Abstract: Drilling and acoustic wave observation is often used to determine the disturbance of excavation on rock quality. However, it is difficult to determine the boundary of rock quality partition by using conventional methods because there are more than one intersection between the curve of acoustic velocity changing rate and the threshold line. In this research, we optimize the conventional method by considering the detail features of acoustic velocity changing rate curve, and redefine the criteria of rock quality partition, which is made unique. Furthermore we apply the optimized method to the 4^# diversion tunnel project of Baihetan hydropower station, and divide the rock quality of 4^# diversion tunnel into 3 zones: the first disturbed zone, the second disturbed zone, and non-disturbed zone. Along with the increase of the depth from the excavation face, the disturbance of excavation on surrounding rock gets weaker and the rock quality becomes better. Subsequent excavation steps could worsen the rock quality, and the disturbed area will increase.

Key words: acoustic wave, surrounding rock of chamber, rock quality partition, optimization method, Baihetan hydropower station