Influence of roof cutting depth on stability of gob side entry in thick coal seams with a bulky limestone roof

For the gob-side entry retained by roof cutting, the roof pre-splitting depth is directly related to its stability, especially under the condition of fully mechanized mining in 4.5 m thick coal seam underneath 13.7 m thick limestone roof. Due to insufficient investigation, the Yongning coal mine 10202 gob-side entry retained by roof cutting experienced large rock pressure, causing severe prop deformation and sharp reduction in the cross-section of the entry, which greatly affects its reuse. Extensive field measurements were conducted to analyze the severe rock pressure in the 10202 gob-side entry retained. Two typical models of overburden structure after roof pre-splitting were established, and they were used to study the effect of pre-splitting depth on the stability of gob-side entry retained in terms of the roof subsidence and the support resistance, respectively. A numerical investigation of the effect of roof pre-splitting depth on the stress distribution near the gob-side entry retained was carried out. Afterwards, the roof pre-splitting parameters for the 10204 gob-side entry retained were optimized, and they were validated through field experiments. The results indicated that insufficient pre-splitting depth is the main factor causing the severe rock pressure of the 10202 gob-side entry retained. The roof subsidence and support resistance after main roof pre-splitting were reduced by 35.2% and 24.8% compared with immediate roof pre-splitting, respectively. As pre-splitting depth increased, the stress-relaxation area expanded, but the pressure-relief effect weakened beyond 20 m. As the roof pre-splitting depth increased, the weighting interval increased, but the working resistance decreased, which indicates that the goaf is almost densely filled by the collapsed roof and the main roof has no space for subsidence movement. Thus, the rock pressure behaviors of the 10204 gob-side entry retained were milder than that of the 10202 gob-side entry retained.