Challenges of simulating undrained tests using the constant volume method in DEM

Abstract

Liquefaction during earthquakes can cause significant infrastructural damage and loss of life, motivating a fundamental study of undrained sand response using discrete element modeling (DEM). Two methods are widely used in DEM for simulating the undrained response of soil. One approach is to numerically couple the DEM code with a fluid model. Alternatively, if the soil is fully saturated and water is assumed to be incompressible, the volume of the sample can be held constant to simulate an undrained test. The latter has the advantage of being computationally straightforward, but the assumption of a constant volume can cause some issues which are discussed in this paper. Depending on the contact model selected, extremely high deviatoric stresses and pore water pressures can be generated for dense samples using the constant volume approach which are not observed in corresponding laboratory tests. Furthermore the results of these constant volume simulations tend to be sensitive to the strain rate selected. The evolution of particle size distribution caused by grain crushing is also ignored in most undrained simulations. For these reasons, authors often restrict the extent of the data presented to physically-realistic ranges and report results in non-dimensional terms, e.g., using stress ratios (q/p’) or stresses normalized by the initial confining pressure. This paper aims to highlight some of these issues, explore whether the constant volume approach is appropriate and make recommendations for future analysis of undrained soil behavior using DEM. © 2013 AIP Publishing LLC