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Equivalent pier theory for piled raft design
Équivalence de la théorie de la jetée pour la conception de radeau empilé
Balakumar V.
Simplex Infrastructures Limited, Chennai, Tamil Nadu, India.
Huang M.
Arup Geotechnics, Brisbane, Australia.
Oh E., Balasubramaniam A.S.
GriffithUniversity Gold Coast Campus, Brisbane, Australia.
ABSTRACT: The objective of generating an economical and safe foundation system reducing the settlement rather than eliminating
has led to the change in the design philosophy recognising the fact that most structures can tolerate a certain amount of settlement
The piled raft analyses and design is three dimensional interaction problems, wherein, the applied load is transferred by a
complicated interaction process between the piles and the raft.. The necessity to have a relatively simple design procedure so that the
preliminary design can give adequate but reasonably accurate data for the final analyses is explained. The paper presents such a
simple design process in the form of equivalent pier approach by establishing its applicability by applying it to two cases.Also the
study has brought out the effect of deep compressible deposit sandwiched between two dense layers.
RÉSUMÉ :
L’objectif de créer un système de fondation économi
que et fiable, par la réduction des affaissements plutôt que de leur
éradication, a entraîné un changement dans la philosophie de conception, admettant le fait que la plupart des structures sont capables
de tolérer un certain nombre d’affaissements. Les a
nalyses et la conception du radier sur pieux comportent des problèmes
d’interaction tri dimensionnels, où la charge appliquée est transférée par un processus d’interaction complexe entre les pieux et le
pilier. Il est expliqué la nécessité de disposer d’un
e procédure de conception relativement simple, de manière à ce que la conception
préliminaire puisse apporter des données appropriées suffisamment précises pour les analyses finales. L’article présente un
processus de conception aussi simple, sous forme d
’approche de pilier équivalent, en prouvant sa faisabilité par son application
dans deux cas. L’étude a également révélé les effets du dépôt fortement compressible coincé entre deux couches épaisses.
KEYWORDS: Piled raft, Pier, Peat.
1.
INTRODUCTION
The design of foundation system for structures that cannot
tolerate settlements, the aspect of balancing the performance
and cost, had always been a challenge for the foundation
designers. Due to the complexity involved in the soil structure
interaction analyses, required for an optimum design, designers
have so far been resorting to the traditionally designed pile
foundations system permitting very small limiting settlements.
Even though this approach produces a safe design, the
economics of the design becomes questionable. The objective
of generating an economical and safe foundation system
reducing the settlement rather than eliminating, has led to the
change in the design philosophy Keeping the above objective in
mind researchers like Burland (1995) and subsequently Polous
(2001) had brought out the use of piles with the raft to reduce
the settlement of the raft.This had led the advent of the
combined piled raft foundation system, which provides a skilful
geotechnical concept to design the foundation for structures
which are sensitive to large settlements. The piled raft analyses
is a three dimensional interaction problems, wherein, the load
transfer mechanism is a complicated interaction process by
which the load is shared The interactive process between the
Various procedures based on observational study (Katzenbach
etal.,2000a) small scale model studies such as centrifuge models
(Horikoshi 1995) 1g model studies (Balakumar,2008) and the
resulting interactive process with the numerical modeling
(Clancy 1993; Russo,1998;) supported by the development of
new geotechnical computational facilities (Polous and Small
2007) has led to the the piled raft foundation system being
extensively used to support tall and heavily loaded structures in
a successful manner permitting larger settlements close to the
permissible value (Polous,2008;., Yamashita et al .,2010).
2.
DESIGN PROCESS
The satisfactory performances of piled raft largely depend upon
the performance of the pile group of piled raft in providing the
initial stiffness and then allow the raft to have a higher capacity
by functioning as settlement reducer. Hence after ascertaining
the feasibility of the piled raft to support the structure, a
preliminary analyses has to be done to finalize the
computational details of the constituent elements. Primarily the
number, length of the piles, the load shared by the pile group are
the essential parameters in addition to the properties of the
supporting soil layers. In the case of the piled raft the pile
group capacity and the overall capacity of the piled raft play an
important role. The second stage of analyses has to produce
these data in a reliable manner such that when used in the final
analyses, the analyses will produce a design which need not be
subjected to any iteration process. This requirement makes the
procedure to be more realistic and simple enough such that the
computational efforts are minimum and economical. Even
though the existing methods can provide a design approach,
these involve a very detailed computational efforts, not really
warranted for the second stage of design, from the commercial
design organisation point of view.. Therefore it is essential to
have a relatively simple design procedure so that the second
stage of work can give adequate but reasonably accurate data
for the final analyses.
