Evolution of Overburden Fractures and the Water-Conducting Fracture Zone in Closely Spaced Coal Mine Seams under Highly Water-Rich Aquifers

Risks of water-related mine incidents are related to water source, pathways, and water inrush intensity. This study aims to clarify the evolution of overburden fractures under repeated mining in closely spaced coal seams and to identify the presence of a connection between the damaged overburden and the overlying highly water-rich aquifer. Longfeng Coal Mine in northern Guizhou was taken as a case study. A coupled UDEC-DIC methodology was applied to identify fracture evolution and the development of water-conducting fracture zones during the downward mining of closely spaced coal seams. This approach allows an integrated study of fracture patterns, displacement, stress, and strain fields. Four key strata from the working face upward were identified, with breaking intervals of 22.72, 22.78, 24.08, and 100.79 m, respectively. The uppermost layer is a highly water-rich aquifer. During the early mining stage, risks of overburden collapse and fracture development were relatively minor. However, after advancing 30 m, overburden collapse, subsidence, and the vertical and horizontal development of fractures gradually intensified. After 110 m, fracture development gradually stabilized. The combination of UDEC numerical simulation and DIC digital speckle technology showed increasing vertical displacement and vertical stress of the overburden with the advance of the working face. During the evolution process, the overburden showed "saddle"- and "arch"-shaped patterns. The amplitudes of both the stress peak and displacement peak first showed a rapid increase, followed by a slow increase, and then finally a rapid increase. However, the displacement peak eventually stabilized, whereas the stress peak continued to increase. The strain peak fluctuated between increasing and decreasing trends over two cycles. The height of the water-conducting fracture zone first gradually increased, then rapidly increased, and lastly followed a stable increasing pattern. With the downward advance of the mine shaft, the increase in the fracture zone height showed a temporary halt and progressed again after the overburden of the lower coal seam collapsed and connected to the upper goaf. During the mining of the #5 coal seam, fracture height reached 41.7-42.8 m, thereby connecting to the highly water-rich Changxing Formation aquifer, resulting in a water inrush risk. Subsequent to the mining of the #5 and #9 coal seams, fracture height ultimately reached 90.2-91.2 m, connecting two highly water-rich aquifers and presenting a serious water hazard risk. The research findings hold significant theoretical and practical importance for water hazard prevention and water resource protection in coal mines.