Three dimensional (3D) DEM (discrete element method) simulations of drained triaxial compression and plane strain tests are presented for both dense and loose assemblies of polydisperse spheres using a periodic cell.In the work reported,drained tests were modelled by deforming the samples under constant mean stress conditions.The drained behaviour is shown to be qualitatively similar to published physical experimental results.The Bishop's formula for the estimation of the intermediate principal stress is evaluated.The existence of critieal density is shown to be independent of initial packing densities and strain conditions.Different failure criteria have been compared based on the DEM simulation results,and the Lade criterion is found to be the most appropriate one.A new microscopic fabric parameter is introduced to give insight to structural anisotropy under general 3D fabric conditions.It is found that two parameters characterize the evolution of the stress and fabric respectively independent of strain conditions.