Disaster-inducing mechanism of fracture of high-position thick and hard key strata and their weakening and disaster-mitigation effects

At present, studies on the disaster-inducing mechanism of high-position thick and hard key strata mainly focus on the fracture behavior of key strata and their influence on strong ground pressure in the mining face, while relatively little attention is paid to the effect of variations in the distance between the key strata and the coal seam (interlayer distance) on the transformation of fracture modes of high-position key strata and the associated strain energy "accumulation-release" process. To address this issue, theoretical analysis, numerical simulation, and similarity simulation experiments were comprehensively employed to reveal the transformation mechanism of fracture modes of high-position key strata and the evolution laws of energy accumulation and dissipation, to compare the attenuation characteristics of ground pressure intensity on the working face after weakening of high-position key strata at different horizons, and to clarify the weakening and disaster-mitigation effects of high-position thick and hard key strata. The results showed that an increase in the distance between the key strata and the coal seam led to changes in the area of the suspended region of the key stratum, altered the magnitude and growth pattern of bending moments at the strike and dip boundaries of the key stratum, and promoted the transformation of its fracture mode from vertical fracture to horizontal fracture. Due to its strong bearing capacity, a typical high-position thick and hard key stratum had a larger suspended area and a greater accumulation of elastic strain energy than low-position key strata, and the rapid release of elastic strain energy during fracture posed a higher risk of inducing strong ground pressure disasters. The hydraulic fracturing method significantly altered the fracture step distance of the key strata and the intensity of ground pressure manifestation in the working face. After hydraulic fracturing weakening of a high-position key stratum located 125 m above the coal seam, the fracture step distance was reduced by 40.94% compared with the unweakened condition, the accumulated elastic strain energy was reduced by 98.40%, and the roof pressure of the working face was reduced by 48.77%, effectively controlling the strong ground pressure manifestation induced by fracture of high-position thick and hard key strata.