provides
most of the stiffness, and the raft elements provide the additional capacity at ultimate loading.

Extensive research has been done so far on the behaviour of piled raft and various aspects of
behaviour relating to the load settlement and the load sharing response have been studied in detail
by various researchers like Reul [1],Poulos[2] and so on. Based on such works the piled raft
foundation has been used to support structures not only on favorable ground conditions as shown
by Poulos[3] but also on unfavorable ground conditions with ground improvement techniques as
reported by Yamashita etal.,[4].However it appears that the phenomenon of the interaction
between the constituent elements have not been studied in detail. As pointed out by Lee and Chung
[5], the complex interaction can become favorable like increase in the group capacity or
unfavorable like causing additional settlement. The study on the interaction behaviour gains
importance as in the case of piled raft the interaction takes place between the pile raft and the
soil.
THE STUDY
The study was carried out by conducting a series of 1g model tests on circular, square and
rectangular shaped piled raft. The tests included tests on un-piled raft, free standing pile group and
the piled raft. The tests were carried out in steel tank by preparing the bed adopting sand raining
process in pre-calibrated manner so that the required densities could be achieved. Although
extensive parametric studies were carried out this presentation is restricted to circular piled raft on
medium dense sand. The 8mm thick raft diameter was kept as 200mm; the piles were 10mm dia
160mm long and the pile raft area ratio was kept as 5.2% and the 21 piles were placed in radial
grid pattern. Loading was applied in small increments till the settlement reached 20mm namely 10
% of the raft diameter. The bed material was poorly graded palar sand.
RESULTS ANALYSES AND DISCUSSIONS
Figure 1 presents the load settlement response of free standing pile group and pile group of piled
raft. In the case of free standing pile group the raft was not in contact with the soil and was kept
40mm above the bed level. Load was applied in small increments till the settlement reached
20mm.
The response of pile group of piled raft was obtained from the load settlement response of un-piled
raft and pile raft; the load taken by the un-piled raft was deducted from the load taken by the piled
raft at each settlement level to get the load taken by the pile group of piled raft. It is seen from the
8MM RAFT - 10MM PILE - 200MM LONG
MEDIUM DENSE
0
2
4
6
8
10
12
14
16
18
20
22
0 1 2 3 4
LOAD, kN
SETTLEMENT, MM
FREE STANDING GROUP
PILED RAFT GROUP
d = 8mm
L = 200mm
N = 21
R.A. = 3
D = 200mm
Bed = Mediumdense
d = 10mm
t = 8mm
Figure 1. Comparison of load-settlement response of free standing pile group and pile
group of piled raft

figure that in the initial stages till the settlement reached 1.2mm, the free standing pile group was
exhibiting a very high stiffness. The load corresponding to this settlement level was 1.2kN which
is almost 75% of the load taken by the free standing pile group corresponding to 20mm settlement.
Beyond this level, even for a small increment in the load, the pile group of piled raft settled
rapidly, which means that once the friction is overcome the pile group of piled raft failed
instantaneously. The free standing pile group took only 25% of load further. In the case of pile
group of piled raft till the settlement level reached 2mm, the response was very stiff and the load at
this level was 1.8kN which is nearly 50% more than the load taken by the free standing pile group.
Thereafter as the as the load increase, the stiffness of the pile group of piled raft was falling down
rapidly although it was taking further load.
The characterized load settlement response presented in Figure 2 clearly shows that the
freestanding pile group has a two phase response with the initial stage showing a stiff response till
the friction is overcome. Once the friction is overcome the pile group loses its stiffness
instantaneously. In the case of pile group of piled raft although the response is similar to that of
freestanding pile group, till major part of the friction is overcome. Even though the pile group of
piled raft started losing its stiffness rapidly it continued to take further load which is similar to
elastic work hardening behaviour.
QUALITATIVE ASSESSMENT
The interaction between the raft pile and the soil can be explained based on the interesting
hypothesis proposed by Chin [6].The mobilization of stress in a soil with increase in the strain is a
function of an increasing number of effective soil contacts rather than of a general increase in the
intergranular stress on a constant number of grain contacts. It may be visualized that when the soil
mass is under a compressive stress the load is transmitted by internal columnar grain structures and
that as they reach the limiting loads more and more columns begin to support the load each having
approximately same yield load. It is seen that as the pressure applied from the raft increases, the
assembly of columnar structure become denser creating higher confining pressure. Once the
applied stress increases beyond the limiting value , the grain structure tends to collapse or does not
resist the applied stress but pile soil system stiffness remains at the enhanced value, helping the
raft to take higher load. It is seen that in the initial stages the stiffness is very high. The loss of
stiffness is gradual till the settlement reaches 6mm and at the final settlement of 20mm the
Figure 2. Characteristic response of freestanding pile group and pile group of piled
raft
0.1
1
10
0 2 4 6 8 10 12 14 16 18 20 22 24
SETTLEMENT, mm
LOAD, kN
FREE STANDING
PILED RAFT
d=8mm
L=200mm
N=21
R.A. = 36deg
D=200mm
Bed Medium dense
d = 10mm
t = 8mm
L = 200mm
N = 21
R.A. = 36º
D = 200mm
Bed = Medium
dense

