Rock failure modes under uniaxial compression, Brazilian, and point load tests

Rock failure is a serious problem in rock engineering environments. Rock failure modes, however, are complex and difficult to quantify or predict. A comprehensive study on rock failure modes at laboratory scale is, therefore, potentially important as it helps recognize the adequacy of the support designed on the basis of the nature of an engineering work. With due need, this paper analyzes the failure modes of granite, schist, and sandstone under uniaxial compression, Brazilian, and point load tests in relation to corresponding strengths. The nature of the principal failure mode changes from axial splitting to shearing along a single plane to multiple fracturing in the case of both granite and sandstone specimens as uniaxial compressive strength (UCS) increases. In the case of schist, specimens failed at low UCS show failure along foliations whereas specimens which do not fail along foliations portray high strength. The relation between failure modes of all three rocks under uniaxial compression and corresponding UCS values was broadly explained in terms of damage evolution of the rocks. Granite and sandstone specimens failed mainly following central or central multiple type of fracturing whereas schist specimens principally failed by layer activation in combination with either central or non-central fractures over the entire range of determined Brazilian tensile strength. In the case of granite and sandstone, central multiple failure mode corresponds to high tensile strength. Descriptions of different failure modes under point loading were presented. It was found that granite and sandstone specimens generally fail through the rock materials in one or more extensional planes containing the line of loading. Failure patterns showing triple junctions correspond to high point load strength indices. In the case of schist, specimens failed along foliations show a low point load strength index whereas specimens failed through material with a single extensional plane result in high strength.