Micromechanical characterization and prediction of undrained response from drained triaxial shear tests in granular media under monotonic loading: Simulations using PFC3D

When a stress increment is applied quickly to a saturated cohesionless soil, undrained conditions develop with an increase in pore pressure and no change in volume. Once steady state is reached, the soil continues to distort at a constant rate. Generally, to evaluate the steady state of undrained loading conditions, triaxial undrained tests with pore water pressure measurements are done. But the steady state is influenced by a number of parameters including the method of preparation of the sample, loading pattern, membrane penetration effects, type of triaxial equipment. Norris et al. (1997) proposed a technique to predict the undrained response of sands from drained triaxial tests carried out from isotropic rebound paths based on the effective stress concept. In this paper, the discrete element application for predicting undrained response from drained test results will be validated. Numerical simulations of triaxial tests considering 1000 spherical particles have been carried out using PFC3D. These results show an excellent correlation between the predicted undrained behaviour from drained triaxial tests and the observed response without explicitly incorporating pore water pressures. Further, micromechanical analyses are presented to understand the micro to macro behaviour.