Mechanical property and permeability of raw coal containing methane under unloading confining pressure

Based on domestic-developed triaxial servo-controlled seepage equipment for thermal-hydrologicalmechanical coupling of coal containing methane,an experimental study was carried out to investigate mechanical property and gas permeability of raw coal,under the situation of conventional triaxial compression and unloading confining pressure tests in different gas pressure conditions.Triaxial unloading confining pressure process was reducing confining pressure while increasing axial pressure.The research results show that,compared with the peak intensity of conventional triaxial loading,the ultimate strength of coal samples of triaxial unloading confining pressure was lower,deformation under loading was far less than unloading,dilation caused by unloading was more obvious than loading.The change trend of volumetric strain would embody change of gas permeability of coal,the permeability first reduced along with volumetric strain increase,and then raised with volume strain decrease,furthermore,the change trends of permeability of coal before and after destruction were different in the stage of decreasing volume strain due to the effect of gas pressure.When gas pressure was greater,the effective confining pressure was smaller,and the radial deformation produced by unloading was greater.When the unloading failed confining pressure difference was smaller,coal would be easier to get unstable failure.

Experimental Study on Seepage Characteristics of a Soil-Rock Mixture in a Fault Zone

A mixture of fault gouge and rubble taken out from a fault zone is used to prepare a S-RM(Soil-Rock Mixture)sample with rock block proportions of 20%,30%,40%,50%,60%and 70%,respectively.A GDS triaxial test system is used accordingly to measure the seepage characteristics of such samples under different loading and unloading confining pressures in order to determine the variation law of the permeability coefficient.The test results show that:(1)The permeability coefficient of the S-RM samples decreases as the pressure increases,and the decrease rate of this coefficient in the initial stage of confining pressure loading is obviously higher than in the semi-late period;(2)The permeability coefficient at different confining pressure levels presents a common trend as the rock block proportion is increased,i.e.,it decreases first then it increases(the permeability coefficient of the sample with rock block proportion 40%being the smallest,70%the largest);(3)In the stage of confining pressure unloading,the recovery degree of the permeability coefficient grows with the increase of rock block proportion(the recovery rate of S-RM sample with rock block proportion 70%reaches 50.2%);(4)In the stage of confining pressure loading and unloading,the sensitivity of the permeability coefficient to the rock block proportion displays the inverse“Z”variation rule(when rock block proportion reaches 60%,the sensitivity is highest);(5)In the stage of confining pressure loading,the relationship between the permeability coefficient and confining pressure can be described by an exponential relationship.