Theory and technology of partial backfill non-pillar mining in longwall faces of steeply dipping coal seam

The structure and stress environment of the surrounding rock at the longwall mining area in the steeply dipping coal seam are alienated, and the composition factors and disaster modes of the ”support and surrounding rock” system in the working face are affected. This leads to some frequent safety accidents in the working face, low coal extraction rate, and high roadway excavation rate. This paper analyzes the mining mechanics behavior of surrounding rock of in the steeply dipping coal seam and proposes a technological concept of partial backfill non-pillar mining in the longwall faces of steeply dipping coal seam. During the process of advancing the working face, partial backfill is carried out along the inclined direction in the lower part of the goaf. The filling body not only forms a gob-side roadway with the support effect of the roadway side, but also eliminates the protection coal pillar, achieving a non-pillar mining. At the same time, the length of the filling and compaction area at the lower dip of the working face is increased, enhancing the stability of the ”support and surrounding rock” system of the working face. Based on the characteristics of the longwall mining area in the steeply dipping coal seam, the partial backfill process of the steeply dipping coal seam paste was optimized and determined. And the partial backfill recovery system and mining and filling process in the steeply dipping coal seam were designed. By using a combination of theoretical analysis, simulation experiments, and numerical calculations, the regulatory mechanism of partial backfill on the mechanical behavior of surrounding rock at the longwall mining area in the steeply dipping coal seam was analyzed. The results show that the filling body affects the deformation and failure of the main roof rock beam, as well as the bearing characteristics of the coal and rock mass on the lower side of the mining area. The deformation of the main roof and the roof of the head entry, as well as the constraints on the coal and rock mass on the lower side of the head entry, all decrease with the increase of filling length. In order to prevent the hanging roof disaster in the unfilled area of the goaf, the filling length should not exceed 1/3 of the working face length. The partial backfill body restricts the fracture of the roof in the lower area of the working face, reduces the formation layer of the critical zone of the overlying rock, forms a stable roadway, and reduces the deformation of the surrounding rock gob-side roadway retained. At the same time, the length of the filling area at the lower part of the working face increases, and the length of the unstable area of the middle and upper surrounding rock structures decreases, improving the stability of the ”support and surrounding rock” system. The filling body changes the stress transmission path of the surrounding rock in the mining area, bearing part of the overlying rock load. The side abutment pressure on the lower side of the working face and the front advanced pressure both decrease with the increase of filling length, and the front support pressure in the lower filling area of the working face has the greatest decrease. The stress state of the gob-side roadway and the working face is improved. Technology of partial backfill non-pillar mining in the longwall faces of steeply dipping coal seam has the advantages of effectively improving resource recovery rate, reducing the roadway drivage ratio, alleviating the tension of mining and replacement, and strengthening the stability of the ”support and surrounding rock” system in the working face. It is of great scientific significance for guiding the green, safe, and efficient mining in the steeply dipping coal seam.