## 2 Engineering Background  
Xinhu Coal Mine is located in Woyang County, Anhui Province, China, approximately 14 km from Woyang County (Figure 2a). The coal mine has a designed capacity of 3 million tons per year and an expected service life of 63.4 years. Mining area 81 is situated in the central-western part of Xinhu Coal Mine. This is the first extraction panel in the mine. The designed length of the number 815 longwall panel is 1770 m (horizontal distance), with a longwall length (inclination distance) of 239 m. A fully mechanized cave mining of the coal seam was used. Based on drilling data and three-dimensional seismic exploration, 195 combined faults have been identified in the mining area, including 3 faults with H \(\geq\) 100 m, 3 faults with 50 m \(\leq\) H \(<\) 100 m, 4 faults with 20 m \(\leq\) H \(<\) 50 m, 21 faults with 10 m \(\leq\) H \(<\) 20 m, 77 faults with 5 m \(\leq\) H \(<\) 10 m, and 87 faults with H \(<\) 5 m. Among them, fault 815F16 is a high-angled fault with a displacement of 7.5 m. It is activated by factors such as ground stress, abutment pressure, overburden deformation, and resulted in active water inflow. Through a statistical analysis of exploration well data in the number 81 mining area, the predominant lithology has been identified as fine sand and clayey sand. Additionally, the bottom layer contains gravel, clay, or loam. The upper part of the number 81 mining area is weakly water-bearing (\(-\)450 m) and consists of gray-green, brown-red, gray-white, clayey sandstones, and limestone clays, with localized occurrences of marl. The thickness of the impermeable layer remains relatively stable, which prevents surface water and atmospheric precipitation from establishing a hydraulic connection with the 81 mining area and the coal-bearing underground water. During the mining of the 815 longwall, a filtration window formed due to a fracture in the sandstone in overburden rock. The longwall mining began in March 2023, and, when 314 m had been extracted, water inflow occurred (Figure 2b). At the time of the incident, the volumetric water inflow rate was approximately 200 m\({}^{3}\)/h, reaching a peak of 300 m\({}^{3}\)/h. The water contained a high concentration of sediment, which led to the blockage of the water pumps. Figure 2b illustrates the layout of four longwall panels with the location of the water window. In addition, Figure 2b shows the location of monitoring wells. A total of 11 wells were drilled to identify aquifers and monitor water levels. The primary hazard is posed by the aquifer, as shown in Figure 3a. The aquifer water intrushes in the underground mine roadways through a water-conducting fracture zone. Figure 2: Geographical location of Xinhu Coal Mine (**a**), layout of longwall at the time of an accident (**b**). Figure 3: Vertical section (**a**) and plan (**b**) of the water inrush control method by horizontal regional grouting through multiple directional wells. The red lines it is faults, blue area it is flooded area. Two-level horizontal anti-filtration barriers were constructed to control water inrush in the mine. As part of the project, two groups of wells, D1 and D2, were drilled, with a total length of 4447 m (Figure 3). The concept of the water inrush control method was based on sealing the water inflow channels [24]. This method involved constructing a double horizontal water-protection barrier with two groups of multiple wells in a crosswise pattern. The creation of two interstratal horizontal anti-filtration barriers was carried out by horizontal drilling. A system of multiple injection wells was established and filled with a cementing mixture. Initially, grouting was applied to seal exposed rock fractures to prevent the loss of drilling fluid. Subsequently, large water-conducting channels were filled, followed by the injection of cement under pressure to seal the remaining rock fractures. Dense cement slurry with a density of 1.5 g/cm\({}^{3}\) was used for water blocking.