Experimental study of coal deformation and permeability characteristics under loading-unloading conditions

Making raw coal as research objects,used domestic-developed triaxial servo-controlled seepage equipment for thermo-fluid-solid coupling of coal containing methane,with one pressure control mode of increasing the axial compression and unloading confining pressure to carry out coal and rock loading and unloading test,and analyzed the evolution of coal deformation properties and permeability characteristics under loading and unloading conditions.Results show that coal peak intensity under loading and unloading conditions is significantly lower than that under conventional triaxial compression test.The principal stress difference shows an apparently increasing trend during experimental process.The bigger the rate of the second unloading phase is,the greater the curve slope becomes,correspondingly,the faster the radial strain and volumetric strain increases,whereas,the smaller the peak intensity becomes.At the failure stage after peak value,both radial strain and volumetric strain decrease.Relations between coal permeability,principal stress difference and strain can be divided into three stages respectively,initial compaction and yield phase,the stage after yield phase,and failure stage.After the peak,coal permeability showed a sudden small increase,which once again appeared dramatically sharp increase after some time.Changes of coal permeability are closely related to the coal deformation damage evolution,and the relation between them can be expressed by a quadratic polynomial function.