Acoustic emission based mechanical behaviors of Beishan granite under conventional triaxial compression and hydro-mechanical coupling tests

Abstract

Beishan area is now considered as a potential site of high-level radioactive waste (HLW) repository. In order to understand the mechanical behavior of Beishan granite, conventional triaxial compression (CTC) and hydro-mechanical (HM) coupling tests are conducted on Beishan granite. An acoustic emission (AE) monitoring system is employed to estimate the rock damage evolution during the tests. The testing results show that the compressibility of the rock specimens increases and expansibility decreases with increasing confining pressure under CTC tests, resulting in failure pattern transforms from splitting failure to shearing failure. Under HM coupling, the compressibility decreases and expansibility increases with increasing pore pressure, and the micro cracks can keep relatively equilibrium state due to the existence of fluid. Based on AE hit counts, the crack closure stress σcc, crack initiation stress σci and crack damage stress σcd are 12.3%, 48.7% and 81.1% of the peak stress σc under CTC tests, respectively. Under HM coupling, the crack closure stress σcc is vanished and the crack initiation stress σci is higher than that of CTC tests. In this sense, pore pressure can facilitate crack development once the crack damage stress σcd is reached. The fluctuation intensities of AE amplitude and frequency centroid (FC) increase during the loading process. The amplitude fluctuation range increases and FC decreases after peak during CTC tests. However, the amplitude fluctuation range reaches its maximum at peak stress under HM coupling tests, and FC decreases. Moreover, simplified models of failure process for crystalline rocks under compression and HM coupling are proposed.