When a sand or clay is sheared to large strains, it will pass through a peak shearing resistance and will generally decline in resistance until a value is reached where deformation can continue with no further change in resistance. This is the residual strength and an important characteristic of the residual strength is that it is independent of the original state of the soil. When a rock is sheared to large strains it too will pass through a peak resistance which will decline to a value where deformation can continue with essentially no change on shearing resistance, at least on a scale of displacement larger than the asperities on the resulting discontinuity. While this has also been called the residual strength, this terminology is mistaken because the resulting resistance is a function of the surface roughness developed along the rock discontinuity during the failure process. The term ultimate frictional resistance is preferred. The same rock brought to failure in different ways, under different environments, will have different ultimate frictional resistances that depend upon surface roughness. In the laboratory, surfaces of the same rock prepared in different ways that result in different roughness will also display different ultimate frictional resistances. Laboratory data on the ultimate frictional resistance of limestone from the Frank Slide, Alberta, are given. Alternate measures of quantifying surface roughness are discussed and correlations between surface roughness and ultimate friction are presented. The influence of wear on surface roughness is also discussed. Evidence from natural discontinuities in support of the concept of ultimate frictional resistance is provided by noting that the ultimate resistance along a natural bedding plane in the limestone is much greater than along a flexural slip surface. Flexural slip folding has reduced the roughness along certain bedding planes to values much less than those produced by shearing underformed bedding planes. © 1979.
