Study On Mining Method Of Residual Concave-convex Thick Coal Body In Thin Bedrock Pre-mining Strip Top Strata

The coal pillar left by the top layer strip mining in the third underground mining has a good effect on controlling the surface movement and deformation.The special mining environment composed of the top layer coal pillar and the lower layer coal seam presents a "concave-convex"coal body in the longitudinal section.When the economic and technical conditions are mature and all the coal resources need to be mined,the abnormal distribution of stress in concave-convex briquette affects the safe mining of lower strata.Taking Jinqiao Coal Mine as the engineering background,this paper studies the concave and convex coal body under thin bedrock by means of theoretical analysis,empirical calculation,numerical simulation and field observation,and completes the selection of groove position and mining technology of 1308 working face.The observation of mine pressure on the spot is completed,and the selection of mining method of concave and convex briquette is reasonable through the field industrial test.(1)the development height of water diversion fracture zone after mining of concave-convex coal body is calculated by using the empirical formula in "Three-underground Mining Code(201 7 Edition)",based on the prediction method of key strata position and the engineering analogy method,and the development height of water diversion fracture zone after mining of concave-convex coal body is calculated.For the sake of safety,the maximum height of the water conductivity fracture zone is 50 m.After exploration,it is concluded that there is a thick clay layer at the thin bedrock,which can ensure the safety during the mining period.(2)Through the analysis of the mechanical model,it is found that when the groove is arranged in the middle of the top stratified coal pillar,it is least affected by the superposition of the additional load;In order to reduce the influence of additional load on the working face support,the mining height of the lower coal seam face should be small,and the fully mechanized top coal caving technology should be considered.(3)The comparison of the results of UDEC numerical simulation scheme shows that when the lower layered working face is arranged along the slot in the middle of the top layered coal pillar,the working resistance of the whole stope support is smaller than that of other arrangements,and the minimum height of the water diversion fracture zone is 42.55m.When fully mechanized top coal caving and layered mining are selected for concave and convex type coal,the working resistance of support is relatively small,and the resistance distribution is also relatively uniform.When the fully mechanized mining technology with large mining height is adopted,the development of water diversion fracture zone is larger,which is not conducive to the control of water diversion fracture zone.however,the development of conductive height of layered mining is basically consistent with that of fully mechanized top coal caving mining,and the fully mechanized top coal caving technology is finally selected according to the amount of roadway excavation and the complexity of technology.(4)According to the analysis of the measured results of the field mine pressure,the working resistance of the whole support in the working face is generally saddle-shaped,and the working resistance of the support in the middle of the working face(under the top layered goaf)and along the trough at both ends is relatively low,with the acceleration of the advancing speed of the working face,the resistance of the support under the solid coal and the top layered goaf decreases,but the working resistance of the support under the top layered goaf changes more significantly by the advancing speed.Due to the use of fully mechanized top coal caving technology,the periodic pressure is relatively relaxed,the average periodic pressure step distance is 18.7m,the overall dynamic load coefficient is 1.12,and the utilization rate of the front pillar of the integral support(79.89%)is close to 80%.There is no water gushing accident in the working face during mining.The results show that the selection of working face location,mining technology and support are better.