Strength anisotropies in mudrocks

Abstract

Mudrocks exhibit a variation in compressive strength when the direction of the plane of weakness is varied with respect to the direction of the principal stresses. Uniaxial compression tests were performed on two anisotropic mudrocks (siltshale and mudshale). The sample laminations were at orientation angles of 0, 15, 30, 45, 60, 75 and 90° relative to the direction of loading. The anisotropy strength ratio (the ratio of the maximum compressive strength to the minimum strength) over the full range of the lamination orientations was determined. The results of the uniaxial compressive strength tests were compared with the strength anisotropy index ratio [IS(50)] determined from the point load strength tests. The difference between the anisotropy ratios from the individual point load test results and from the uniaxial compressive test results indicates the difficulty of determining the anisotropy from point load tests.