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1 INTRODUCTION
A piled raft foundation is a composite structure with three
components: subsoil, raft and piles. These components interact
through a complex soil-structure interaction scheme, including
the pile-soil interaction, pile-soil-pile interaction, raft-soil
interaction, and finally the piles-raft interaction.
The construction of a piled raft foundation system generally
follows the same practices used to construct a pile group
foundation in which a cap is normally cast directly on the
ground. Although the construction of the cap in this manner
should allow a significant percentage of the load to be
transmitted directly from the cap to the ground, the pile group is
usually designed conservatively by ignoring the bearing
capacity of the raft (i.e. the pile cap). In many cases, the raft
alone can provide adequate bearing resistance; however, it may
experience excessive settlement. Therefore, the concept of
employing piles as settlement reducers was proposed by
Burland et al. (1977), where the piles are used to limit the
average and differential settlements.
The vertical load applied to a piled raft foundation is
assumed to be transmitted to the ground by both the raft and
piles, which differentiates the design of a piled raft from a pile
group. The percentage of load taken by each element depends
on a number of factors, including: the number and spacing of
piles, subsoil conditions, and the raft thickness.
A piled raft design offers some advantages over the pile
group design in terms of serviceability and efficient utilization
of materials. For a piled raft, the piles will provide sufficient
stiffness to control the settlement and differential settlement at
serviceability load while the raft will provide additional
capacity at ultimate load. The raft in a piled raft design
transmits 30% to 50% of the applied load to the soil (Clancy
and Randolph, 1993). Typically, a piled raft design will require
fewer piles in comparison to a pile group design to satisfy the
same capacity and settlement requirements. Additionally, if any
of the piles in a piled raft becomes defective, the raft allows re-
distribution of the load from the damaged pile to other piles
(Poulos et al. 2011). Furthermore, the pressure applied from the
raft to the subsoil may increase the confining pressure for the
underlying piles, which in turn increases the pile load carrying
capacity (Katzenbach et al. 1998).
Analytical, numerical and physical modeling approaches
were employed to evaluate the performance of piled raft
foundations. The simplified Poulos-Davis-Randolph (PDR)
method (Poulos 2001) combines the analytical methods
proposed by Poulos and Davis (1980) and Randolph (1994) for
the analysis of piled rafts. Clancy and Randolph (1993)
proposed a plate-on-spring method in which the raft is
represented by a plate and the piles are represented by springs.
Additionally, there are methods that combine the finite element
analysis for the raft and the boundary element analysis for the
piles (e.g. Ta and Small, 1996), and methods that are based on
three-dimensional finite elements modeling (e.g. Katzenbach et
al.,1998). Piled raft behavior was also investigated employing
physical modeling such as centrifuge testing (e.g. Horikoshi et
al., 2002, 2003a and b; Matsumoto et al., 2004a and b).
Performance of Piled-Raft System under Axial Load
Performance du système radier pieux sous chargement axial
Alnuiam A.
University of Western Ontario, London, ON, Canada and affiliated with King Saud University, Riyadh, Saudi Arabia
El Naggar H.
University of New Brunswick, Fredericton, NB, Canada
El Naggar M.H.
University of Western Ontario, London, ON, Canada
ABSTRACT: Many high rise buildings are founded on piled raft foundation systems, which generated a significant interest in
understanding the performance characteristics of this foundation system. The soil-structure interaction for piled raft foundations
involves the interaction of different components including: the pile-soil interaction; pile-soil-pile interaction; raft-soil interaction; and
piles-raft interaction. To account for this complicated interaction scheme, three-dimensional consideration of the problem is
necessary. In this paper, a 3D finite element model is established to analyze the response piled raft foundation system installed in
cohesionless soil with linearly increasing stiffness with depth. The model was calibrated/verified using geotechnical centrifuge test
data. The calibrated model was then employed to investigate the effect of raft dimensions and piles diameter and spacing on the load
sharing between piles and raft.
RÉSUMÉ : Durant ces dernières années, un grand nombre de projets ont été construit en utilisant le concept du système de fondation
de radier sur pieux. C’est ainsi que de nombreuses études ont été menées afin d’examiner et de comprendre la performance de ce
système soumis à des charges verticales. L’interaction sol-structure pour ce système implique différentes interactions comme
l’interaction pieux-sol, pieux-radier, radier-sol et finalement l’interaction pieux-radier. Par conséquent, une analyse numérique
tridimensionnelle est nécessaire dans un cas aussi complexe. Dans cet article, une modélisation 3-D par éléments finis a été effectués
pour analyser la réponse d’un système radier pieux sur un sol sans cohésion et qui présente une rigidité qui augmente linéairement
avec la profondeur. Ce modèle a été calibré à partir de résultats expérimentaux obtenus sur des essais en centrifugeuse. Les résultats
numériques ont été utilisés pour étudier l’effet de plusieurs paramètres, comme l'épaisseur et la largeur du radier, l’espacement et le
diamètre des pieux, sur le transfert des charges entre le radier et les pieux.
KEYWORDS: piled raft foundation, soil-structure interaction, 3D FEM, centrifuge, cohesionless soil, piled-raft load sharing.
