Modelling of Granular Materials Using the Discrete Element Method

With the Discrete Element Method it is possible to model materials that consists of individual particles where a particle may role or slide on other particles. This is interesting because most of the deformation in granular materials is due to rolling or sliding rather that compression of the grains. This is true even of the resilient (or reversible) deformations. It is also interesting because the Discrete Element Method models resilient and plastic deformations as well as failure in a single process.The paper describes two types of calculations. One on a small sample of angular elements subjected to a pulsating (repeated) biaxial loading and another of a larger sample of circular element subjected to a plate load. Both cases are two dimensional, i.e. plane strain.The repeated biaxial loading showed a large increase in plastic strain for the first load pulse at a given load level. Additional load pulses at the same load level gave decreasing plastic strain rate, in agreement with what is normally observed on granular materials. The resilient modulus was much lower than the stiffness of the elements and was decreasing with increasing deviator stress. At high deviator stresses the stiffness of the assembly of elements was less than one percent of the stiffness of the elements. This is also in good agreement with observations on granular materials.Plate loading showed a distribution of vertical stress that was close to the stress in an elastic continuum. Very little stress concentration was observed, but this might change if angular elements were used. The horizontal stresses on the other hand were quite different from the horizontal stresses in an elastic continuum. Modulus and Poisson's ratio calculated at different points of the particulate medium, from the stresses and strains, showed large variations. Dilation of the material was frequent.