stiffness is very low but the system takes a higher load due the stiffness gained by the grain
columns.
QUANTITATIVE ASSESSMENT
Although the interaction behaviour has been qualitatively established, quantification has to be
done through a suitable parameter in non-dimensional form mainly because the interaction is a
measure of varying relative stiffness. Randolph [7,8] in his studies had expressed the interaction
between the pile and the cap considering the average displacements beneath the cap due to the
loading on the pile, and down the center line of the pile due to the cap. Clancy [9] had later
established the raft pile interaction factor pr and based on this, more rigorous analyses were
conducted to show that the value is constant as the number of piles increases. The value suggested
was around 0.8.The interaction factor is expressed as
(1)
Where, Kpr is the stiffness of the piled raft, Kp is the stiffness of the pile group, and Kr is the
stiffness of the raft. All measured at the same settlement level. pr is the interaction factor.
Based on the parametric studies conducted through the 1g model tests the stiffness of the
individual elements namely the raft (kr) and the pile group as pier (kp) and the piled raft (kpr)
were computed at different settlement levels using Excel programme and the value of pr was
computed for various pile lengths, pile-raft area ratio in the case of circular piled raft and pile
spacing in the case of circular piled raft. It is to be noted here that the number of piles were varied
keeping the raft dimension constant.
Effect of pile length
Figure 3a presents the bilinear variation of the interaction factor with the pile length at different
settlement level. At a settlement level of 2mm the interaction factor is unity mainly because the
pile group shares the maximum load due to the high intergranular stress created. As the settlement
level increases the interaction factor reduces and at 20mm the value is the least.
K
pr
=
K
p
+
(1 - 2
pr
) K
r
1 -
pr
2
(K
r
/ K
p
)
Figure 3. Effect of pile length and pile raft area ratio on
pr
a
0.00
0.40
0.80
1.20
1.60
0 4 8 12 16 20 24
α
pr
Settlement
Variation in Radial Angle / Pile Raft area ratio
9.25 5.25 4.25
b

Citations
More filters
Journal Article
TL;DR: In this article, a piled-raft foundation with disconnection cap and a sand cushion between the pile and raft was investigated to compare the influence of ultimate bearing capacity and settlement, and load-settlement relation curves were used to evaluate the ultimate bearing capacities.
Abstract: The piled-raft foundation, a new design concept, is one of the most effective kinds of foundation for reducing settlement of structures. An alternative piled-raft system with disconnection cap and a sand cushion between the pile and raft was also investigated to compare the influence of ultimate bearing capacity and settlement. Load-settlement relation curves were used to evaluate the ultimate bearing capacity. In the numerical analyses, a plane strain elasto-plastic finite element model (Mohr-Coulomb model) was used to present the response of the piled-raft foundation.

