Stability control of coal wall in ultra-large-mining-height working face with hard roof based on advance speed fffect

Addressing the critical challenges of coal wall spalling, roof collapse, and support crushing, which significantly impede the safe and efficient production of the Caojiatan 122104 ultra-large-mining-height working face characterized by a hard roof during rapid advancement, this study utilizes theoretical analysis to develop a comprehensive mechanical model of the support-surrounding rock interaction. Grounded in the Winkler elastic foundation theory and incorporating the influence of advancement speed, the analytical solutions for the deflection of the embedded coal wall and the corresponding roof load exerted on the coal wall are rigorously derived. To evaluate the stability of the coal wall, a safety factor is introduced, and the maximum depth of coal wall spalling is quantified across varying advancement speeds. A nonlinear regression analysis is conducted to establish the functional relationship between advancement speeds and the corresponding depth of coal wall spalling. Based on the roof instability patterns corresponding to various advancing speeds and the working resistance of the supports, the lower bound of the rational advancing speed was determined. The regulatory role of advancing speed and initial roof breaking step in governing coal wall stability is elucidated, and hydraulic fracturing is recommended as a targeted control measure for coal wall spalling in hard roof working faces characterized by high advancing speed. The findings reveal that as the working face advancement speed escalates from 1 m/d to 20 m/d, the ultimate span of the initial fracture extends by 4.04 m, the distance required for the coal wall to attain critical steady state lengthens, and the maximum spalling depth surges from 0.06 m to 1.85 m. Moreover, the rational advancing speed, determined in conjunction with the support working resistance under different advancing speeds, should not be less than 11.14 m/d. Taking a 5% increment in the coal wall spalling depth as the threshold, when the advancing speed exceeds 12 m/d, its regulatory impact on the stability of the coal wall diminishes. When the regulation of advancement speed attains its upper limit, taking into account the in-situ initial breakage span, the corresponding depth of coal wall spalling, and the ultra-long initial breakage spans of the previously exploited fully-mechanized caving faces in Caojiatan, the breakage span of the hard roof exerts a notable influence on the stability of the coal wall. Hydraulic fracturing can be employed on-site to weaken the hard roof, thereby reducing the roof weighting span and intensity, and improving the operating conditions of the support, with the aim of effectively curbing coal wall spalling.