Pre-peak damage thresholds of different rocks in confined and unconfined conditions

Abstract

Evaluation of pre-peak damage threshold values of rock under different loading conditions is important in civil engineering and mining development projects. The main thresholds in rock shearing include crack closure, crack initiation and crack damage. In underground development projects the lower limit of the longterm strength is given by the crack initiation stress, while crack damage stress predicts the upper limit of the long-term strength. These thresholds may be affected by a number of factors such as confining pressure and rock strength. A large number of uniaxial and triaxial tests on different rocks with uniaxial compressive strength values ranging between 7-278 MPa were performed in this study. Axial and lateral strains were measured throughout the test from the beginning of loading until post-peak state. Several criteria were investigated to obtain rock damage threshold values accurately. Statistical analysis of the results showed that, volumetric strain is the most reliable parameter to estimate threshold values when plotted versus axial strain and axial stress. Furthermore, it was found that, in rocks having different strength values, on average, ratios of crack closure, crack initiation and crack damage to peak strength are 0.22, 0.37 and 0.73 respectively. Crack closure and crack initiation values which were normalized by peak strength generally increases with an increase in peak strength, however, the normalized damage stress decreases with peak strength increase. Effect of confining pressure on threshold values was found to be different in different rocks. In addition, crack damage stress decreases significantly during systematic cyclic loading.