Study on Unloading Relaxation Characteristics for Rock Masses under High Geostress

China is rich in hydropower resources that are mainly distributed in the southwest region. The continuous uplifts of Qinghai-Tibet Plateau in the recent hundreds of years result in geomorphologic landscapes of alpine valleys and complex initial geostress field. Constructing large-scale underground cavern groups in such complex geological conditions will lead to some prominent problems. In particular, the unloading and relaxation deformation of rock masses has become one of the key technical problems that restrict the design and construction of underground caverns in southwest China. Based on the engineering practices of hydropower construction in southwest China in recent years and actual engineering geophysical prospecting detection results, typical unloading relaxation characteristics of underground cavern surrounding rock under high geostress are summarized and analyzed. Differences between areas are analyzed and arranged for the rock unloading relaxation area. By reference to the qualitative classification table for the rock mass intactness in the GB 50218-1994 Standard for engineering classification of rock masses, a standard classification methodology for unloading relaxation areas of rock masses under high geostress is initially formed, which is of great significance to the design and construction of underground caverns under high geostress.