Field Investigation and Stability Analysis of Gob-Side Roadway Retention with an 8 m Coal Pillar in Extra-Thick Coal Seam Mining

To improve the stability of gob-side entry in the mining of extra-thick coal seams, we examined the field practice of reserving a narrow coal pillar with a width of 8 m in a 17 m thick coal seam. A combination of roof directional hydraulic fracturing and pouring concrete to consolidate the coal pillars was proposed and applied to reduce the deformation of the gob entry. First, roof directional hydraulic fracturing inhibits the vertical stress transmission to the key roof strata, and it transfers more vertical stress to the gob and reduces the lateral abutment stress in the roof. Subsequently, the narrow coal pillar is strengthened with a reinforced concrete wall, forming a strong–weak coupled bearing structure capable of bearing the overburden load, and the higher stiffness of the reinforced concrete wall effectively resists the lateral deformation of the coal pillar into the roadway. Additionally, the feasibility of this application was verified by comparing the results with the roadway convergence for a 43 m coal pillar on site. The results show that the maximum convergence, stability time, and advanced influence range were all significantly decreased by the proposed method, and the gob-side entry stability was improved. The proposed method and the results obtained provide a valuable reference for mining in similar conditions.