Deformation and seepage characteristics of gas-containing coal under true triaxial stress

Conventional triaxial tests were mostly conducted to explore the coal and gas outburst mechanisms based on the moulded coal specimens, leading to the difference of three-dimensional unequal stress condition between the simulated coal seam and the real coal seams. A true triaxial gas-solid coupled coal seepage test system was developed to fulfil the gas seepage under the true triaxial stress. Moreover, the deformation-failure characteristics and gas seepage behaviours of moulded coal and raw coal were investigated under true triaxial stress. By analysing the relations among stress, strain and permeability in true triaxial tests, it has established the important effect of the intermediate principle stress (sigma(2)) on the occurrence of coal and gas outburst. The coal's yield strength increased and permeability decreased when sigma(2) was at the early and middle stage of the load, while the coal's permeability increased and instable failure happened when sigma(2) was at the late stage of the load. Particle flow code software was applied innovatively to simulate the seepage experiment, proving the reliability of experimental results. Meanwhile, the test standard based on raw coal was proposed. Furthermore, the present research results provide a new insight and approach for the prevention and monitoring of coal and gas outburst.