Abstract:

The geometry and stability of stochastic blocks are fundamental to support design of underground powerhouse of Wudongde hydropower station. In this paper, the characteristics of stochastic blocks in the underground powerhouse on the right bank of Wudongde hydropower station are studied in detail. Firstly, measured fractures in the study area were grouped and statistically analyzed to determine the distribution type and magnitude of orientation，trace length and some other parameters of each fracture set. Three-dimensional fracture network is subsequently built by an inverse method in order to determine the diameter and volumetric density of each fracture set. A general block method was employed to detect stochastic blocks and evaluate the stability of blocks. Ten implementations were conducted and the results were analyzed and discussed. Results of the study show that stochastic blocks formed by discontinuities and excavations are located in the vault of the powerhouse. The average number of stochastic blocks formed in entire powerhouse of ten implementations is 414；the average volume of stochastic blocks is 2.9 m³ and the maximum block is 152.0 m³  in volume；most removable blocks are stable while unstable blocks collapse under the gravitational loading；most removable blocks have 3 or 4 discontinuities and the maximum value is 12.Fractures with west dip direction and medium dip are the most dangerous discontinuities in forming blocks and making them unstable;the average depth of stochastic blocks is 1.2 m and the maximum depth can be up to 8.8 m. It is suggested that the anchor bolts should go through the large fractures with west dip direction and medium dip as far as possible and their lengths should be greater than 8.8 m.

Key words: underground powerhouse, general block theory, fracture, stochastic block, block stability