Analysis and Forecast of Rock Block Instability about Large Underground Cavern under Complex Geological Conditions

Tailrace surge tank has following characters typically deep embedding, high wall, large section, during the excavation, falling of rock blocks often occurred. After analyzing with engineering geological theory and block theory, three conditions causing the big block falling were put forward, The first condition was that the rock block had to be cut by joint No. (1) No (4) and the fault, The second was that the scale of rock block would be decided by the extended length of joint No. (1) and No (4) The third was that the searching work was carried on the three conditions. Finally a big block was found between stake 0+45 degrees similar to 0+120 degrees , its volume would be about 16 m3, Block theory was used to calculate, the safety factor of the block is 1.05. It would be dangerous, so some relevant countermeasures are suggested.