RESPONSE CHARACTERISTIC ANALYSIS OF THE DEFORMATION AND FAILURE OF SURROUNDING ROCK MASS IN A LARGE UNDERGROUND POWERHOUSE UNDER HIGH IN-SITU STRESS

The hydropower station in southwest China usually has large-scale underground powerhouse. Subjected to high in situ stress and complex geological condition, the construction of these powerhouses often encounters deformation and failure of surrounding rock mass. This paper takes the Baihetan right bank underground powerhouse as a case study, which is large-scale and under high in situ stress. Via site monitoring and field investigation, we analyzed the response characteristics of deformation and failure of surrounding rock mass in the underground powerhouse. With the factors including in situ stress and rock mass property, we discussed the mechanism of deformation and failure of surrounding rock mass. The results show that the second principal stress with a direction approximately perpendicular to the powerhouse axis plays a dominant role in the stress adjustment of the main powerhouse. When the main powerhouse was excavated, stress concentration zones formed at the upstream spandrel and the foot of downstream sidewall. In these two areas, stress-dominated failures occurred, such as spalling, surrounding rock mass fracture and shotcrete cracking. The underground excavation under high in situ stress causes strong unloading of surrounding rock mass, which leads to the large deformation with time effect. In the parts of arch and sidewalls, the surrounding rock mass deformation increased continuously as the stratified excavation progressed.