Development and Application Research of a NovelBorehole Protection Screen Pipe for Preventing Borehole Collapse in Soft and Fragmented Rock Strata

High-level roof borehole is one of the core technologies for gas control in high-gas mines in China. However, in soft and fragmented rock strata, the influence of mining-induced stress disturbance often causes compression-torsion deformation, borehole wall collapse, or dislocation when the borehole passes through the coal-rock interface due to lithological differences. This results in blockage of the gas flow channel or even borehole failure. Existing borehole protection technologies generally suffer from issues such as heavy screen pipes, low construction efficiency, and difficulty in large-scale application. To address these problems, this study, based on the engineering background of Xin’an Coal Mine, developed a stainless-steel socket-type screen pipe with an “upper large, lower small” structure by systematically analyzing the necessity of borehole protection and the stress characteristics of protective pipes. A stepwise insertion method driven by the drill rig’s jacking system was adopted to achieve full-length borehole protection in soft rock strata. Meanwhile, the YZT-Ⅱ rock formation borehole detector was used to analyze borehole wall stability, and a comparative experiment between protected and unprotected boreholes was carried out at the 14230 working face of Xin’an Coal Mine. The results indicate that the rock formation detector identified the coal-rock interface as the high-incidence zone of borehole collapse, whereas the novel protective screen pipe effectively maintained borehole wall integrity in this zone. The gas drainage concentration of protected boreholes remained stable above 80%, with a pure extraction flow rate of ≥0.2 m3/min, and the total extracted gas volume was 8–10 times higher than that of unprotected boreholes (with extraction concentrations of 30%–45% and pure flow rates of 0.03–0.06 m3/min). Furthermore, based on a fluid-solid coupling model and field data, the optimal spacing between high-level roof boreholes under these geological conditions was determined to be 3.0–3.5 m, with an optimal number of three boreholes. The proposed novel screen pipe and corresponding construction technology effectively solve the problems of borehole collapse and blockage in high-level roof boreholes within soft and fragmented rock strata, significantly improving gas drainage efficiency and borehole utilization. This provides reliable technical support for gas control in mines with similar geological conditions and demonstrates broad application potential.