Triaxial shearing of an assembly of sand grains is numerically simulated using X-ray CT imaging and FEM concurrently. The actual morphology of particles, geometric contacts, and arrangement of solid, void phases are captured using X-ray CT imaging. A novel framework is introduced wherein a realistic assembly of sand grains is discretized using shell-based finite elements to solve the boundary value problem. The shell elements are based on Mindlin-Reissner flexural theory and corotational velocity strain formulation which are well suited for problems involving large rotation. This approach is computationally less expensive compared to the finite discrete element approach where sand grains are discretized using solid finite elements. The latex membrane is modeled as hyper-elastic material to provide a flexible boundary condition for applying confining pressure.