Calculation model of rock mass permeability in coal mine goaf and its numerical simulation analysis

Permeability of rock mass is a basic parameter of CBM extraction design in coal mine goaf. Starting from the analysis of deformation and failure characteristics of rock mass in coal mine goaf,through theoretical analysis,the cou- pling relationship and model between the permeability and stress of rock mass was studied,then the distribution regu- larity of stress,strain and permeability in the coal goaf was revealed. Research results show that the permeability of dif- ferent lithology rocks is considered as a function of strain in the stress-strain process,which depends on both the dam- age degree and the openness of fracture. Based on the openness of fracture caused by the stress-strain of rock mass in the goaf,the three dimensional relationship and model between the permeability and stress was deduced. Using the cal- culation software FLAC3D ,the simulated calculation was carried out in the stress-strain-permeability of rock mass in the goaf,the segmental characteristics of the deformation and failure of rock mass in the goaf was analyzed,which was formed into the bending subsidence zone,fractured zone and carving zone from top to bottom in the vertical direction and divided into the original in-situ stress area,abutment pressure compression area,unloading stress area and recovery area in the horizontal direction. The characteristics of the permeability distribution which is consistent with the stress- strain and failure law was revealed. Both the coefficient ratio of vertical permeability ( Kz / Kz0 ) and that of horizontal permeability (Ky / Ky0 ) increase with the increase of the vertical distance from mining coal seam,the permeability of rock mass in the goaf gradually decreases,and the permeability coefficient on the edge of the goaf is larger,while the permeability on both sides of the coal pillar zone decreases significantly.