Postpeak Strength of Interfaces in a Stress-Dilatancy Framework

Laboratory sand-steel interface tests, using a range of sand sizes on a wide range of surface roughnesses, have been conducted using a direct shear apparatus modified to enable reliable measurements of both friction and dilation. The paper looks at the minimum interface strength after peak, termed here the postpeak strength, and assesses its dependence on roughness, density, and stress level. Its upper limit is the large displacement direct shear friction angle, related to but not equal to the critical state friction angle. When data are normalized by this value, they show linear dependence on the logarithm of relative roughness in the intermediate zone between smooth and rough. Once the roughness dependence of the postpeak strength has been allowed for, dilatant interfaces are found to follow classical stress–dilatancy relationships. It appears that there is no fundamental difference in the responses of sand-onsteel or sand-on-sand interfaces.