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Deformation behavior of single pile in silt under long-term cyclic axial loading
Comportement
d’un
pieu isolé sous chargement axial cyclique de longue durée dans un limon
Chen R.P., Ren Y., Zhu B. , Chen Y.M.
MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering, Zhejiang University, Hangzhou, China
ABSTRACT: Evaluating the response of piles to cyclic loading is a crucial part in the design of piled-embankment over soft ground.
In this paper, a series of large-scale model tests were performed to investigate the response of pile in silt under cyclic axial loading.
Heavily instrumented piles were used in the tests. The study is focused on the accumulation of permanent dispalecemtn of the piles
under long-term cyclic loading. Piles were tested at differernt cyclic loading levels and subjected up to 50,000 cycles of loading in
each test. The accumulated settlement was found to be strongly dependent on the characteristics of the applied cyclic loads. The piles
were found not to produce any increase in accumulated settlement if the cyclic loading amplitude is less than a certain threshold
value. A simple method is proposed to predict the accumulated settlement of single pile due to very large number of loading cycles.
The idea of a cyclic deformation diagram for analyzing the influence of charcteristics of cyclic loads on the deformation behavior was
also developed.
RÉSUMÉ : L'évaluation de la réponse des pieux vis-à-
vis d’un
chargement cyclique est un élément essentiel dans la conception des
remblais sur sols mous. Dans cet article, une série d'essais sur maquette à grande échelle a été réalisée pour étudier la réponse de pieux
sous chargement axial cyclique dans le limon. L'étude est centrée sur l'accumulation des déplacements permanents des pieux sous
chargement cyclique de longue durée. Les pieux ont été testés à différents niveaux de charge cyclique et soumis à 50000 cycles pour
chaque test. Les tassements accumulés sont fortement dépendants des charges cycliques appliquées. Aucune augmentation de
tassements n’est constatée
si l'amplitude de chargement cyclique est inférieure à un certain seuil. Une méthode simple est proposée
pour prédire le tassement cumulé
d’
un pieu isolé en
fonction d’un très
grand nombre de cycles de chargement. L'idée d'un diagramme
de déformation cyclique pour analyser l'influence des caractéristiques des charges cycliques sur le comportement en déformation a
également été développée.
EYWORDS: Pile; model test; accumulated settlement; cyclic loading
1 INTRODUCTION
Piles are commonly used to support high-speed railway
embankment in soft ground, which are exposed not only to the
heavy loads from superstructure self-weight, but also to the
long-term
“
one-way
”
cyclic loads induced by high-speed trains
throughout their service life. However, available design
experiences on the long-term response of the pile in silt to
cyclic axial loading are very limited, due to the fact that the
existing data obtained from laboratory tests and field
measurement are insufficient. This results in uncertainty in the
design and always leads to an over-conservative design of the
pile foundation.
The response of pile subjected to cyclic loading is very
complex and model test is the most effective and reliable way to
study it and its influencing factors. Laboratory and field tests
(Chan and Hanna 1980; Lee and Poulos 1991; Karlsrud et al.
1993) have shown that there are two main effects of cyclic axial
loading on piles: (1). a reduction in load capactiy and pile-soil
system stiffness; (2). an increase in settlement of piles. Poulos
(1989) reviewed several cyclic loading tests performed in sand
and stated that the accumulation of permanent displacement
with increasing load cycles was expected to dominate under
“one
-
way” cyclic loading, particularly if “stra
in-
softening”
behavior can occur at the pile-soil interface. The accumulation
of the permanent displacement principally depends on the cyclic
load level. Briaud and Felio (1986) analyzed the published data
and concluded that a load threshold exists above which failure
occurs by plunging due to cyclic loading and the value of this
threshold is 80% of the ultimate pile capacity on average.
In previous studies, most of the piles were loaded less than
500 cycles of loading. There have been few researches on the
response of pile to long-term cyclic loading at present.
This paper describes the results of a series of large-scale
model tests on single stiff piles in saturated silt to study the
accumulation of permanent displacement of the piles. The tests
have been performed using heavily instrumented model piles. A
simple method for predicting the accumulation of permanent
displacement of pile to long-term cyclic axial loading is
proposed. The idea of a cyclic deformation diagram for
analyzing the influence of charcteristics of cyclic loads on the
deformation behavior was also developed.
2 DESCRIPTIONS OF EXPERIMENTS
2.1 Test site and soil characteristics
The present large-scale model tests were carried out in a big soil
tank at Zhejiang University (Fig 1). This soil tank has an
dimension of 15×5m in plan view and a depth of 6m.
The soil used in the tests is low cohesive silt. Grading tests
showed that the soil contains 10% sand, 85% silt and 5% clay.
The characteristics of the soil are summarized in Table 1.
Laboratory tests show that the prepared soil had an average
water content of 28.5% and an effective internal friction angle
of 30
o
.
