Study On The Deformation And Failure Of Overlying Strata And The Evolution Of Permeability In Shallow Coal Seam Mining

The mining of shallow coal seams often causes great damage to the overlying strata.The damaged area provides Water-conducting for groundwater flow into the working face,and cause surface deformation,which poses a serious threat to the safety of mine production.However,there is a huge challenge in monitoring and mastering the damage height and permeability changes of the subsurface overlying rock.In this study,taking the work face of S12003 in Ningtiaota coalfeild as an example,a three-dimensional geological model has been established by numerical simulation and a coupled strain-fracture-seepage calculation is carried out.Combining with the remarkable characteristics of surface deformation in the process of shallow coal seam mining,the reliability analysis of the model was carried out in two aspects:engineering mechanics calculation and seepage calculation based on surface settlement and water level monitoring data.On the basis of calculation results after verified,the ground surface deformation,overburden mechanical failure,permeability change and pore pressure change were analyzed,and the following research results were mainly obtained:(1)The ground subsidence area of the goaf is distributed in an elliptical shape as a whole,and the subsidence area is larger than the scope of the goaf.After the mining face advances 1000 m,the theoretical maximum settlement value of the center is about127 mm.When the distance from the mining face is less than 340 m,the ground surface begins to respond with obvious settlement.After the mining face passes 80 m,the surface settlement rate reaches the maximum value of 2.65mm/d,and then the settlement rate gradually decreases.The surface settlement process of the mining area can be divided into three stages: no impact,increase in settlement rate and decrease in settlement rate.The settlement process is a long-term change process.(2)The compressive stress concentration area in the overburden in-situ stress field of the mined-out area is distributed in the shape of an arch bridge,and the tensile stress concentration area is distributed in a saddle shape.According to the stress state,the entire stress change area can be divided into two-way compressive stress area,tensile-compressive stress area,bidirectional tensile stress zone and initial stress zone.(3)During the advancing process,the area of the plastic failure zone increases with the advancing length.Combined with stress and displacement field analysis,the height of the water-conducting fissure zone is 57 m,which is basically consistent with the calculation result range of the empirical formula for fully mechanized caving.The research method is more accurate.In terms of distribution,the shape of the plastic failure area is a regular trapezoid above the goaf.(4)Under the condition of Ningtiaota coalfeild,according to the characteristics of permeability changes,the entire overlying rock permeability change area can be divided into a obviously permeability increased area,a slightly increased permeability area,a surface permeability increased area,a horizontal permeability increased area,and an initial permeability zone.The vertical and horizontal permeability have the largest changes in the area where the permeability has greatly increased,reaching two orders of magnitude.(5)After mining,the pore pressure of the surrounding rock in the goaf is significantly reduced,and the range continues to expand as the length of the mining face advances.The entire pore pressure reduction area is distributed in an inverted trapezoid shape,and there is a leading influence area in front of the mining face.