Incorporating the Effect of Pore Pressure in Undrained DEM Simulations

Liquefaction in undrained soils coincides with the development of significant excess pore water pressures. The undrained behaviour of soils has been studied extensively using laboratory testing, but these tests cannot give any insight into the micromechanical changes that cause the observed macro-scale response. One way to obtain insight into the micromechanical behaviour is to use a suitable numerical technique such as the discrete element method (DEM). The ‘constant volume’ method, in which the sample volume is maintained constant throughout shearing, is often used to simulate the undrained test condition. In this method the soil sample is assumed to be perfectly saturated with an incompressible liquid. The constant volume method has the advantage of computational simplicity. However, some problems arise when simulating dense samples such as the generation of unrealistically high stresses and excessively large interparticle overlaps. There is a clear need to develop an alternative to constant volume simulations which retains the method’s computational efficiency but without the unphysicality. In this paper, several reasons are proposed for the inability of constant volume simulations to quantitatively capture a real soil’s undrained behaviour. Alternatives to the constant volume method are discussed, all of which allow the sample volume to vary during the simulation by incorporating the effect of pore pressure. One method was selected and implemented in the open-source LAMMPS DEM code, and its appropriateness for simulating undrained soil behaviour is explored with reference to monotonic simulations of sand.