Coal Pillar Size Determination and Surrounding Rock Control for Gob-Side Entry Driving in Deep Soft Coal Seams

Abstract

In response to the large-scale instability failure problem of designing coal pillars and support systems for gob-side entry driving (GSED) in high-stress soft coal seams in deep mines, the main difficulties in the surrounding rock control of GSED were analyzed. The relationship between the position of the main roof breaking line, together with the width of the limit equilibrium zone and a reasonable size for the coal pillar, were quantified through theoretical calculations. The theoretical calculations showed that the maximum and minimum widths of the coal pillar are 8.40 m and 5.47 m, respectively. A numerical simulation was used to study the distribution characteristics and evolution laws of deviatoric stress and plastic failure fields in the GSED surrounding rock under different coal pillar sizes. Theoretical analysis, numerical simulation, and engineering practice were comprehensively applied to determine a reasonable size for narrow coal pillars for GSED in deep soft coal seams, which was 6.5 m. Based on the 6.5 m coal pillar size, the distribution of deviatoric stress and plastic zones in the surrounding rock of the roadway, at different positions of the advanced panel during mining, was simulated, and the range of roadway strengthening supports for the advanced panel was determined as 25 m. The plasticization degree of the roof, entity coal and coal pillar, and the boundary line position of the peak deviatoric stress zone after the stability of the excavation were obtained. Drilling crack detection was conducted on the surrounding rock of the GSED roof and rib, and the development range and degree of the crack were obtained. The key areas for GSED surrounding rock control were clarified. Joint control technology for surrounding rock is proposed, which includes a combination of a roof channel steel anchor beam mesh, a rib asymmetric channel steel truss anchor cable beam mesh, a grouting modification in local fractured areas and an advanced strengthening support with a single hydraulic support. The engineering practice showed that the selected 6.5 m size for narrow coal pillars and high-strength combined reinforcement technology can effectively control large deformations of the GSED surrounding rock.