Experimental Study of Mechanical and Permeability Characteristics of Raw CoalUnder Loading-unloading Stress Conditions

Taking the natural coal drilled from C1 coal bed of a coal mine of Sichuan Coal Group Furong Company as research object, we investigated the mechanical and permeability characteristics of raw coal under different loading-unloading stress conditions based on the triaxial servo-controlled seepage equipment for thermal-hydrological-mechanical coupling of coal and rock(THM-2). The results showed that: under the same loading-unloading control point, with the increasement of loading-unloading rate ratio, the volumetric strain decreased, but the axial strain and the radial strain increased, the failure pattern of coal gradually developed from several macroscopic fracture to single macroscopic fracture. Under the same loading-unloading rate ratio, with the increasement of loading-unloading control point, the intensity of coal increased, but the failure angle of coal decreased. During the yield stage, with the increase of loading-unloading rate ratio, the response variation of permeability decreased with a linear relationship. However, the response variation of strain from loading-unloading control point to permeability transformation point increased with a linear relationship.