Evaluation of undrained response from drained triaxial shear tests: DEM simulations and Experiments

摘要

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 \u26 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 \u26 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 \u26 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.