Abstract：Xiaonanhai Hydropower Project is the third large grade I hydropower project on Yangtze River. Its rock foundation is composed of typical red beds，which have been weathered and demarcated clearly. The average thickness of the overburden and strongly weathered zone is 18 m，and the average thickness of the weakly weathered zone is about 6 m. If all the weakly weathered rock masses are excavated，the excavating depth will be equal to one third of the total one. Obviously，the excavating volume is huge. The wave velocity distribution of rock mass in a large area of dam site，engineering classification of rock mass quality，deformation and strength properties of rock mass are analyzed. The differences between weakly and slightly weathered rock masses are researched，and the probability of using the weakly weathered rock mass as foundation plane is discussed. The experimental results of this stage show that the qualities of weakly and slightly weathered rock mass are grade IV，the mechanical parameters of weakly weathered rock mass approach to those of slightly weathered rock mass，and the weakly weathered rock mass can be used as foundation plane.
