Risk prediction of surface-water bursting in the gully-crossing mining section of shallow coal seams within thin bedrock zones

Background The gully-crossing mining of shallow coal seams within thin bedrock zones is prone to connecting hydraulically conductive fracture zones in the overburden with surface water bodies, significantly increasing the risk of water disasters, such as water inrushes and surface-water bursting in mines. Some mining faces in the Shandong Coal Mine, Shaanxi Province are located beneath the Shajie Gully basin, where storm runoff is highly likely to flow into the underground mine through mining-induced fractures, posing a serious threat to mine safety. Therefore, there is an urgent need to research risk identification and assessment of surface-water bursting.

Methods This study investigated the No. 3 coal seam and its gully-crossing mining faces in the Shandong Coal Mine. Based on measured mining heights and borehole data, this study plotted the contour map showing the heights of the hydraulically conductive fracture zone in the No. 3 coal seam. Using the differences between the heights of the hydraulically conductive fracture zone and the burial depths of the coal seam as a critical criterion, this study divided the coal seam into high-risk, moderate-risk, and safe zones. In combination with the structural characteristics of surface mining-induced fractures and the storm runoff process in the gully, this study constructed a prediction model of the flow rates of surface-water bursting via mining-induced fractures. Using this model, this study simulated and assessed the flow rates of surface-water bursting along mining faces 3108 and 3109 under storm rainfall with 5-, 10-, and 20-year recurrence intervals. By integrating the classified flow rates of surface-water bursting with the classified caving-fracture safety, this study established a dual-factor six-level surface-water bursting risk classification system. To improve the engineering applicability of the classification system, a simplified three-level risk classification scheme was proposed to re-identify the risk levels of the mining face.

Results The optimal threshold for the difference between the burial depth of the No.3 Coal Seam and the heights of the hydraulically conductive fracture zone is −20 m, allowing an effective identification of high-risk zones of surface-water bursting beneath and around the Shajie Gully. The prediction results of the model indicate that under different recurrence intervals of storms, mining face 3109 exhibited consistently higher flow rates of surface-water bursting compared to mining face 3108, reaching a maximum of 0.335 m³/s (1205.169 m³/h). According to the six-level risk classification system, mining face 3109 was divided into extremely high-risk (20- and 10-year recurrence intervals) and high-risk (5-year recurrence interval) zones, while mining face 3108 was divided into high-risk (20-year recurrence interval) and moderate-risk (10- and 5-year recurrence intervals) zones. Under the simplified three-level risk classification scheme, mining faces 3109 and 3108 were classified as high-risk zones across all recurrence intervals of storms.

Conclusions By integrating surface mining-induced fracture structures with the storm runoff process in the gully, this study constructed a prediction model of the flow rate of surface-water bursting via mining-induced fractures in the gully-crossing mining section of shallow coal seams within thin bedrock zones. Moreover, this study established a comprehensive risk assessment system combining the levels of flow rates of surface-water bursting with the caving-fracture safety zones. The results of this study will provide theoretical support and an engineering reference for the identification and control of surface-water bursting risks along mining faces under similar geological conditions, holding great practical value.