Mechanical behaviors and permeability characteristics of granite considering through-crack inclination and seepage gas pressure

Abstract

In the construction process of tunnel excavation in a deep stratum, the cracked surrounding rock is prone to instability, deterioration and harmful gas overflow under the action of high ground stress and high osmotic pressure. To identify the interaction and influencing mechanism between excavation unloading and high-pressure gas seepage. Firstly, mechanical tests under the coupling effect of gas seepage and triaxial loading were carried out on the specimens with different through-crack inclination angles (TCA) and seepage pressure of crack (SPC). The load and permeability of the specimens were monitored, and the strength degradation, deformation characteristics, and failure mode evolution of the specimens were analyzed. The results show that the risk of the deterioration and the damage for the cracked granite during unloading excavation is positively correlated with TCA and SPC. While, the deformation velocity of the cracked granite is negatively correlated with TCA and positively correlated with SPC. As TCA increased, the failure mode evolves from a single weak plane shear failure mode to a double parallel plane shear failure mode with end-face initiation. Furthermore, SPC accelerate the propagation and development of new cracks along the direction of the increased stress when TCA = 70°and 80°. Secondly, when TCA < 90°, before the failure of the cracked rock mass a sharp increasing in the permeability during axial loading and radial unloading has been occurred. Meanwhile, the growth rate of gas permeability increased significantly during the rock mass destruction process, and the risk of toxic, gas outbursts and accumulation increased. Finally, a permeability evolution model considering the damaging effects of crack inclination, unloading confining pressure, and seepage gas pressure was established based on the Weibull distribution theory. The reliability of the proposed model was verified by combining the experimental results and the existing model, the evolution law of the specimen crack development degree’s sensitivity coefficient (β) to coupled damage was analyzed.