DEM Simulations on the Energy Dissipation in Soil under Static and Cyclic Loading

Nothing can freely vibrate forever because energy has to be dissipated in the end. The term, damping ratio, is often used to quantify the energy loss or dissipation in soil. However, the detailed process of energy dissipation in soil and the associated underlying mechanisms that regulate the damping ratio remain unclear at present, especially from the micromechanics point of view. In this context, numerical simulations by the discrete element method (DEM) were carried out in this study to gain insights into this subject. The software, PFC2D(Particle Flow Code in 2 Dimension) was used in the simulations to trace the energy dissipated as well as stored in soil while the sample is subject to static biaxial tests. It has been found that the energy dissipation is mainly due to the frictional loss between particle contacts (quantified by the Hertz-Mindlin contact model). The stored energy is in the form of elastic strain energy at the particle contact. The difference between the total applied energy and the stored energy is the dissipated energy by friction. Simulation results also indicate that the strain energy is not fully released when the soil becomes plastic, which suggests the existence of frozen energy hidden at particle contacts. PFC3D(Particle Flow Code in 3 Dimension) was used to model the energy dissipation in soil under cyclic simple shear tests. The damping ratios obtained from the simulations, when considering both frictional and viscous loss, are very similar to the experimental findings, not only the increasing trend with increasing shear strain but also the magnitude. Most of the energy dissipation by friction takes place in those contacts of weak forces because the contact normal forces and the associated shearing resistance are small, which in turn enhances the sliding to occur between contacts.