Abstract: The unloading collapse and the falling of the columnar jointed rock mass have challenged the excavation and supporting design of underground tunnel in Baihetan Hydropower Station project. This paper presents the results of the characteristics and the mechanism of the columnar structural surface and the irregular shaped rock joints based on the detailed field rock mass investigation results. The insitu rock features were revealed by the borehole camera and ultrasonic testing in order to develop the jointed rock supporting and excavation methods. These studies indicate that, after the excavation (unlading), the structural planes are opened and the results provide experience in the supporting and excavation of the similar rock mass.

Key words:  Columnar Rock Joints, Structured Rock Mass, Unloading Loosen, Rock Mass Support