Study On Mechanical And Seepage Characteristics Of Gassy Coal Under True Triaxial Loading And Unloading

In the actual mining process of the coal mine,the coal body under three-way stress experienced the stress evolution process from the original rock stress,axial stress increases and the confining pressure decreases until the coal rock failure.The energy stored in coal rock will be sudden released due to the change of rock stress,which leads to gas outburst and casualties.Therefore,it is necessary to analyze the mechanical and percolation characteristics of gassy coal with different test conditions.In this paper,conventional triaxial loading,fixed axial unloading confining pressure and increased axial unloading confining pressure on gassy coal under different conditions were carried out,and the effects of confining pressure,gas pressure,unloading and loading rate and different stress paths on the mechanical parameters and percolation characteristics of gassy coal were systematically analyzed.The main research are as follows:(1)Conventional triaxial loading and different stress paths loading and unloading tests were carried out.The effects of different confining pressures,gas pressures and loading and unloading rates on the mechanical properties of gassy coal were analyzed.The results showed that when confining pressure and loading rate increased,the peak strength and peak axial strain of gassy coal increased,the expansion point of coal samples appeared late,the deformation modulus increased and Poisson’s ratio decreased.When gas pressure and unloading rate increased,the peak stress and axial strain of gassy coal decreased,the expansion point of coal samples appeared earlier,the deformation modulus and Poisson’s ratio began to decrease and rise rapidly under small strain conditions,respectively.In the conventional triaxial loading and increasing axial unloading confining pressure test,the coal sample was mainly shear failure;in the fixed axial unloading confining pressure test,the coal sample was dominated by tensile-shear composite failure.(2)Analyzing the energy evolution characteristics of confining pressure,gas pressure,loading and unloading rate about the deformation and failure of gassy coal under different stress paths,and then clarifying the influence of different conditions on the deformation and failure of coal samples.Based on the energy dissipation theory,the damage constitutive model of gassy coal under true triaxial loading and unloading was constructed.The influence of three-way stress and gas pressure on the deformation and failure of coal was considered,and the rationality of the model was verified.And the damage evolution curve of coal was obtained,with the increase of axial strain,it showed an ’ S ’ type growth.(3)The effects of confining pressure,gas pressure,loading and unloading rate and stress path on the mesoscopic failure and seepage characteristics of gassy coal were studied by numerical simulation.The results showed that,the permeability showed a downward trend before the yield point,then reached the minimum at the yield point,lastly,the permeability almost rises rapidly in the ’ plumb line ’ type in the post-peak stage.From the yield point to the peak point,the permeability increased slightly.In the post-peak stage,the permeability increased almost in a ’ plumb line ’ type.When confining pressure and loading rate increased,the time and mutation rate of permeability increase became slower,and the plastic strain of gassy coal at yield point and peak point decreased gradually.When gas pressure and unloading rate increased,the permeability of coal samples increased,and the plastic strain of gassy coal at yield point and peak point increased gradually.In the process of coal from yield point to peak point and then to post-peak failure,the seepage channel in coal gradually changed from vertical to irregular deformation around.The degree of crack penetration in the coal body determined its macroscopic failure mode.The better the internal connectivity of the crack,the wider the failure plastic zone was formed after failure.