Abstract: Field survey on the left abutment slope of a hydropower station on the Lancang River reveals a type of special rock mass with fragmented structure and significant loosening deformation synchronously. On this basis, we put forward the concept of cataclastic loose rock mass. Study on its development characteristics unveils that the cataclastic loose rock mass features with granular structure, cataclastic structure, and block structure from the slope surface to the interior, and its development depth corresponds well to the extremely-strong unloading zone. In view of this, we put forward the division standard: the depth of the rock mass with granular structure, cataclastic structure and block structure is regarded as the development depth of cataclastic rock mass, while the depth of the extremely-strong unloading zone is the depth of its unloading zone; and the average value of the two is determined as the ultimate development depth. In order to prove that the result acquired by this standard can be used to guide the excavation of the slope, we performed simulations using UDEC. Results demonstrated that after the excavation of the slope, besides the dangerous rock in a small scale near the excavation face, the overall deformation is small, and the stability is good. Therefore, we recommend to excavating the slope along the division depth of the cataclastic rock mass, and the excavation slope ratio above and below 2 880 m height is 1∶1.5 and 1∶1, respectively.The results prove that the division of cataclastic loose rock mass provides reasonable reference for the excavation depth in such special slope rock mass.

Key words: cataclastic loose rock mass, development characteristics, standards of division, numerical simulation, response of excavation deformation