Abstract
To investigate the evolution of grain crushing during extremely large shear under constant confining pressure, a series of rotary shear tests was performed for two types of sands with various confining pressure, shear strain rate and total shear strain. Grain size distribution after the test was also measured for each sample. It turned out that (1) all the specimens reached an identical critical state with the same shear stress ratio, void ratio and gran size distribution described by a fractal (or power-law) distribution after a sufficiently large shear strain application, (2) the plastic compression occurred rapidly in the early stage of the test, and (3) the shear rate (from 0.21 to 210 (1/s)) did not affect the overall behavior. Based on such observation, a micromechanical model that connects the single grain crushing behavior with the bulk shear behavior was proposed. The model successfully described that a decrease in porosity with shear strain is a straight line in log - log plot, and that the threshold strain from where the plastic compression dominates is related to the ratio of confining pressure to the single grain crushing strength.
