Shear strength, interparticle locking, and dilatancy of granular materials

The effect of particle angularity on the strength and dilation of granular materials is investigated through a series of laboratory tests on two materials, Ottawa standard sand (Sand O) and crushed limestone (Sand L), that are made up of rounded and angular particles, respectively. Triaxial tests on both materials at different confining pressures and initial void ratios show that particle angularity has a substantial effect on both the peak friction angle phi(p) and the mobilized friction angle at the onset of dilation, phi(f). It is found that phi(f) is smaller than the critical friction angle phi(cv) for Ottawa sand; nevertheless phi(f) is larger than phi(cv) for Sand L owing to interparticle locking induced by particle angularity. The experimental results clearly show the contributions to shear resistance from both dilation and interlocking, with interlocking still largely existing at the peak stress ratio but not at the critical state. Suggestions are made to modify the stress-dilatancy formulations for sand to take into account the effect of interparticle locking associated with particle angularity.