Experiment on deformation failure and permeability evolution law of gas- containing coal under multi-stage unloading confining pressure

Abstract：Focusing on the No.7 coal seam of Datong mine,Chongqing Songzao C ＆ E Co.,Ltd.,by using self-made ＂thermo-fluid-solid coupling with triaxial servo-controlled seepage equipment of gas-containing coal＂,an experiment was carried out to investigate the deformation failure and permeability evolution law of multi-stage unloading confining pressure of gas-containing coal under different initial confining pressures and various gas pressures.The research results show that the coal is not immediately destroyed and out of stability after the unloading process begins,but maintains over σu1 for some time,and coal is out of stability after n stages of unloading confining pressure.Before coal instability occuring,the variable speed of deformation and permeability of coal is not the same during each stage of unloading confining pressure,and shows a trend of increasing speed.At each stage of constant confining pressure,with the lower confining pressure,creep speed and permeability of coal also are increasingly accelerated.The deformation and permeability in unloading confining pressure stage increases faster than inconstant confining pressure stage.Whether it is the process of unloading confining pressure or constant confining pressure stage,the confining pressure to reduce the transverse deformation caused by change speed is greater than the axial deformation change speed.