When sliding takes place on a foundation with significant, asymmetric topography, the resulting effective friction angle is dependent upon this three dimensional “roughness” in the same way that a 2-D strength envelope is dependent on dilatancy angle i. In both cases, there is considerable strength gain over a smooth test pair. The addition of lateral constraints further increases test sample shear strength in the 3-D case, because the block is forced to dilate upward when it would prefer to dilate laterally. In this paper, we examine this strength gain as experienced by an asymmetric three-plane “monolith” sliding over its matching aluminum foundation block. The test pair is first sheared without any lateral constraint, next with moderate lateral constraints, and finally with rigid lateral constraints, which force the monolith to move linearly forward. The practical implications of the increase in available friction angle are discussed in the context of a gravity dam foundation.
The linear envelope known as the Mohr-Coulomb failure criterion is a simple, practical, relationship between normal and shear forces. Patton’s (1966) introduction of the roughness parameter i enabled the model to handle an entirely new range of surfaces, namely, those with symmetric macro-scale topography. The roughness coefficient i loses its physical meaning when sample topography is strongly asymmetric. However, if we assume that the failure envelope is still reasonably linear, the interpretation of envelope slope as the tangent of the effective friction angle continues to hold. The question of whether a linear failure envelope is appropriate for strongly asymmetric samples is beyond the scope of the present discussion (see Grasselli & Egger 2003). The point is that by making this assumption, we retain in feff a convenient means of comparing the sliding resistance of different sample configurations.