Variability of crack initiation and crack damage for various rock types

Abstract

The failure process of rocks can section the stress-strain relation to several stages from crack closure to crack propagation. Two important threshold stresses can be identified, which are normally referred to as crack initiation stress (CI) and crack damage stress (CD). The effects of rock type, grain size, porosity, and confinement on these two stresses are investigated in this study by statistically analyzing 926 sets of test data which are collected from previously published compressive tests. The CI and CD to peak strength ratios are found to be in a narrow range for various rock types in terms of igneous, metamorphic, and sedimentary, indicating that the ratios of the two stress against the peak strength are not greatly influenced by the rock type. The CI and CD decrease as the grain size increases for igneous rocks in uniaxial compression. The two threshold stresses for low porosity igneous rocks are not significantly affected by porosity. However, the two stresses show an obvious decreasing trend with the increase in the porosity for high porosity sedimentary rocks. The CI to peak strength ratio in confined condition is much higher than that in unconfined condition. In addition, the ratio under confinement higher than 50 MPa is much increased when compared to that under confinement lower than 50 MPa. The results imply that the confining stress constrains the microcracking process. Overall, the CI and CD to peak strength ratios are both in a narrow range, which may be used as an intrinsic property of rocks to evaluate the damage/failure behavior of rocks in physical laboratory test and estimate the spalling strength of rock mass in underground openings.