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Post Cyclic Behaviour of Singapore Marine Clay
Le comportement post-cyclique de l’argile marine de Singapour
Ho J., Goh S.H., Lee F.H.
National University of Singapore
ABSTRACT: In this paper, the post-cyclic behaviour of remoulded Singapore Marine Clay is examined. Cyclic triaxial tests,
followed by monotonic loading to failure, were performed on normally consolidated specimens (38mm diameter by 76mm height)
within a cyclic strain range of approximately 0.7% to 1.4%. Results herein reveal that the effective stress paths under post-cyclic
monotonic loading may take on different forms depending on the mean effective stress state of the specimen at the end of the cyclic
loading phase. By normalizing the mean effective stress (p’) against the effective consolidation pressure (p
c
’) of the specimen, the
effective stress paths during the post-cyclic monotonic loading may be approximately grouped into three different regimes, according
to the normalized mean stress (p’/p
c
’). Within each normalized mean stress regime, the monotonic soil response is independent of the
effective consolidation pressure, the cyclic strain amplitude and number of cycles applied during cyclic loading. The results suggest
that the normalized mean stress after cyclic loading may be an important parameter in determining the subsequent stress path under
undrained monotonic loading to failure.
RÉSUMÉ : Dans ce papier, le comportement post-cyclique de l’argile marine remaniée de Singapour est étudié. Des essais triaxiaux
cycliques suivis de chargement monotone jusqu’à la rupture ont été effectués pour des déformations cycliques comprises entre 0,7%
et 1,4% sur des échantillons normalement consolidés. Les résultats démontrent que les chemins de contrainte durant le chargement
monotone post-cyclique peuvent être de formes différentes et dépendent de l’état de contrainte moyenne effective de l’échantillon à la
fin de la phase du chargement cyclique. En normalisant la contrainte moyenne effective (p’) par la contrainte de consolidation (p
c
’),
les chemins de contrainte pendant le chargement monotone post-cyclique peuvent être regroupés en trois différents groupes selon la
contrainte moyenne normalisée (p’/p
c
’). Dans tous les cas, la réponse monotone du sol est indépendante de la contrainte effective de
consolidation, de l’amplitude de la déformation cyclique et du nombre de cycles appliqué pendant le chargement cyclique. Les
résultats suggèrent que la contrainte moyenne normalisée après le chargement cyclique pourrait être un paramètre important pour
déterminer, sous chargement monotone non-drainé, les chemins de contrainte jusqu’à la rupture .
KEYWORDS: Post-cyclic clay behaviour, Mean effective stress, Cyclic stress reversal
1 INTRODUCTION
Previous studies have shown that, during undrained
compression loading, the effective stress path of a normally
consolidated clay after cyclic loading is similar to the effective
stress path of an overconsolidated clay (Hyde and Ward 1985,
Matsui et al. 1992, Yasuhara et al. 1992). Researchers have
adopted different frameworks in the analysis of the post-cyclic
undrained shear strength of clays. One common approach is to
estimate the undrained shear strength based on the maximum
shear strain during the applied cyclic loading (Thiers and Seed
1969, Sangrey and France 1980). Another method of analysis is
to relate the post-cyclic undrained shear strength with the
apparent overconsolidation ratio induced by the cyclic loading
(Matsui et al. 1992, Yasuhara et al. 1992). Apart from the post-
cyclic undrained shear strength, this induced apparent
overconsolidation ratio from cyclic loading was also used to
determine how the subsequent monotonic effective stress path
approaches the critical state line (Yasuhara et al. 1992).
However, one possible limitation in past studies is the
relatively fast rates of cyclic loading used, which typically
ranges from 0.5Hz to 1 Hz. At such loading rates, it is uncertain
if excess pore pressure in the clay specimens will be able to
dissipate. For this reason, the reliability of pore pressure
measurements during cyclic loading phase may be doubtful.
Fast loading rates on normally consolidated clays during
undrained triaxial tests prevents equilibration of excess pore
pressure within test specimens, which results in a higher pore
pressure within the middle one-third portion of the specimen
(Wood 1982). This can lead to errors in effective stress
calculations. Few attempts had been made to overcome the issue
of unequalized pore pressures during cyclic loading. For
example, Matsui et al (1992) the specimen to stand in an
undrained state for 1 hour after cyclic loading and prior to post-
cyclic monotonic loading to allow for equalization of pore
pressures. On the other hand, Diaz-Rodriguez et al (2000) used
a longer equalization period of 12 hours. Another approach is to
allow drainage before post-cyclic loading to allow for pore
pressures accumulated during cyclic loading to be dissipated
(Yasuhara et al. 1992). The drawback of this approach is that it
alters the pore pressures within specimens, leading to
discontinuities in effective stress paths between the cyclic
loading and post-cyclic loading phases. Intuitively, the effective
stress response of a clay undergoing cyclic loading should be
indicative of its post-cyclic behavior if post-cyclic monotonic
loading is conducted immediately after cyclic loading. Due to
possible errors effective stress measurements and discontinuities
between cyclic and post-cyclic effective stress paths, a direct
comparison between the cyclic and post-cyclic behavior of clays
has hitherto not been possible. This objective of the study
reported herein is to re-visit the issue of post-cyclic behaviour
of soft clay, while ensuring adequate equilibration of excess
pore pressure.
2 CYCLIC AND POST CYCLIC TESTING PROGRAM
A series of two way strain-controlled undrained cyclic triaxial
tests were performed on remoulded specimens (38mm diameter
by 76mm height) of normally consolidated Singapore Upper
