Influence of true triaxial loading-unloading stress paths on mechanical property and permeability of coal

During the process of deep coal mining,coal seams are generally under the true triaxial stress state (σ1 > σ2 >σ3 ),which is influenced by the crustal stress and strong excavation disturbance. The results of conventional traxial stress tests (σ1 >σ2 = σ3 ) could not correctly describe the in-situ condition. In order to more accurately replicate the stress environment of coal seams,the self-developed new multi-functional true triaxial fluid-solid coupling experiment system was applied to investigate the mechanical properties and permeability evolution of coal under true triaxial load- ing-unloading stress paths. The results show that the true triaxial loading-unloading stress path has a great influence on the deformation and failure behavior,strength characteristics,permeability evolution. Compared with the CCT stress path,the LUT and LUUT stress paths reduced the peak strength of coal. The octahedral shear stress and the effective mean normal stress of coal can be linearly fitted. The failure modes of coal under different true triaxial loading-unloa- ding paths were composite tensile-shear failure. Additionally,true triaxial stress path has great influence on the perme- ability evolution of coal.