Numerical simulation of liquefaction and pore pressure generation in granular materials using DEM

A qualitative understanding of the liquefaction process and the pore water pressure generation in sandy soils has considerably enhanced by various researchers for the last few decades. Laboratory investigation carried out by these researchers have highlighted that liquefaction and pore water pressure generation in granular materials depends on many factors, such as sample preparation, initial anisotropy, confining pressure, void ratio, loading condition, amplitude of shear strain etc. Also, in these laboratory experiments there are limitation of repeatability (with in lab) and reproducibility (lab to lab) in preparation of sample. In this paper three dimensional discrete element method (DEM) was employed to understand the liquefaction and pore pressure generation of granular materials. In DEM modeling a good control of sample preparation can be exercised and it is easy to prepare samples repeatedly with similar fabric. A series of strain controlled undrained cyclic triaxial test has been carried out on isotropically compressed assembly of spheres. The undrained tests have been simulated by maintaining a constant volume condition of the sample throughout the cyclic triaxial tests. The pore water pressure has been computed taking the difference of mean pressure between the undrained (effective) and drained (total) stress path during strain controlled cyclic triaxial tests. The study has been planned to emphasize the effect of confining pressure, void ratio, initial anisotropy and amplitude of shear strain on the liquefaction and pore water pressure generation in the sample. As observed from the results, numerical simulation using DEM has captured liquefaction and pore water pressure generation very similar to the experimental results. Further, evolution of micromechanical parameters such as average coordination number, deviator anisotropic coefficients during cyclic triaxial loading has also been reported and discussed.