Parameter determination and effect evaluation of gob-side entry retaining by directional roof cutting and pressure releasing

Cutting the roof and releasing pressure to form a roadway is a crucial technical measure to guarantee the stability of the gob-side entry, and the parameters for cutting the roof are the core issue that requires consideration in the technique. This paper studies the engineering case of the gob-side entry on the 3405 working face of the Danyang coal mine. Theoretical calculation and analysis were conducted to obtain the range of roof cutting heights and roof cutting angles, numerical simulation method was used to simulate different roof cutting parameters. It is found that the roof cutting and pressure releasing technique can sever the cantilever beam structure of the main roof, segregate stress transmission between rock bodies, diminish deformation of the roadway, alleviate stress on the flexible formwork concrete wall, and enhance the stability of the gob-side entry. Furthermore, the appropriate height and angle of the roof cutting were verified to be 15 m and 15 degrees in the case and the effect of cutting was checked by using borehole viewer. On the basis, flexible formwork concrete wall stress and roadway deformation in uncut and cut roof were measured and analyzed in field test. The results show that the maximum stress on the flexible formwork concrete wall is reduced from 160 kN to 20 kN after the roof cutting, total roadway deformation reduced by around 40 %, the stability of the gob-side entry is effectively ensured. Meanwhile, the transformation laws of stress on the flexible formwork concrete wall were changed by cutting roof. It is transitioning from a three-stage pattern of rapid rise, slow rise, and stabilization before the roof cutting to a three-stage pattern of rapid rise, gradual decrease, and stabilization after the roof cutting.