Experimental research on the mechanical character of deep mining rocks in THM coupling condition

The phenomenon of three "high" and one "disturbance" in deep rock masses is a prominent problem with the increase of the mining depth, which seriously threatens the stability of the rocks surrounding roadways and mining workplaces. For this study under thermo-hydro-mechanical (THM) coupling effect, the deep gneiss rocks in Hong Tou Shan Copper mine were chosen as the subject to explore the mechanical character of deep mining rocks. The experiment involved triaxial loading and unloading on the rocks under different conditions, and the failure mechanism of deep mining rocks under THM coupling effect was revealed. The research showed that the mechanical responses of gneiss in different experimental conditions were significantly diverse. The maximum strength, damage stress strength, and residual strength of gneiss during triaxial loading and unloading under THM coupling effect were the smallest. Also, under conditions of triaxial loading and unloading under hydro-mechanical coupling effect, thermo-mechanical coupling effect, and the conventional condition of triaxial loading and unloading, the strength of gneiss increased successfully. However, the deformation parameters of gneiss shifted clearly towards failure phase when this experiment was performed using THM coupling effect. That is, the drop in elastic modulus and the increase in Poisson's ratio were the largest. Meanwhile, a reduction in the shifting of deformation parameters of gneiss towards failure was seen in the following order: when the experiment was performed under triaxial loading and unloading with hydro-mechanical coupling, then under thermo-mechanical coupling and then under conventional triaxial loading and unloading. Also, the gneiss failure characteristics in every experimental condition were markedly different. Shear failure of conjugate oblique section happened in conventional triaxial loading and unloading condition, shear failure of the X-conjugate oblique plane happened in triaxial loading and unloading condition under thermo-mechanical coupling effect; tensional shear failure happened in triaxial loading and unloading condition under hydro-mechanical coupling effect, and shear failure of a single slope happened in condition of triaxial loading and unloading condition under THM coupling effect. Based on the coupling effect, the three fields formed on various conditions such as temperature, stress, and seepage impact, interact and constraint, influence each other mutually during deep mining of rocks. Reduction coefficient of confining pressure in relation to water pressure is introduced to build a mechanical model of THM coupling in deep mining rock. Then, gneiss failure characteristics are further discussed and analyzed, which provides a theoretical basis for deep engineering rock mass control under complicated environments.