Study on surface pre-grouting reinforcement layer of stratified regenerated roof under bifurcated coal seam

Affected by the mining of the upper layer, bifurcated and merged coal seams cause the roof of the lower layer to be broken and difficult to support, which seriously affects the safe and efficient mining of the lower seam. Pre-grouting reinforcement technology is often used to reconstruct the regenerated roof in advance to improve the integrity of the roof. The grouting reinforcement section has a direct impact on grouting effect. In order to provide a scientific basis for the selection of surface pre-grouting zone, the bifurcated coal seam of Xutuan coal mine in Huaibei mining area has been used as a study object. The damage height and depth of coal roof and floor during the upper seam mining were calculated by theoretical analysis and numerical simulation methods and the structure characteristics of regenerated roof of lower seam were obtained. Based on the numerical simulation, porosity and permeability coefficient at different positions of the gob were calculated and the spatial distribution law of porosity and permeability coefficient was achieved and the grouting characteristics and range of different positions in gob were determined. The results show that the development height of the roof collapse zone of the upper seam gob and fracture zone (“two zones”) in the upper layered mining is about 21 to 28 m, and the failure depth of floor is about 10 m. The lower layer regenerated roof is composed of the upper layered floor failure zone, collapse zone and fracture zone, and the rock mass structure is a cataclastic loose structure with poor integrity. The porosity of the caving zone in the gob is the largest at the open cut, end caving line and the roadways on both sides, which can reach more than 50%, and gradually decreases to less than 10% towards the middle part of gob and the fracture zone above the gob with “O”-shaped distribution on the plane. The permeability coefficient of the collapse zone and the fracture zone can differ by more than 3 orders of magnitude. The plane of the gob can be divided into easy grouting area, medium grouting area and general grouting area. It is determined that the distance between the coal seams on the plane is less than 10 m between the two sides of the roadway. The open cut and final mining line are the main grouting sections, and the vertical upper collapse zone is the main grouting section. The research results can provide a basis for the layout of grouting reinforcement holes and the selection of grouting zone in similar close distance coal seams.