| Three-dimensional micromechanical computational studies of assemblies of ellipsoidal particles have been presented. Uniform or they are specified size distributions of particle assemblies are pluviated into the analysis domain by means of gravity with specified boundaries or isotropically compressed under displacement or traction boundary conditions. The technique for detecting and defining contacts between the ellipsoidal particles is described. The constitutive or stiffness behavior at the particle contacts is described by standard nonlinear elastic Hertz and load-history dependent elastic-plastic slip Mindlin models. A technique for handling both displacement and traction boundaries is developed. The traction boundary is devised by means of a simulated flexible membrane system, while the rigid displacement boundary is implemented either in terms of force or displacement control. A new integration algorithm is devised specifically for static analysis of granular materials, which shows increased performance in obtaining accurate and stable positions for all particles in the assembly. Numerical simulations of true-triaxial cubical, axisymmetric triaxial-prismatic, direct simple shear, strip footing tests and lateral earth pressure tests are performed, where the intermediate principal stress (traction) is varied between the major and minor stresses, or where different pure displacement or mixed traction-displacement conditions are applied. The number of ellipsoidal particles present in the various three-dimensional experiments were in the range of 300 to 1,000. Most of the simulations required between 20 to 30 hrs. to perform on modem, high-clockspeed and large memory PC-machines. The nominal stress-strain, strength and volume change behavior for the various tests are reported. The effects of both smooth and frictional walls (displacement boundaries) are studied. In general, the analysis results compare favorably with data obtained in related laboratory tests. Sensitivity studies involving varying particle properties (Young's modulus, Poisson's ratio, and Coulomb friction) and time-step sizes are described. This development is used to simulate the strength, stiffness and dilatancy behavior of cohesionless granular materials subjected to a wide range of confining stresses and to gain insight to the role of particle rotation at very low stresses. |
