Quantifying Rock Strength Variability Under Different Tests and Failure Modes

Abstract The influence of rock strength variability on discontinuity stability is often not considered, because slope stability and discontinuity shear strength models generally assume uniform strength conditions. In addition, a lack of budget and access to material often restricts engineers from adequately characterising rock variability with extensive laboratory testing. The strength of rock is highly variable, and slope stability analysis must account for variability for accuracy, and to minimise over-conservativism in design. An extensive laboratory testing regime comprised of unconfined and triaxial compressive strength tests, Schmidt hammer tests, and tilt tests was conducted on three rock types: a limestone, a sandstone, and a granite. The purpose of extensively testing these materials was to compare the variability of each test type to find economic methods for determining variability, and to compare how different tests react to material strength variability. The Schmidt hammer and tilt tests were found to be useful tests to predict the variability of UCS tests and triaxial tests at high confining stress, respectively. The variability of triaxial compressive strength was observed to decrease as the failure mechanism transitioned from brittle failure to the transition zone of brittle–ductile failure when confining stress had increased sufficiently. Whilst some parameters’ variability was characterised with many tests, the variability of the Hoek–Brown intact material constant was determined by a statistical bootstrap analysis of a single data set. Finally, a brief review of current literature found that the basic friction angle, the Hoek–Brown intact compressive strength, and the Hoek–Brown intact material constant can be assumed to be independent of each other during slope stability analysis, where extensive site-specific calibration or tensile testing cannot validate any relationship between each parameter.