Deformation law of overlying rock strata during the initial mining period of a coal mining face: mechanical analysis

The deformation and failure behaviour of overlying rock layers during the initial stage of coal seam mining directly affects the stability control of surrounding rock and the prevention of dynamic disasters in the mine. To address the unclear mechanism of overlying rock structural instability and insufficient quantitative characterization of the evolution process under high-gas and low-permeability coal seam mining conditions, mechanical models were established-based on the assumption of elastic foundation, beam deformation, and key layer theory-for the deflection curve and bending moment equations of key layer deformation during the initial mining period under two boundary conditions of broken and unbroken coal walls in the mining face. The relationships among displacement, bending moment, mining advance influence distance, stiffness, coal rock mechanical properties, and working face advancement are quantified. The calculation of the mechanical performance parameters reveals that the vertical displacement of the key layer above the solid coal in front of the coal wall alternately increases and decreases towards the deep part of the coal rock mass, with the peak value gradually decreasing and approaching zero. The influence distance of horizontal advance mining on the working face is positively correlated with the bending stiffness of the key layer. Under the boundary of the fractured zone, the peak value is smaller than that of the nonfractured zone because of the greater range of coal wall pressure relief in the fractured zone. The vertical displacement of the key layer above the goaf section increases as it moves farther from the coal wall and shows a polynomial variation with increasing distance from the coal wall. This study reveals the mechanical mechanism by which the deformation of key layers during the initial mining period influences the evolution of overlying rock fractures, providing a theoretical basis for achieving coal and gas co-mining.