An Experimental and Analytical Research on the Evolution of Mining Cracks in Deep Floor Rock Mass

The evolution of rock mass cracks is particularly important for the study of the rock mass fracture instability and catastrophic mechanism of underground engineering. To investigate the fracture characteristics of deep rock mass and formation mechanism of the water inrush channel and based on the real measured data of water-inrush mines, this paper divides floor mining cracks into shear cracks, layer cracks, and vertical tensile cracks according to the location of the cracks. Knowledge of mechanics is utilized to analyze the crack-initiating and growing principles of floor mining cracks to disclose the coupling relationship between mining-induced cracks and principle stress and hydraulic pressure. Based on a self-developed simulation test system for high-pressure floor water inrush, a model of crack evolution and channel formation for the deep floor above confined water is established. Through installing stress sensors and displacement-monitoring spots at different depths of the floor, the stress and displacement variation characteristics of floor rock mass during mining are obtained, and the development and evolution patterns of floor mining cracks are deduced. This research shows that the mining-induced cracks of the deep floor rock mass are mainly produced in three parts: open-off cut, the middle of the goaf, and the actual mining face; the crack types mainly include vertical tensile cracks, shear cracks, and layer cracks, and some small cracks have developed near the main cracks; in the process of mining, the mining-induced cracks evolve and penetrate through each other along the direction of the principle stress, and finally the water inrush channel is formed. In an actual coal seam mining process, it is necessary to monitor the crack state of these three parts of the rock mass and the pressure variation of the confined water. The research results can provide new guidelines for the management of deep water inrush disasters and the mining safety of coal resources.