In order to reveal the unloading relaxation characteristics of foundation rock mass after excavation in the middle-ground stress region,supplement the unloading relaxation theory of hydropower projects in western China,and provide scientific basis for the latter engineering treatment measures of Dagangshan hydropower station,the Dagangshan hydropower station was studied. Based on the study of geological background conditions and relying on field investigation and a large number of exploration drillings,geophysical exploration technics,hole camera technique,et al.,the characteristics of foundation rock mass after excavation in the middle-ground stress region were discussed,including the supergene and the endogenous unloading relaxation phenomenon,which assume the fissures opening,rock mass loosening and entirety relaxation, but without any new rock failure.Meanwhile,the unloading relaxation characteristics of foundation rock mass were markedly controlled by the structure of rock mass in the middle ground-stress area.
