Optimization and Field Application of Key Parameters for Gob - Side Entry Retaining by Roof Cutting Pressure Relief at Longfeng Coal Mine

Abstract

To mitigate resource waste caused by coal pillar - protected entries, tight mining - excavation replacement schedules and severe stress concentration in coal pillar zones, this study takes the No.120902 auxiliary air entry of Longfeng Coal Mine as the engineering prototype and adopts the gob - side entry retaining technology by roof cutting pressure relief on the goaf - side roof of the entry. To address the large surrounding rock deformation of entries induced by unreasonable roof cutting parameters, theoretical calculations are first performed to obtain the theoretical ranges of roof cutting height and cutting angle, and the internal support scheme for the No.120902 entry is formulated accordingly. Subsequently, FLAC3D numerical modelling is adopted to characterize roof displacement of the retained entry and vertical stress distribution within the solid coal rib under various roof cutting parameter schemes, and the optimal roof cutting parameters are ultimately determined. Field industrial trials are carried out to verify the reliability of the proposed parameter optimization strategy.The numerical results show that under vertical roof cutting conditions, roof subsidence can be reduced by 29.6% when the cutting height increases from 7 m to 9 m, while only an additional 13% reduction in roof subsidence is achieved as the cutting height rises from 9 m to 11 m. The roof cutting angle imposes a marginal effect on vertical stress of the solid coal rib, and the minimum roof subsidence of 30 mm is captured at a cutting angle of 15°. Therefore, the combined scheme with a cutting height of 9 m and a cutting angle of 15° is identified as the optimum configuration to sustain the stability of entry surrounding rock. Field observations reveal two symmetric upward - extending through fractures developed at a borehole depth of 8.67 m, and the strike of fracture propagation is consistent with the advancing direction of the working face, which confirms the satisfactory forming quality of on - site pre - split roof cutting. Measurements recorded by roof separation monitors indicate that rock deformation at two monitoring stations is fully stabilized at 190 m and 220 m behind the working face, respectively, with the steady separation magnitude of deep rock strata controlled below 33 mm. The effective containment of entry surrounding rock deformation substantiates the validity of the optimized roof cutting parameters .