2771
Numerical Simulation of the Load Tests on Bearing Capacity of Piled Raft
Foundations
Simulations numériques d'essais de chargement pour établir la capacité portante des fondations
mixtes radier sur pieux
Kaneda K., Honda T., Shigeno Y., Hamada J.
Takenaka Corporation, Chiba, Japan
ABSTRACT: Field tests on the bearing capacity of pile, raft, and piled raft foundations are compared with simulations of the same
tests. Those simulations used the SYS Cam-clay model developed by the Soil Mechanics Group of Nagoya University as the
constitutive equations for material behavior. In this study, the material parameters and initial conditions were determined by
laboratory testing performed on specimens collected in the area of the field tests. We found that the numerical simulations reproduced
the experimental data well. Furthermore, in our experiments, the total bearing capacity of the raft and pile were found to be equal to
that of the piled raft foundation. This was also confirmed via the simulations.
RÉSUMÉ : Cet article, qui traite de la capacité portante des pieux, des radiers et des fondations mixtes de type radier sur pieux,
présente des résultats issus d’une comparaison entre des essais
in situ
et ceux obtenus par simulations numériques. Ces simulations
utilisent le modèle de comportement SYS, développé par le groupe de mécanique des sols de l’Université de Nagoya. Les paramètres
relatifs aux matériaux et aux conditions initiales ont été déterminés sur la base d’essais en laboratoire d’échantillons prélevés au même
endroit où les essais
in situ
ont été exécutés. Ces simulations numériques ont convenablement reproduit les données expérimentales.
Par ailleurs, lors des essais
in situ
, la capacité portante totale du radier et du pieu a été trouvée similaire à celle de la fondation mixte.
Ce résultat a été aussi confirmé par les simulations.
KEYWORDS: piled raft foundations, numerical simulation, bearing capacity
1 INTRODUCTION
A significant number of studies have been carried out on the
settlement and bearing capacity of pile, raft, and piled raft
foundations. The designed bearing capacity for pile and piled
raft foundations is considered to be the load at 10% settlement.
In the present study, prototype vertical load simulations were
carried out on such foundations to investigate the relationship
between their settlement and bearing capacity at loads near their
designed limits. This paper reports the results of these
simulations and compares them to experimental data.
In this research, a combined soil and pile analysis was
carried out. In the simulations, the SYS Cam clay model
developed by the Soil Mechanics Group of Nagoya University
was used as the constitutive mathematical model for the soil
(Asaoka
et al.
(2000, 2002)). Since settlement exceeded 10% in
the simulations, and since large soil deformations made the
influence of geometrical non-linearity important, finite
deformation analysis based on finite deformation theory was
performed (Asaoka
et al.
(1994)). In addition, because the
bearing capacity of saturated soils changes with loading rate, a
soil-water coupled analysis was also carried out.
The remainder of the present paper is organized as follows. A
brief outline of the experiments is first provided, followed by a
discussion on the determination of material parameters and
initial conditions for the vertical loading experiments based on
laboratory tests. Next, the use of these parameters to perform
soil-water coupled analyses based on simulated loading rates is
described, and the results are compared to experimental data.
Differences in the bearing capacities of pile, raft, and piled raft
foundations are then discussed.
2 EXPERIMENTAL CONDITIONS
Details of the experiments and results are given in Honda
et al.
(2012). The experimental conditions and soil profile used in this
paper are shown in Figure 1. From the top surface downwards,
the soil layers consist of fill, loam, clay, two types of sand and
silt. The N value around the pile is from 10 to 60
in the sand
layer. Cases 1, 2, and 3 correspond to the raft, pile, and piled
raft foundations, respectively. The raft has a square shape with 4
m sides, and the pile is 12 m long.
0
5
10
15
20
25
30
0 10 20 30 40 50 60 70
4m
4m
No.1
Raft
No.3
Piled raft
No.2
Pile
N Value
fill
loam
clay
sand
sand
silt
Depth (m)
Figure 1: Experimental conditions
3 ANALYSIS MESH AND DETERMINATION OF
MATERIAL PARAMETERS
Axisymmetric conditions are assumed for all simulations.
Figure 2 shows the mesh used to analyze the piled raft
foundation and the boundary conditions that were applied. The
left edge corresponds to the central axis of a cylindrical section
taken through the soil. Overall, the mesh contained 9,845
elements and 10,080 nodes, and it was made as fine as possible
around the pile. It extended about four times the pile length in
the vertical direction.
