Characterization of damage evolution in granite under compressive stress condition and its effect on permeability

Abstract

With the aid of 3D acoustic emission (AE) monitoring system, the cracking process of granite under compressive stress condition and its effect on the hydro-mechanical properties is experimentally studied. The granite is taken from Beishan area, a preferable region for high-level radioactive waste (HLW) disposal in China. The experiment results suggest that the rock failure and degradation of mechanical properties are essentially related to the propagation and coalescence of induced cracks. Using an anisotropic damage tensor proposed by Shao et al., the damage evolution during the whole loading process is studied according to the experimental data. It is revealed that the damage evolution is mainly initiated with the appearance of nonlinear mechanical behaviour, and accelerated close to the failure point and in post-peak region. The estimated damage variable in lateral direction is found be globally higher than the value in vertical direction. The recorded AE events indicate that cracking process of granite could be accelerated due to the existence of hydraulic pressure. As a result, much lower compressive strength is obtained under same effective confining pressure in hydro-mechanical coupling tests. A similar tendency of damage and permeability is noticed, and the permeability variation in granite is found to be negligible before the coalescence of microcracks. Finally, an empirical relation is proposed to describe the influence of damage evolution and confining pressure on permeability variation.