Application of rock brittleness indices

Abstract

Rock brittleness determines the failure behavior under loading. Various rock material properties were introduced, such as rock strength, rock toughness, and fracture toughness. These properties are of different definitions and practices. From the previous literature, forty-eight brittleness indices were summarized and characterized into eight groups based on different approaches: (1) strength parameters, (2) stress-strain curves, (3) elastic constants, (4) mineral compositions, (5) conventional well logging, (6) internal friction angle, (7) indentation tests, and (8) content of fines after impact. In Hong Kong, the most common rock tests are direct shear, triaxial, uniaxial, compaction, and various types of stress and strength tests. Two data sets are analyzed. These are uniaxial and triaxial compression test of Sichuan Marble at different temperatures and direct shear test and uniaxial compression test of granite of different maximum strength. Available parameters were obtained from the stress-strain curves, and seven brittleness were selected for brittleness calculation. B18, B19, B22, and B23 are based on compression stress-strain curves. B25, B27, and B28 are based on elastic constants. B18 and B22 decrease, while B23 increases with the increasing temperature, in the range of 50 􁂏 to 150 􁂏. The results show that B27 and B28 were in a high correlation while the others are not. B19, B27, and B28 of granite show that it is not accurate to correlate uniaxial compressive strength with rock brittleness. B25 increases with the increase of compressive strength, which is a reliable index in this context.