Discrete Simulations of Laboratory Loading Conditions

It is well known that all three principal stresses play a role in the stress-strain-strength-volume response of solids and granular materials, yet conventional triaxial compression tests and direct shear tests are typically used for the determination of design parameters for granular materials, even when field conditions may be plane strain (e.g., behind a long retaining wall). The effects of loading conditions on the macroscale response of granular materials have been studied extensively using experimental methods and it is relatively straightforward to quantify the variations in material macroresponse using continuum constitutive equations. However, these methods provide relatively little insight into the driving micromechanics that govern macroscale behavior, particularly in soils that fail via regions of high localized strain (e.g., shear banding). Thus, it is desirable to develop a set of models that reproduce the expected macroscopic behavior and allow insight into the governing microscale mechanics. To this end, a series of numerical experiments has been performed to assess the effects of loading (i.e., boundary) conditions on particulate material response. Three-dimensional discrete numerical specimens, with the same material and model properties but different problem geometries, were assembled to similar void ratios and confining stresses and then subjected to plane strain compression, conventional triaxial compression, and direct shear loading conditions. Macroscale responses, including deviatoric stress (plain strain and conventional triaxial compression) or stress ratio (direct shear), volumetric strain, friction angle, and dilation angle of the specimens are investigated. Shear strength relationships of the three different loading conditions are explored. Small-strain response and the volume change response are studied. Simulated material response is generally in good agreement with previously published results from laboratory investigations though some significant differences are noted and discussed. Given that the models reasonably produce boundary condition effects, it may be possible in the future to exploit the discrete nature of the simulations to observe changes in material microstructure under different boundary conditions and to infer how micromechanics influences the macroresponse of the assemblies.