A three-dimensional discrete element (DE) code is developed based on the history-dependent Hertz-Mindlin contact model with the capacity to operate ellipsoidal particles with translational and rotational degrees of freedom and to handle rigid boundary conditions; relevant visualization technology is also accomplished. Fundamental mechanical behavior of particle contact in normal and tangential directions is investigated through interactions such as centric/eccentric impacting, rolling/sliding, depositing and collapsing of particles, parameters including time step, contact damping, background damping and mass scaling are carefully examined with dynamic and static simulation.

Numerical tests with up to several thousand particles, including isotropic, odometer and triaxial compression, are conducted to study macroscopically mechanical behavior of granular materials. The results agree well with those from laboratory tests qualitatively. In particular, a simplified pile penetration process is simulated to test the boundary effect for follow-up research.

Furthermore, discrete-continuum coupling techniques are reviewed from the perspective of computational multiscale modeling. In order to overcome the high requirement on computational resources raised by purely particle-based simulations and to study the discrete particle mechanics across several orders of magnitude in length scale, a granular-continuum coupling algorithm is proposed to incorporate DE and finite element (FE) domain calculations concurrently through "ghost" particles, which not only allows large domain of relatively "small" deformation to be accounted for, but also is capable of modeling localized regions with relatively "large" deformation. DE code and FE code are wrapped and assembled using object-oriented programming methodology to implement the coupling algorithm.

A benchmark problem is studied to verify the validity of granular-continuum coupling computation. The pile penetration test is again performed with the newly developed granular-continuum coupling scheme. It is promising to find that boundary effect has been alleviated significantly with tuned elastic continuum parameters at the moment. The granular-continuum coupling technique can be expected to be an effective method in providing a nearly seamless DE-FE transition region and lowering computational cost to simulate granular problems across several orders of magnitude in length scale.