7 citations

Book ChapterDOI
01 Jan 2020
TL;DR: In this paper, a typical case of a piled raft with relatively smaller width, supported on two rows of piles on the edges was studied with small scale 1g model tests for understanding the basic behavior and a numerical study to obtain the details of raft settlement and the shaft stress distribution along the pile length.
Abstract: When the bearing capacity is not adequate to provide isolated footings, columns are grouped and supported combined footing. However, if the settlement increases beyond the permissible limits, piles are provided below the combined footing, applying the concept of piled raft. In this case the piles function as settlement reducers. Such systems can be categorized as a particular case of the combined piled raft system. In the present study a typical case of a piled raft with relatively smaller width, supported on two rows of piles on the edges was studied with small scale 1g model tests for understanding the basic behavior and a numerical study to obtain the details of raft settlement and the shaft stress distribution along the pile length.
References
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors discuss the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful and some of the characteristics of piled raft behavior are described.
Abstract: In situations where a raft foundation alone does not satisfy the design requirements, it may be possible to enhance the performance of the raft by the addition of piles. The use of a limited number of piles, strategically located, may improve both the ultimate load capacity and the settlement and differential settlement performance of the raft. This paper discusses the philosophy of using piles as settlement reducers and the conditions under which such an approach may be successful. Some of the characteristics of piled raft behaviour are described. The design process for a piled raft can be considered as a three-stage process. The first is a preliminary stage in which the effects of the number of piles on load capacity and settlement are assessed via an approximate analysis. The second is a more detailed examination to assess where piles are required and to obtain some indication of the piling requirements. The third is a detailed design phase in which a more refined analysis is employed to confirm the op...

379 citations


"Parametric Study on the Interaction..." refers background in this paper

  • ...Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1],Poulos[2] and so on....

    [...]

01 Jan 1994

296 citations


"Parametric Study on the Interaction..." refers background in this paper

  • ...Randolph [7,8] in his studies had expressed the interaction between the pile and the cap considering the average displacements beneath the cap due to the loading on the pile, and down the center line of the pile due to the cap....

    [...]

Journal ArticleDOI
TL;DR: In this article, a load test was carried out on an isolated single pile, single-loaded center piles in groups, a footing without any piling, free standing pile groups, and piled footings.
Abstract: The interactions among closely located piles and a cap in a pile group are complex. The current design practice for vertically loaded pile groups roughly estimates their overall behavior and generally yields conservative estimations of the group capacity. For a proper pile group design, factors such as the interaction among piles, the interaction between cap and piles, and the influence of pile installation method all need to be considered. This paper presents the results of the model test, which can be used to better understand the interactions of vertically loaded pile groups in granular soil. Load tests were carried out on the following: an isolated single pile, single-loaded center piles in groups, a footing without any piling, free standing pile groups, and piled footings. The influences of pile driving and the interactions among bearing components on load–settlement and load transfer characteristics of piles and on the bearing behavior of a cap in a group are investigated separately by comparing the...

64 citations


"Parametric Study on the Interaction..." refers background in this paper

  • ...As pointed out by Lee and Chung [5], the complex interaction can become favorable like increase in the group capacity or unfavorable like causing additional settlement....

    [...]

Dissertation
01 Jan 2000

8 citations


"Parametric Study on the Interaction..." refers background in this paper

  • ...Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1],Poulos[2] and so on....

    [...]

  • ...ISBN: 978-981-07-4948-4 :: doi:10.3850/978-981-07-4948-4 259 1 Extensive research has been done so far on the behaviour of piled raft and various aspects of behaviour relating to the load settlement and the load sharing response have been studied in detail by various researchers like Reul [1], Poulos [2] and so on....

    [...]

Frequently Asked Questions (2)
Q1. What are the contributions mentioned in the paper "Parametric study on the interaction factor of piled raft author" ?

This paper presents a qualitative and a quantitative assessment of the interaction behaviour through a non dimensional interaction factor, through the results of a series of 1g model studies. The effect of various important parameters like pile length, pile raft area ratio and spacing on the interaction factor are discussed. 

Even though the pile group of piled raft started losing its stiffness rapidly it continued to take further load which is similar to elastic work hardening behaviour.