Pre-splitting weakening failure characteristics of hard overburden and height control mechanism of water-conducting fracture zone

In order to address the issue of high development of hard overlying rock water-conducting fracture zones in deep coal mining and achieve water-preserved coal mining by reducing the height of water-conducting fracture zones, a new method is proposed in this study, which combines the techniques of pre-fracturing and weakening of hard main controlling overlying strata. This study comprehensively investigates the modification effect of pre-fracturing and weakening of hard main controlling overlying strata through laboratory experiments, theoretical analysis, and numerical simulation. The control mechanisms of different weakening layers on water-conducting fracture zones are elaborated and validated through field measurements. The results demonstrate that pre-existing fractures can induce the rock mass to shift its failure mode from intact rock fracturing to tension-shear failure controlled by pre-existing fractures, leading to a reduction in the severity of rock mass failure. The weakening effect of pre-existing fractures on the hard rock mass is revealed, and the damage variables coupled with stress and fractures are calculated. The relationship between the degree of weakening and the transformation of rock properties is qualitatively analyzed. It is found that the energy storage capacity of the rock mass is reduced while the dissipation capacity is enhanced after the modification. Numerical simulations are conducted to evaluate the control effect of different weakening layers on water-conducting fracture zones. By comparing the dynamic evolution of overlying rock failure modes, fracture counts, and failure heights, it is observed that the “saddle-shaped” failure mode gradually weakens with the increase of weakening layer height, and the evolution trend of fracture counts follows a pattern of “slow increase-sudden increase”. The dynamic evolution of failure height for unweakened and moderately to highly weakened overlying strata approximately exhibits an “S” shape, while the moderately weakened overlying strata exhibit a “semi-ejection” shape. Based on the analysis of the characteristics of failure in hard rock layers at different positions, the mechanisms of controlling the development of water-conducting fracture zones by different pre-fracturing weakening layers are revealed. Furthermore, using the borehole measurement method, the development height of the “two zones” under the condition of top plate fracturing in Mengcun mine is obtained. It is observed that the fracture extraction ratio is reduced under the condition of pre-fracturing weakening, thus preliminarily validating the inhibitory effect of top plate pre-fracturing weakening on the development of water-conducting fracture zones.It provides theoretical and scientific basis for coupling disaster prevention and control, water resources and ecological protection.