Study on overburden and surface damage characteristicsof shallow-buried coal seam mining in loess gully area

The discontinuous cracking damage on the surface of the shallow-buried coal seam mining in loess gully area not only caused many ecological and environmental problems,but also increased the difficulty of ecological restoration,which seriously threatened the ecological security of the mining area. The discontinuous deformation of the surface of shallow coal seam mining is the result of overlying strata failure movement under special geological conditions. In order to more systematically study the influence law of overlying strata failure on surface discontinuous damage,taking No.125203 working face in Shanbei Anshan minefield as the geological prototype,by using the method of combining discrete element numerical simulation and physical similar mate-rial simulation,the characteristics of overlying rock and surface failure,rock block size distribution and force chain evolution were simulated and analyzed. The law of influence of the evolution of the overlying force chain on the distribution of the blocks and the development of ground fissures was revealed,and the controlling effect of the overlying broken blocks on the discontinuous damage of the surface was clarified. The research results show that the vertical deformation and horizontal movement of the overlying strata in the shallow-buried coal seam mining in the gentle slope area of the loess gully area are greatly affected by the terrain. The failure of the overlying strata forms a double-layered block structure. The stratified interface is 11.0 m from the coal seam floor. The average length of the lower stratified block is less than 5.0 m,and the upper stratified block is more than 5.0 m. The average length of the overlying strata block increases exponentially with the distance between the rock strata and the coal seam floor. The average length of the surface blocks increases gradually from the bottom of the slope to the top of the slope.The rock block structure of layered caving and compaction in the lower strata and the force chain structure of stress concentration affect the distribution of blocks and force chains in the upper strata and the development of ground fissures,and the average spacing of ground fissures is basically consistent with the average spacing of upper stratified stress concentration points.The average spacing of ground fissures along the slope,ditch bottom and reverse slope in the main ditch is about 1.10 times,0.75 times and 1.95 times the average spacing of the stress concentration points in the lower strata,respectively,which is about 2.93 times,2.00 times and 5.18 times the average length of the rock blocks in the lower strata. The displacement model of “block voussoir beam” can well describe the subsidence and movement process and characteristics of the rock block at the bottom of the lower strata,and then reflect the double-stratified structure characteristics of the overlying strata block and the damage effect on the surface. The research results can provide a new scientific basis for subsidence control and restoration under the condition of shallow coal seam mining in loess gully area.