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. Taking the Sanhejian closed mine area as an example, this study comprehensively evaluates the stability of the goaf using numerical simulation, Analytic Hierarchy Process (AHP), and Vulnerability Index (VI). Firstly, the numerical model of the goaf was built using FLAC3D software to obtain the stress field, displacement field, and characteristics of plastic zone development. Based on the simulation results, stability evaluation criteria for the goaf were formulated, and stability levels were determined. Secondly, a vulnerability assessment model was established using AHP, selecting geological factors, mining factors, and hydrological factors as primary indicators and further determining eight secondary indicators, including geological structural complexity, roof lithology and thickness, geostress, stop mining time, depth-to-coal ratio, goaf width, goaf area, and water volume in goaf. The weights of each indicator were determined, and the indicators were quantified to calculate the VI value of the vulnerability assessment model. The stability zoning threshold of the goaf was obtained using a natural breakpoint classification method and verified against the numerical simulation results to enhance the accuracy of stability evaluation. By integrating the results of both methods and adhering to a conservative risk assessment principle, the stability level of the goaf was ultimately determined, providing reference for the stability evaluation of related underground spaces.