EXPERIMENTAL STUDY FOR UNLOADING AND RELAXATION|OF DAM BASE DURING EXCAVATION AT XIAOWAN HYDROPOWER STATION

During the dam base excavation of Xiaowan Hydropower Station, intensive unloading and relaxation happened to the superficial rock mass. Main types of the phenomena included crack slip, opening and development, in addition relaxation from new fracture. The latter mainly showed onion skin phenomenon, plate fracture phenomenon and rock burst phenomenon. Experiment results showed that slightly weathered rock nearby excavation surface was hard-extremely hard rock. But it had clear initial damage, and its critical stress ranged from 5.2 to 12.4MPa. Its tension strength changed from 3 to 6MPA. This was the material base on intensive unloading and relaxation of superficial dam base rock; Stress concentration from explosion excavation of dam base surface and effect from explosion were the lithology conditions of intensive unloading and relaxation of dam base rock.