Degassing a coal seam with in‐seam boreholes is an important method for mitigating the gas hazards in the underground coal mine. However, the low strength of the outburst‐proven coal limits the borehole sealing performance and borehole space maintaining, and thus influences the drainage performance of in‐seam boreholes. This study was conducted to seek a method to improve the sealing performance and borehole space maintaining for high‐efficiency CMM drainage. A visco‐elastic plastic model involving the plastic softening and dilatancy features for soft coal was proposed, and the deformation, shrinkage, and fracture characteristics of the coal surrounded a borehole and a roadway were analyzed. An enormous amount of connective fractures generated in the failure zone and the plastic zone, and the plastic zone develops timely for the creep behavior of coal, which aggravates the difficulties of borehole sealing. A comprehensive approach including the theoretical method and the technical method was developed to determine the proper sealing depth which can significantly influence the sealing performance. The sieve tubes made of high‐density polyethylene has been used to maintain the borehole space during the scheduled pre‐drainage period. A series of field tests were conducted in an outburst‐proven coal seam to verify the feasibility of the comprehensive approach and the borehole space maintaining method. Field tests showed that the comprehensive approach to determine the proper sealing depth and the installing of sieve tubes to protect the borehole space can provide favorable conditions for maximum CMM pre‐drainage from outburst‐prone coal and maximum utilization of the in‐seam boreholes.

Background: The pressure reduction range in traditional protective coal mining is often set conservatively, resulting in diminished actual pressure reduction effects near the mining boundary. Therefore, analyzing the stress and permeability evolution patterns at the mining boundary is particularly essential. Method: A three-dimensional numerical model was established according to the mining conditions of the 864 working face in T Mine to study the stress evolution, fissure development, and permeability evolution of the coal and rock mass overlying the protective seam mining, especially those near the mining boundary. Results: The overlying coal and surrounding rock mass near the mining boundary are in the state of increasing vertical stress and decreasing horizontal stress, and under this mechanical path of “increasing axial pressure and decreasing peripheral pressure”, the coal mass is damaged and destroyed, fissures are developed, and the permeability is increased; as a result, the coal and surrounding rock mass near the mining boundary mainly produce vertical longitudinal fissures, and the permeability can be increased 900 times compared with that of the overlying coal and surrounding rock mass in the mining boundary. After ground drilling and enhanced depressurization, the measured maximum gas content of the coal mass at the strike boundary is 3.25 m³/t, and the measured maximum gas content of the coal mass at the inclination boundary is 2.63 m³/t. Conclusions: After mining the protective layer, the permeability enhancement effect diminishes from the center toward the sides, yet remains sufficient to eliminate the risk of gas outbursts. This validates the importance of verifying permeability enhancement effects at coal seam boundaries.

As an efficient and low-power drilling process, the mechanisms of rock breakage and efficiency enhancement in rotary water jet technology remain unclear. Existing research has predominantly focused on steady-state effects, neglecting transient physical details such as turbulence evolution and vortex dynamics. Furthermore, quantitative data on how rotational speed affects near-wall flow and impact range are lacking, limiting precise process optimization and engineering applications. To address this, numerical simulations were conducted to investigate dual rotating water jets under varying speeds. By analyzing the development process, velocity distribution, and vortex evolution of the instantaneous jet, the dynamic characteristics and efficiency mechanisms of spin-type water jet drilling were systematically explored. Results indicate that impact efficiency is nonlinearly influenced by rotational speed, primarily through centrifugal-driven flow field restructuring. Increasing speed (n = 1–7) disrupts flow symmetry, induces tangential velocity, and enhances vortex stretching and breakup, thereby strengthening local shear. At 6 ms, the wall velocity gradient under n = 7 is markedly higher than under n = 0. In this study, a 10 MPa jet at n = 4 optimally balances impact range and intensity, improving rock-breaking efficiency. However, excessively high speeds increase energy dissipation; although impact coverage expands, effective impact capacity saturates, limiting further improvement in rock breaking and reducing energy utilization efficiency. These findings provide an important theoretical basis for parameter optimization and energy efficiency enhancement in spin water jet drilling technology.

In deep coal seam groups, pressure relief induced by lower protective seam mining does not necessarily lead to synchronous gas-drainage enhancement, making it difficult to define effective protection ranges and drainage timing. This study investigates the mechanical–hydraulic mismatch in the III1022 working face of Luling Coal Mine by integrating FLAC^3D numerical simulation, field verification of pressure-relief boundaries, roof–floor deformation monitoring, and surface-borehole drainage data within a unified advance-distance framework. An initial compaction compensation ratio, C_n(x), is proposed to normalize the formation criterion of strong pressure relief by comparing mining-induced expansion release with the initial normal compaction strain of the protected seam. The onset of drainage enhancement is further identified using a continuous piecewise-growth model of the total drainage rate Q(x). Results show that the protected seam exhibits staged and zoned responses. Once established, the strong pressure-relief zone evolves within a finite stable range rather than expanding indefinitely. The criterion C_n(x)=1 identifies the onset of strong pressure relief at 128.8 m, with a continuous strong pressure-relief interval of 128.8–1216.6 m. In contrast, working-face-scale drainage enhancement begins at 280.4 m, indicating a spatial lag of 151.6 m. Lag-corrected local conversion intensity reveals that drainage conversion is highly nonuniform and concentrated in preferential windows. These findings demonstrate that strong pressure relief is necessary but insufficient for effective drainage; hydraulic effectiveness depends on fracture connectivity, pressure-field reorganization, gas-source mobilization, and borehole interception.