Abstract: Some mining methods such as caving method, which have short construction period and low mining cost, are often selected for mines changing from open-pit to underground. However, as ecological and environmental protection getting more and more attention, surface collapse induced by underground mining is not permitted so that upward backfilling method with relative longer construction period and larger investment tend to be adopted by mine, which is harmful to mine management in transition period. Open-pit mining has already caused damage to the surface, so collapse is permitted inside the open-pit limit, therefore Sangyuan mine decide to combine caving and backfilling method for its transition to underground mining. Firstly, the engineering characteristic of Sangyuan Mine is introduced. Then, engineering analogy method and Laubscher method are used for preliminary choosing the depth of cave mining. Finally, based on the coupling analysis of block distinct element method (DEM) and particle DEM, the depth of cave mining and the thickness of covering layer are determined. Economic analysis showed that, with the combined mining method Sangyuan mine can not only overcome difficulties in transition period, but also save mining cost up to nearly 100 million Yuan.

Key words: Transition from open-pit to underground, Caving method, Backfilling method, Gravel covering layer, Combined mining method