2909
Load Tests on Full-Scale Bored Pile Groups
Essais de chargement sur des groupes de pieux forés
Zhang Y., Salgado R.
Purdue University, USA
Dai G., Gong W.
Southeast University, China
ABSTRACT: Pile groups are commonly used in foundation engineering. Due to the difficulties and cost of full-scale load tests, most
pile group tests are scaled down regardless of whether performed in the field or laboratory. Very limited experimental data are
available on the loading of full-scale bored pile groups in the field. This paper reports the results of axial static load tests of both full-
scale instrumented pile groups and single piles. Experiments vary in the number of piles in the group, the pile spacing, the type of pile
groups and pile length. All piles have a diameter of 400 mm. Two-pile groups, four-pile groups and nine-pile groups with pile lengths
of 20 m and 24 m are tested. Since the isolated piles and some piles in the pile groups are instrumented, the load transfer curve and the
load-settlement curve of both of piles in isolation and individual instrumented piles in the groups are obtained. The interaction
coefficient for each pile in the group is back-calculated from the measured data by optimization. The interaction coefficients are
shown to depend on pile proximity, as usually assumed in elastic analyses, but also on settlement and on the size of the group.
RÉSUMÉ : Les groupes de pieux sont couramment utilisés dans les travaux de fondation. En raison des difficultés et du coût des
essais de chargement à grande échelle, la plupart des essais de groupe de pieux sont réalisés à petite échelle indépendamment de la
réalisation des essais en laboratoire ou in-situ. Très peu de données expérimentales sont disponibles sur le chargement en pleine
échelle groupes des pieux forés in-situ. Cet article présente les résultats d'essais de chargement statique axial à grande échelle des
groupes de pieux et des pieux simples instrumentés. Les expériences varient en nombre de pieux dans le groupe, l'espacement des
pieux, le type de groupes et de longueur des pieux. Tous les pieux ont un diamètre de 400 mm. Des groupes composés de deux, quatre
ou neuf pieux, avec des longueurs de 20 m et 24 m sont testés. Comme les pieux isolés et quelques uns des pieux dans les groupes
sont instrumentées, la courbe de transfert de charge et la courbe de charge-tassement des pieux isolés et en groupe sont obtenus. Le
coefficient d'interaction pour chaque pieu dans le groupe est évalué par calcul inverse à partir des données mesurées par
l'optimisation. Il a été montré que les coefficients d'interaction dépendent de la proximité des pieux, comme habituellement supposé
dans des analyses élastiques, mais également du tassement et de la taille du groupe.
KEYWORDS: pile groups, load transfer, interaction coefficient
1 INTRODUCTION
Settlement analyses of pile groups (e.g., Poulos 1968; Randolph
and Wroth 1978, 1979; Poulos and Randolph 1983; Poulos
1989; Randolph 2003; Leung et al. 2010) are based on a variety
of approaches, which include boundary-element methods, the
hybrid load transfer approach, and the finite element method.
Despite some theoretical advances in the analyses and
prediction of pile group behavior in the last few decades,
analyses are still largely based on simplifications of the problem
and of the constitutive behavior of the soil. Due to the
difficulties and cost of full-scale load tests, most pile group tests
were scaled down regardless of whether performed in the field
or laboratory. There are few
in situ
, full-scale bored pile group
load tests reported in the literature.
The present paper aims to start filling this knowledge gap
by reporting the results of
in situ
, full-scale bored pile group
tests. The aim of the tests was to investigate the following
crucial issues in particular: (
i
) what the rates of pile head and
pile base load mobilization with settlement are; (
ii
) how the
shaft resistance, which is responsible for the difference between
these two rates, varies between single piles and piles in a group
in various arrangements; (
iii
) the proportion in which load
applied on a pile cap is shared between the piles in the group;
(
iv
) how pile group efficiency varies with settlement.
2 EXPERIMENTAL PROGRAM
The field load tests on bored piles were performed on: (
i
) an
isolated single pile with length L = 20 m; (
ii
) an isolated single
pile with L = 24 m; (
iii
) a two-pile group with spacing s
p
= 2.5B
(B, the pile diameter) and L = 20 m; (
iv
) a two-pile group with
s
p
= 3.0B and L = 24 m; (
v
) a four-pile group with s
p
= 2.5B and
L = 20 m; (
vi
) a four-pile group with s
p
= 3.0B and L = 24 m;
(
vii
) a nine-pile group with s
p
= 2.5B and L = 20 m; and (
viii
) a
nine-pile group with s
p
= 3.0B and L = 24 m. All piles in the
experiments had a diameter B of 400 mm. The concrete strength
(
f
’
cd
) was 25 MPa for both the piles and the caps. The concrete
reinforcement cover was 70 mm in the caps and 35 mm in the
piles. In this paper, tests on isolated single piles are denoted by
DZ. Pile group tests are denoted by QZ. The suffix L is used to
indicate that the pile length L is 24 m. A dash after the pile
group reference followed by a number indicates a specific pile
within that group.
One auger boring was drilled at the test site to a depth of
29.50 m. This boring showed a uniform, thick soft clay layer
starting at 17m and extending all the way to the bottom of that
boring. This auger boring depth was 11B deeper than the test
pile base for piles with 24 m length. Static cone penetration tests
(CPTs) were performed in the vicinity of the boring to give a
continuous record of the soil resistance with depth. The subsoil
profile includes multiple layers of silt and clay. The ground
water level was found at a depth of 2.60 m. The detailed soil
