Abstract
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The undrained shear response of monotonically loaded isotropically
consolidated saturated sands can be characterised
by a change in the excess pore pressure generation in the
sample. The generation of positive and negative excess pore
water pressures is related to contractive and dilative responses.
The increase or decrease in pore water pressure
continues until it reaches critical state (known as steady state
for undrained tests). In general, Casagrande’s definition of
critical state for sands has been utilised in most commonly
used liquefaction analyses, and is referred to as the steadystate
procedure (Castro, 1969; Casagrande, 1977; Castro &
Poulos, 1977; Poulos et al., 1985). In the laboratory, consolidated
undrained triaxial tests on both reconstituted and
undisturbed samples are generally carried out to evaluate the
steady state of sands (Poulos et al., 1985). However, it is
very clear that steady-state determination from consolidated
undrained tests with pore pressure measurements is sensitive
to parameters such as initial effective confining pressure and
initial fabric (Poulos, 1981; Been & Jefferies, 1985; Been et
al., 1991; Castro et al., 1992). The most recent findings of
De Gregorio (1990) indicate that critical state is influenced
by the method of soil sample preparation (moist tamping,
moist vibration or dry pluviation). This may be due to the
volume change tendency caused by the difference in the
fabric of the sand, which affects the critical-state response.
Furthermore, such behaviour also depends on the loading
system equipment’s capability to keep up with the potential
for sample deformation, an important point with regard to
potential differences in testing equipment from one laboratory
to another. In this regard, Norris et al. (1997) developed
a methodology to predict the undrained shear response of
sands from drained triaxial tests carried out from isotropic
rebound paths based on the effective stress concept. This
method makes it possible for the majority of geotechnical
firms to participate in the prediction of static liquefaction
and residual strength by performing traditional drained tests
with volume change measurements. Furthermore, this method
also provides the condition and logic for the development
of complete as against limited liquefaction (Norris et al.,
1997).
In the laboratory, drained triaxial shear tests were used to
predict undrained behaviour using samples consolidated to
the desired confining pressure and then rebounded to lower
pressures. However, it is seldom possible in the laboratory to
consolidate the assemblies along identical paths owing to the
difficulty of preparing samples with the same initial fabric.
In this technical note, the method proposed by Norris et
al. (1997) is revisited using discrete element methods
(DEM) (Cundall & Strack, 1979), by which means the
sample preparation problem can be avoided. In addition,
laboratory experiments on clean sands were carried out to
validate the numerical simulation results using DEM.