Abstract: For mines that switch from underground to open-pit mining，the retention of underground goaf areas often leads to sudden roof collapse，which in turn causes the fall of mining equipment and casu‐ alties. To eliminate the safety hazards of open-pit operations in such goaf areas and ensure the safety of equipment and personnel's lives and property，in the actual mining process，a certain molybdenum mine first used a combination of transient electromagnetic method and natural source surface wave method to ac‐ curately locate the abnormal distribution of goaf areas. Then，through drilling means，the elevation data of the top and bottom plates of the goaf areas were verified. Thus，the 3D spatial form of the goaf can be clear‐ ly defined. Based on the above survey results，the FLAC3D numerical simulation technology was further ap‐ plied to calculate and analyze the required thickness of the safe roof in the goaf. Relevant analysis shows that due to the long-term effects of multiple factors such as ground pressure activities，natural weathering of rock masses，rainwater infiltration，and vibration from blasting operations，the roof and side walls of the goaf have experienced varying degrees of instability and collapse. The actual formed spatial morphology has deviated significantly from the initial design parameters. Based on the detection data of the goaf，by using the comprehensive method of theoretical derivation and numerical simulation，when the span of the goaf reaches 18.4 m and the maximum mining height is 23.4 m，it is calculated that the minimum safe thick‐ ness required for the roof of the goaf under this condition is 22 m. The on-site blasting effect shows that the  value of this roof thickness is reasonable. The research provides scientific support for determining the tim‐
ing of goaf management in mines，and also offers a useful reference for setting the safe roof thickness when
structural parameters are selected in similar mining sites.

Key words: underground to open-pit mining, goaf detection, thickness of safe roof, numerical simula‐ tion