Experimental investigation on mechanical behavior and permeability evolution in coal-rock combined body under unloading conditions

Abstract

Coal-gas compound dynamic disasters remain to be unsolved, which has a huge impact on deep mining and coalbed methane (CBM) exploitation. In this research, a series of triaxial tests were conducted on gas-containing coal-sandstone combination samples under two types of unloading conditions. One is unloading confining pressure tests (UCPs), the other is unloading confining pressure-reloading axial stress tests (UCP-RAS). Simultaneously, the permeabilities in the mechanical tests were obtained. Our results demonstrated that the bearing strength decreased under unloading conditions compared to conventional triaxial compression (CTC). Mohr-Coulomb strength criterion was also applicable to gas-bearing coal-sandstone combined bodies under unloading stress paths. For gas-bearing coal-sandstone combined bodies, the peak strain in UCP and UCP-RAS was smaller than that in CTC. The lateral deformation in UCP-RAS was the largest, which showed strong dilatation characteristics. There is no significant difference between degree of damage of combined body under UCP and CTC test scheme. Under UCP-RAS condition, the coal part of combined body was more prone to present deformation and fracture, and the degree of damage was more intense. The permeability of coal-sandstone composite samples under unloading conditions was also mainly determined by the crack evolution in the rock part. Moreover, the crack evolution was synthetically affected by gas deposit condition, mining-induced stress and multiple stress paths.