Study of the Evolution of Water-Conducting Fracture Zones in Overlying Rock of a Fully Mechanized Caving Face in Gently Inclined Extra-Thick Coal Seams

To study the caving of thick hard overburdens and evolution of water-conducting fracture zones in fully mechanized top-coal caving faces of gently inclined extra-thick coal seams, we comprehensively analyzed the 8103 working face of the Beixinyao Coal Mine. We investigated to the caving characteristics of thick hard overburden in fully mechanized top-coal caving faces, fracture information of the internal structure of overburden, and development heights of the “two zones” of overburden after coal mining. Our research methods included those of similarity simulation experiments, such as the use of microseismic monitoring systems, numerical simulations, theoretical analysis, and engineering practice. The results showed that the overlying strata generally experienced stages of roof caving, crack formation, delamination, crack development, and surface subsidence. Due to the influence of overlying strata movement and mining, the separation layer experienced an evolution process called “emergence-development-closure”, where the height of the overlying strata caving envelope increases with the advancing of the working face. When full mining was achieved, the overlying strata caving height was stable, and the height development range of the water-conducting fracture zone was 100–120 m, which is consistent with the height of the overlying strata caving envelope. Most microseismic events occurred near the water-conducting fracture zone, and the water-conducting fracture zone was formed in an area with concentrated energy density. In our numerical simulation, the concentrated distribution area of the fracture field was characterized by a “bridge arch”. The fracture development model in the middle of the goaf was higher than at both ends of the working face, and roof strata deformation was obvious. When the energy value of microseismic event reaches 108.708 J, cracks are produced, and these cracks gradually penetrate to form water-conducting fracture zones. Engineering practice showed that the height range of the water-conducting fracture zone was 98–123 m, and caving of the thick hard overburden and evolution of the water-conducting fracture zone in a fully mechanized top-coal caving face provide a scientific basis for water prevention and control.