Abstract:
In underground coal mining, a large number of safety coal pillars are often reserved to ensure the safe excavation of roadways and the mining of working faces. To improve the overall recovery rate of coal resources in mines, timely coal pillar recovery is required at the end of mining in a region or the entire mine. Among these, arranging wall-type working faces for coal pillar recovery is a critical method. This study summarizes the main types of coal pillars reserved in underground coal mining, identifies the scenarios suitable for arranging wall-type working faces for recovery, expounds the application advantages of wall-type working faces in layout modes, mining technologies, and combined recovery of multi-type coal pillars, and proposes layout methods using reused roadways, coal-pillar roadway driving (wide coal-pillar roadway driving and gob-side roadway driving), and comprehensive approaches. Additionally, six application types of wall-type working faces for coal pillar recovery are summarized: longwall, shortwall, gradient, knife-handle, multi-section, and segmented types. Aiming at shortwall working faces for coal pillar recovery, the characteristics of roof structure fracture evolution are analyzed, and the challenges under self-properties and degraded environments are clarified, including difficult roof fracture, high weighting intensity, large overhanging roof areas on both sides, and high abutment pressure at the ends. Therefore, a recovery method of “pre-pressure-relief before mining” for wall-type coal pillar working faces is proposed based on roof pre-splitting pressure-relief technology. Through theoretical analysis using plate structure theory, the roof fracture structures are found to exhibit a transverse “O-X” type without pre-splitting, a transverse “C-X” type with pre-splitting only at the open-off cut, a transverse “ↀ-X” type with pre-splitting at both mining roadways, and a transverse “D-X” type with pre-splitting at all three roadways. Further comparative simulation analysis shows that the roof fracture distance and stope stress decrease with the increase in pre-splitting position and degree. By introducing Von Mises equivalent stress, the plastic deformation trend before roof caving is obtained, verifying the proposed fracture trend structure. Meanwhile, combined with engineering cases, the pressure-relief effect of roof pre-splitting on the mining of wall-type working faces for coal pillar recovery is analyzed, providing references and guidance for their layout methods, roof fracture theory, and pressure-relief mining approaches.