Exploration and water inrush risk assessment of overlying goaf water to the fisrt working face of lower coal group by fully mechanized caving mining

In order to research the water abundance in the goaf area of the upper coal group and the risk of water inrush to the first mining face of the lower coal group, the first mining face of the lower coal group is taken as the research background, and the lithologic combination and hydrogeological conditions of the top and floor of the upper and lower coal groups in the research area are systematically analyzed. According to the results of geophysical exploration, 20 goaf water boreholes of the upper coal group have been constructed in the roadway of the first mining face of the lower coal group. It is found that the goaf water of upper coal group has the characteristics of large local water volume, high water pressure and uneven water richness. On this basis, the engineering geological numerical model of upper and lower coal seams mining roof and floor is established. The stress field and plastic zone field variation characteristics of upper and lower coal seams after mining are compared and analyzed by numerical simulation method. It is obtained that the floor failure depth of upper coal seam is 11.0 m after mining, and the height of water-conducting fracture zone of lower coal seam is 112.0 m after mining. It does not affect the failure depth range of the upper coal floor, which is consistent with the measured water inflow by in situ of the first mining face. Finally, based on the data of the first mining face and two boreholes nearby, the height of the water-conducting fracture zone in the lower group is calculated by analogy using two relevant empirical formulas, and an empirical formula is proposed with high reliability.