Effect of Pre-Loading with Tensile Stress on Laboratory UCS of a Synthetic Rock

Amongst various index properties used by rock engineers, the uniaxial compressive strength (UCS) is probably the most widely used property of intact rocks. In the field, in situ stresses are always present, and, when samples are extracted from such rocks, the stresses are released. These specimens, when tested in the laboratory, may give low strength values in comparison to what would have been obtained without stress removal. It is difficult to exactly follow the stress path of coring of rock in a natural stress environment; an indirect way of simulating effect of stress removal has been adopted in this study. Specimens of a synthetic rock have been subjected to pre-loading with tensile stress, and qualitative assessment of the reduction in the laboratory UCS of the rock has been made. To induce tensile stresses, conventional Brazilian test apparatus was modified. The results indicate that if pre-loading tensile stress exceeds about half of the Brazilian tensile strength, stress removal has substantial effect on the UCS and conservative results may be expected from laboratory tests on rock specimens.