Development characteristics and risk assessment of roof bed-separation water in thick coal seam fully mechanized mining

The Anxin Coalfield of Ordos Basin is a vital coal production area in China, where the Cretaceous Zhidan Group aquifer—composed of sandstone-conglomerate interbeds—prone to bed-separation development, threatens safe mining operations. Taking the 4506 working face of a certain mine in Anxin Coalfield as an example, this paper employs Universal Distinct Element Code（UDEC） simulations to analyze overburden failure and bed-separation characteristics, integrating hydrogeological conditions. It contrasts bed-separation water hazards with conventional roof water hazards, identifying six key inrush-controlling factors, and establishes a risk assessment system using the entropy weight method. Key findings reveal: bed-separations form at multiple overburden levels, but only the basal separation in the second aquifer attains significant scale, featuring recharge sources and pathways that critically impact mining safety. Bed-separation evolution exhibits cyclic behavior: the primary separation initiates at 280 m advancement, fully closes by 400 m, followed by new bottom-up cycles on the working face side. Six risk indicators across four dimensions （water accumulation, mining disturbance, aquitard integrity, geological structures） are extracted to build an EWM-based risk assessment model, validated with strong field consistency. This research provides a novel methodology for evaluating bed-separation water risks in sandy conglomerate aquifers and offers data-driven guidance for hazard prevention, ensuring safe and efficient coal production.