Abstract:
In recent years, many mines have been closed due to the exhaustion of coal resources in some mining areas and the release of the national overcapacity policy. After a coal mine is closed, the coal rock mass could undergo weathering deterioration and strength reduction due to some factors such as stress and groundwater, which in turn changes the stress and bearing capacity of the fractured rock mass in the abandoned goaf, leading to some secondary or multiple surface deformations over the goaf. This kind of overburden and surface subsidence caused by indirect mining, called secondary subsidence, will pose a huge potential threat to surface infrastructure construction and the safety of people's lives and properties. On the basis of reviewing the development history of closed mines, the geographical distribution of closed mines is firstly analyzed, and then the monitoring methods, prediction methods and goaf stability evaluation methods of closed mines are reviewed respectively, and finally the existing problems and research hotspots on the secondary subsidence disaster of closed mines are prospected. The research results show that:(1) At present, there are many closed mines in China. From 2016 to 2020,5 243 mines have been closed, with an exit production capacity of 875.38 million tonnes.(2) Although the conventional monitoring methods of closed mine subsidence have high accuracy, it has disadvantages such as large workload, high cost, small coverage, and sparse measurement points. The ground subsidence monitoring based on InSAR technology can not only obtain the current time series deformation information with high precision and a large range, but also can trace the origin of the deformation that has occurred. At present, the monitoring of underground rock movement and deformation based on time domain reflectometer and microseismic monitor can obtain real-time and high-density rock movement and deformation information.(3) For some complex geological mining conditions, the prediction results of secondary surface subsidence of the closed mine using the numerical simulation method are highly reliable, but the modeling process is complicated and corresponding parameters selection is difficult. For some simple geological mining conditions, analytical method has higher accuracy and efficiency.(4) The coal pillar stress in pillar mining is related to the goaf size. When the ratio of the goaf size to the mining depth is between 3 and 4,the coal pillar stress can be calculated using the tributary area theory, otherwise, the pressure arch needs to be considered. In terms of coal pillar strength calculation, the stripping failure model considers the change of the pillar size with time, and it theoretically explains the influence of the aspect ratio of the pillar on the long-term stability of the pillar, and the pillar stripping failure model can better distinguish stable and unstable coal.(5) The research on the secondary subsidence disaster of closed mines still faces some key scientific problems such as the indecipherable of subsidence law and its mechanism, the long-term strength variation law of coal pillar, and the unclear mechanism of coal and rock mass weathering and deterioration.