Failure Mechanisms and Evolution Assessment of the Excavation Damaged Zones in a Large-Scale and Deeply Buried Underground Powerhouse

The Jinping I underground powerhouse is deeply buried and is one of the largest underground powerhouses in China. As a result of high levels of in situ stress, complex geological conditions and the effects of excavation in adjacent caverns, the surrounding rock mass has been severely deformed and broken, and excavation damaged zones (EDZs) have become major obstacles to the design of cavern excavation and support. Field investigations and monitoring data indicated that there are two main modes of failure: high tangential stress induced failure and progressive failure, which have occurred on the mountain side and the river valley side of the Jinping I underground powerhouse. These two main modes of failure were due to strong secondary principal stress forces in the sub-parallel directions and sub-vertical directions, acting on the axes of the main powerhouse on the mountain side and on the river valley side, respectively. Deformations and EDZs on the river valley side were generally larger than those found along the mountain side, and the distribution of deformations was consistent with the distribution of EDZs. The evolution of the EDZ on the river valley side has clearly been time dependent, especially along the downstream arch abutment, and the EDZ was considerably enlarged with further excavation. Additionally, the deformation of the surrounding rock mass was first initiated from the edge of the excavation area and gradually extended to deeper areas away from the opening. However, the EDZ on the mountain side was enlarged only during the first two phases of excavation. The extension of pre-existing cracks and the creation of new fractures has mainly occurred in the oldest EDZ section, and the HDZ has been visibly enlarged, whereas the EDZ has shown little change in other excavation phases.