Research on the Fracture Mechanism and Pressure Relief Control Technology of the Thick and Hard Roof in a Coal Pillar Recovery Working Face

Abstract

In coal mining, substantial main roadway pillars are typically discarded post completion, largely wasteful of resources and discordant with the idea of sustainable mining. To enhance the recovery rate of coal resources in a single mining area, it is essential to appropriately plan the main roadway coal pillar recovery (MRCPR) working face for resource recovery at the end. This study delves into the layout and positioning of the MRCPR working face, with particular emphasis on controlling the thick and hard roof (K2 limestone). The transverse O-X roof fracture pattern of the MRCPR working face is outlined, with three roof states before the initial fracture investigated. However, the roof is thick and hard, which creates a long-distance hanging roof structure during the mining process, and a stable structure suitable for withdrawing supports cannot be formed during the stopping process. Additionally, the coal pillars on either side are compressed due to the roof's inability to collapse laterally. Therefore, it is proposed that the thick, hard roof should be precracked and cut off beforehand. After roof cutting, the deflection increases by 157% (at 100 m), and the bending moments of the long and short sides are significantly reduced, which verifies the pressure relief effect and the deformation promotion effect of roof cutting. Simultaneously, the comparative model is established and the deviatoric stress index is introduced to evaluate plastic deformation. It is concluded that the abutment pressure and deviatoric stress of the stope are significantly reduced in the leading section and the coal pillars on both sides after roof cutting. Furthermore, the plastic deformation evolution process of the thick, hard roof is identified. Therefore, the proposal is that the roof is more susceptible to breakage due to its flat elliptical plate structure after being cut. This clarifies the mechanism of roof fracture after cutting. In the field, bilateral cumulative explosion is used to cut the roof, the depth of which covers the thickness of K2 limestone at an angle of 8°. Borehole peeping observations show that the blasting hole has a clear two-way crack after the roof cutting. Also, the coal wall's integrity in the working face is maintained during the mining process, ensuring safe and efficient coal pillar mining. This study will serve as a useful reference for green mining technology in coal pillar recovery, covering mode selection, layout basis, theoretical guidance, and parameter reference.