Stability evaluation of goaf in closed mining area: a case study of Sanhejian closed mining area in Jiangsu Province, China

The stability of goaf is one of the decisive conditions for the redevelopment and utilization of underground spaces after mine closure. This paper focuses on the closed coal mine of Sanhejian in Xuzhou City, Jiangsu Province, China, and proposes a refined classification method for the stability of goaf by integrating numerical simulation and theoretical calculations. Initially, a three-dimensional geological model of the goaf is constructed. Based on field surveys and laboratory tests, a numerical model is established to simulate and obtain the distribution characteristics of stress, deformation, and plastic zones within the goaf. According to the simulation results, classification criteria are set, and the stability of the goaf is evaluated based on its current mechanical and damage characteristics. It is found that the stability of goafs numbered 2, 8, 10, and 12 is the best, while that of goafs numbered 5, 7, and 14 is the worst. Next, eight evaluation indicators were selected from engineering geological factors, mining factors, and hydrogeological factors, including the complexity of geological structures, the lithology and thickness of the roof and floor, in situ stress, the time since mining cessation, the ratio of mining depth to thickness, mining width, mining area, and the volume of water accumulated in the goaf. The weights of these evaluation indicators were calculated using the analytic hierarchy process (AHP). Based on the constructed vulnerability index model, the vulnerability index of each goaf was calculated, and the theoretical stability of the goafs was classified accordingly. It was found that the stability of goafs numbered 1, 2, 8, 10, and 12 is the best, while that of goafs numbered 5, 7, 9, and 14 is the worst. For goafs where the stability levels differ between the two methods, the classification is based on the lower stability level. Among the goafs with consistent evaluation results, priority is given to those with more favorable development conditions. The research findings not only integrate the investigated geological environment and mining techniques, but also capture mechanical characteristics such as stress, deformation, and plastic zones that were not monitored post-closure. This makes the evaluation results more precise and scientific.