复杂地质条件下顶拱大型不稳定块体分析与预测

锦屏一级水电站2#尾水调压室所处位置地质条件复杂，在桩号0+320°~0+360°和0+0°~0+120°之间开挖时局部掉块严重。本文综合分析后，提出了2#尾调室顶拱未开挖域产生大型不稳定块体的必要条件是：(1)由节理①④F183组结构面和开挖临空面切割产生；(2)块体大小主要取决于节理组①和④的延伸长度；(3)较大不稳定块体的产生区域，必然位于F18断层带下盘。对2#尾调室顶拱进行搜索预测，预测结果表明该潜在块体位于未开挖区域桩号0+180°～0+260°之间，规模约为50m3，重达133.5t。根据块体理论分析，其稳定性系数为1.052，处于临界状态，应采取相应的应急支护补强措施。

ANALYSIS AND FORECAST OF LARGE ROCK BLOCK INSTABILITY AT TOP ARCH UNDER COMPLEX GEOLOGICAL CONDITION

The site condition was complex, where the Second Tailrace Surge Chamber for the Jinping First Stage Hydropower Station was built . During the excavation of JPTSC2 between the chainage Nos. 0＋320° to 0＋360° degrees and from 0＋0° to 0＋120° degrees, falling of rock blocks often occurred. They were induced by the complex geological conditions, which was a big issue.   So it was very important to judge and forecast where the big block would fall. After comprehensive analyzed, three conditions causing the big block falling were put forward. The first condition was that the rock block had to be cut by joint Nos. ①, ④ and the fault f18.  The second was that the scale of rock block would be decided by the extended length of joint Nos ① and ④.  The third one was that the block should be found at the lower side of the fault f18. Then searching work was carried on the three conditions. Finally a big block was found, its volume would be about 50 m3, its weight would be about 133.5 tons.  Block Theory was used to calculate,the safety factor of the block of 1.05. It would be dangerous.  So, some relevant countermeasures are suggested.