Abstract: The genetic mechanism of cataclastic loose rock mass in the dam site area of Rumei Hydropower Station on the Lancang River was first analyzed through qualitative analysis of engineering geology. On this basis, a geological generalization model was established using the discrete element UDEC software to simulate the combined influence of self-weight stress field and tectonic stress field on the formation process of river valley downcutting and bank slope cataclastic loose rock mass. Results revealed diverse unloading forms in the dam site area, with “push dislocation” having the largest impact and the most complex mechanism. As the river valley undergoes downcutting, the in-situ stress of bank slope rock mass is redistributed, resulting in decreased principal stress and increased shear stress. This causes significant relaxation and unloading of the rock mass near the slope surface. The bank slope also exhibits large deformation characteristics along the upper slope to the mid dip structural plane, while the rock mass at the lower pinch out part experiences a large thrust. A dislocation of being high outside and low inside is formed on the slope surface in the reverse slope direction, indicating reverse dislocation along steep joints. Bank slope rock mass displays obvious dumping deformation characteristics, forming a typical “push dislocation” unloading mode. With the addition of strong weathering, cataclastic loose rock mass is finally formed.

Key words: cataclastic loose rock mass, river valley downcutting, high geostress, unloading, genetic mechanism