Height of water-conducting fractured zone in a coal seam overlain by thin bedrock and thick clay layer: a case study from the Sanyuan coal mine in North China

The accurate determination of the height of the water-conducting fractured zone (HWCFZ) in a coal seam covered by thin bedrock and thick clay layer is crucial for aquifer protection, mine safety, and eco-environmental conservation. Using the Sanyuan coal mine in North China as a case study, theoretical analysis, numerical simulation, and field measurements were used to analyze the regularity of the HWCFZ in this area. Per mechanical theory, a geometrical model for calculating rock bending and subsidence was established. Two arcs of equal curvature and lengths and opposite curvature direction were used to fit the curve section of intermediate rock layer after bending. The theoretical method for solving the HWCFZ was given by calculating the rock layer's tensile ratio. In addition, rock failure process analysis (RFPA) software was used to simulate the development of the water-conducting fractured zone (WCFZ) during the mining process, and the HWCFZ was measured by the drilling fluid loss measurement, both which verified the feasibility of the theoretical calculation. To determine the primary controlling factors affecting the HWCFZ, the seepage characteristics and plastic properties of the Quaternary clay layer were also analyzed using laboratory tests. Results show that the Quaternary clay with a depth greater than 130 m has a permeability coefficient of E−10 m/s, a liquidity index less than 0.25, and a plasticity index more than 17, indicating the clay layer is impermeable and of great importance to inhibiting the development of WCFZ. The research can provide significant theoretical insights for effectively preventing water hazards on mine roofs.