Risk assessment of water-sand inrush hazards in thin bedrock overlain by thick unconsolidated strata under tectonic influence: a case study from Wanfu Mine Field, China

Abstract Under geological conditions characterized by thick unconsolidated layers and thin bedrock, water-sand inrush from the Neogene basal aquifer into working faces may still occur under tectonic influences, even when the aquifer exhibits weak water abundance and the water-conducting fracture zone remains underdeveloped. During initial development drifts at Wanfu Coal Mine, China water-sand inrush incidents occurred under combined engineering disturbances and adverse geological conditions. Such hazards continue to severely constrain coal extraction operations. To address risk assessment of water-sand inrush from the Neogene basal aquifer in such geological settings, this study focuses on the Wanfu Mine Field, China. Discrete element numerical simulations were employed to systematically investigate the evolutionary patterns of roof collapse during coal extraction. Integrating simulation results with critical geological features—including substantial thickness, deep burial, high water levels of the unconsolidated layer, and pronounced tectonic development—a four-dimensional evaluation index system was established, encompassing unconsolidated layer characteristics, bedrock structure, mining conditions, and tectonic properties. The entropy weight method (EWM) coupled with principal component analysis (PCA) was implemented to mitigate biases caused by indicator redundancy or subjectivity in traditional approaches. Combined with a vulnerability index model, 56 working faces corresponding to borehole locations were classified into five risk levels: safe zone (18 boreholes), relatively safe zone (13), moderately hazardous zone (4), hazardous zone (14), and extremely hazardous zone (7). Spatial analysis revealed banded and elliptical distribution patterns of hazard zones, with principal controlling factors identified as fault displacement, structural dip angle, and thickness of the basal clay aquiclude. These findings preliminarily elucidate the disaster-inducing mechanisms of water-sand inrush in the Wanfu mining area under thick-unconsolidated-layer/thin-bedrock conditions, providing theoretical support for safe mining practices and water hazard prevention design.