2671
A Design Method For Piled Raft Foundations
Méthode de conception des fondations de type radier sur pieux
Balakumar V.
Simplex Infrastructures Limited, Chennai, Tamil Nadu, India.
Oh E., Bolton M., Balasubramaniam A.S.
Griffith University Gold Coast Campus, Brisbane, Australia.
ABSTRACT: The design of piled raft foundation involves two stages namely a preliminary stage and the final stage. The
preliminary design stage involves the identification of the essential parameters namely the number of piles, their diameter and the
length along with E
s
value of the subsoil strata for an optimum design which can produce the required settlement reduction. The
existing design methods, whose accuracy depends upon the accuracy of the evaluation of in-situ parameters like E
s
although can
produce satisfactory results, the computational efforts involved and the time does not justify the use of them for the preliminary
analyses which will involve repetitions. This paper outlines a simple analytical procedure and the effectiveness of the pressuremeter
tests in predicting the piled raft behaviour, so that the results can be used in the detailed design.
RÉSUMÉ :
La conception des fondations de type radier sur pieux implique deux étapes, l’étape préliminaire et l’étape finale.
L’étape préliminaire de conception implique la détermination de paramètres essentiels : nombre de pieux, leurs diamètres et leurs
longueurs ainsi que la valeur de E
s
de la couche de sol superficielle pour un design optimal qui pourra amener à la réduction requise
du tassement. Les méthodes de conception existantes, dont la précision dépend de la précision sur la mesure des paramètres in situ
conduisent à des résultats satisfaisants. Néanmoins les moyens numériques et le temps impliqués pour leurs réalisations ne sont pas
justifiés pour l’analyse préliminaire qui nécessite des répétitions. Cet article synthétise une méthode analytique simple et efficace de
tests de pression pour prédire le comportement des fondation de type radier sur pieux dont les résultats peuvent être utilisés pour des
projet de conception.
KEYWORDS: piled raft, tri-linear pressuremeter.
1. INTRODUCTION.
The combined piled raft foundation system provides a skilful
geotechnical concept wherein the applied load is transferred by
means of a load sharing mechanism which is generated through
a process of interaction between the pile soil and the raft. The
piled raft foundation system differs from the traditionally
designed pile supported raft in the fact that in the case of piled
raft the presence of the raft and its contribution in sharing the
load with the pile group is recognized. The piled raft foundation
utilises the pile group for the control of settlement, with the
piles providing most of the stiffness at the service loads while
the raft elements provide the additional capacity at the ultimate
load levels. The concept of piled raft system was born out of the
fact that any structure has a certain magnitude of permissible
settlement and the foundation system has to aim at reducing the
settlement as close to the permissible value as possible rather
than eliminating the settlement completely. In the last two
decades researchers like Cooke (1986), Burland(1995), and
Poulos(2001) have provided considerable insight into the
behaviour of piled raft .
The development of sophisticated computational facilities
and FEA codes like Defpig,Napra,and HyPr etc have enhanced
the interaction process between the observational methods
(Katzenbach etal., 2000 a; Balakumar and Ilamparuthy, 2007),
small scale model studies (Horikoshi1995; Balakumar
etal.,2005) and numerical and analytical simulations leading to
the improvement of the design process. Consequent to this
number of tall and heavily loaded structures have been
supported on piled raft and the performance of some of these
piled rafts have been monitored and the results may be used to
refine the design in the future. (Poulos, 2008; Yamashita etal.,
2010).
2. THE DESIGN PROCESS
In the design process of piled raft the initial stages of design
involves the determination of the optimum number of piles, pile
length and diameter required to be placed in a strategic manner
to produce the required settlement reduction along with the load
shared by the pile group. This process may require a large
number of trials depending on the nature and requirement.
Hence the analytical procedure has to be computationally
simple so that the efforts and the time will be less. The existing
methods although produce satisfactory results, involve more
complicated computational efforts .Further in solving the
complicated three dimensional problems such as piled raft,
many simplified assumptions are to be made and the
rigourousness of the method may have to be diluted to make the
problem computationally viable. Therefore there is a need for a
simple method that can solved by treating the problem as
axisymmetric or plane strain problem in the case of preliminary
design to establish parameters like pile length, numbers
diameter and the layout to be used in the final design.
