Experimental investigation on crack initiation and damage stresses of deep granite under triaxial compression using acoustic methods

Abstract

Crack initiation stress and crack damage stress are two critical indices for assessing the fracture strength of rock mass. However, understanding the stress characteristics of crack initiation and damage under triaxial compression remains still immature. To address this problem, by acoustic monitoring, i.e. ultrasonic wave transmission and acoustic emission (AE), the integrated triaxial compression experiments were carried out on granitic specimens. The crack initiation and damage stresses were determined by wave velocity, wave amplitude and AE methods, respectively. The discrepancy of stresses for crack initiation and damage identified by these methods were examined. Results showed that the confinement affected the peak stress and corresponding strain, and these two parameters increased with increasing confining pressure. The ultrasonic wave velocity and wave amplitude first increased and then remained relatively stable, and finally decreased with increasing axial compressive stress. The number of AE events stayed at a relatively low extent until axial stress approached the peak; after that, the AE accumulative counts skyrocketed to the maximum. It also shows that for a given confinement, the stresses for crack initiation and damage identified by the wave amplitude method were the smallest, followed those by AE method and wave velocity method. Moreover, the stresses for crack initiation and crack damage identified by these methods increased generally with confining pressures. However, the rate of increment of these two crack stresses decreased with increasing confining pressure. In addition, the slight decrease in these two crack stresses ratios was noticed with increasing confining pressure. The findings are helpful to understand the crack stresses of deep rocks, in terms of support of deep underground engineering.

Keywords

• Crack stress
• Ultrasonic wave
• Acoustic emission (AE)
• Triaxial compression