Study On The Development Law Of Overburden Fracture Under Repeated Mining

China possesses vast coal reserves and has a significant demand for energy consumption due to its continuous social and economic development.This demand is further amplified by the increasing energy requirements of various industries.China has recently imposed stricter standards and requirements for coal mining technology.For the mining of multiple layers of coal,due to repeated mining,the coal face may be affected by nearby coal seam mining during the advancement process.When employing the downward mining method,the rock structure above the working face’s roof in the final coal layer may have already been impacted by previous mining activities.This presents a significant safety risk when attempting to mine the coal seam.When mining coal seams that are subject to repeated disturbances,it is crucial to carefully consider the selection of a suitable mining method and working face layout.In some cases,it may also be necessary to take proactive measures to ensure adequate protection.This paper provides a detailed analysis of the tectonic and hydrogeological conditions of the Yongcheng Xinzhuang Coal Mine,taking into account the geological conditions,coal seam occurrence characteristics,and hydrogeological information.To test the physical and mechanical parameters of the overlying rock in the working face,indoor experimental methods are utilized.By using a combination of theoretical analysis,numerical simulation,and on-site monitoring,in-depth research is conducted on the development laws of overlying rock fractures in two coal mining faces affected by repeated mining in Yongcheng Xinzhuang Coal Mine.The following conclusions and understanding were obtained:(1)In the process of mining multiple coal seams,repeated disturbance affects the stability of the overlying rock structure and the distribution characteristics of the overlying rock fracture field.Among them,the coal seam spacing,overburden lithology and combination structure,coal seam mining thickness,working face size,mining sequence and roof management methods are the main factors affecting the overburden failure and height guide development.(2)The first coal seam to be mined is not affected by repeated mining,and the development pattern of overlying rock fractures in its working face is consistent with that when only one coal seam is mined.The spacing of coal seams has a significant impact on the repeated disturbance effect,and the development height of overlying rock water conducting fracture zones increases with the decrease of coal seam spacing.The stress characteristics of the main key layer evolve from bending subsidence to fracture failure as the thickness of the coal seam increases.(3)Under certain conditions,as the working face advances,repeated disturbance causes secondary damage to the overlying rock of the first mining face.The vertical tensile fracture at the end of the starting mining line on the bottom plate of the first mining face is the main channel that connects the water conducting cracks of the upper and lower working faces.(4)The on-site borehole water level observation results of working faces 36021 and28091 show that the development height of the water conducting fracture zone in both working faces is higher than the theoretical calculation results,further verifying that repeated mining has intensified the damage degree of the overlying rock.