A discrete element code has been developed to simulate assemblies of two-dimensional polygon-shaped particles to study the behavior of granular materials such as rockfill. Two different contact laws (overlap-area and linear) were introduced to model the real interaction between particles. The results of simulations show that both contact laws yield similar macroscopic behavior. The focus of this study is on effects of confining pressure on shear strength and dilatancy of simulated rockfill. It is demonstrated that shear strength and dilatancy decreases as confining pressure increase. The results are similar to data obtained from experiments on rockfills.