It is well known that rock failure modes are complex and difficult to quantify or predict. No straightforward mathematical or numerical analysis model can ascertain the nature of fracture development in rocks. Consequently, investigations of failure modes in the laboratory appear to be the only viable option that could provide useful information concerning rock failure, which is of great concern in any rock engineering environment. In line with the author's involvement in several experimental studies in relation to this issue, this paper presents salient points relating to fracture patterns in granite, schist, and sandstone under uniaxial compression and indirect tensile tests, with reference to their respective strengths. Additionally, the influence of fine drill-holes (a circular channel across the diameter in the middle of the core in the case of uniaxial specimens, and a circular hole aligned along the core axis in case of indirect tensile test specimens) on the failure strength and failure mode of sandstone was explored.

Granite, schist, and sandstone from different parts of India (geologically belonging to Malanjkhand Granitoid, Singhbhum Shear Zone, and Barakar Formation, respectively) were investigated. It was observed that there is an apparent relationship between fracture patterns and corresponding strengths for all three rock types. This can be broadly explained in terms of damage evolution within the rocks under both compression and indirect tension. While exploring the influence of fine drill-holes on the compressive strength of the sandstone, it was found that such fine holes/channels do not play any defining role. This could be attributed to an overriding effect of inherent cracks/pores of a brittle porous rock like the sandstone investigated. Nevertheless, these fine drillholes/ channels seemed to have an influence on the failure patterns of the sandstone specimens. In the indirect tensile tests, it was found that the ring tensile test produces more consistent results than the Brazilian test. It was also observed that the ring tensile strength provides a better measure of the applied energy than the Brazilian tensile strength.

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