高边坡坝基柱状节理玄武岩开挖卸荷时效松弛特性

Time-dependent relaxation characteristics of columnar jointed basalts in high-slope dam foundation during excavation

柱状节理玄武岩在开挖卸荷条件下容易松弛与开裂,是白鹤滩水电站坝基稳定的主要挑战。以白鹤滩水电站左岸坝基柱状节理玄武岩试验区为工程背景,采用单孔声波、钻孔摄像等原位测试技术,研究了分梯段开挖及时间效应条件下Ⅲ_1类和Ⅲ_2类柱状节理玄武岩松弛深度、松弛程度的时空演化规律,探讨了坝基柱状节理岩体松弛特征的原因。分析结果表明:柱状节理玄武岩的松弛深度主要由开挖引起,临近开挖面岩体的松弛程度随时间呈负指数增长,其时效期不超过半年。柱状节理岩体松弛特征与岩体质量关系密切,两类岩体的松弛深度、松弛程度的时效性及松弛程度随孔深的分布规律差异较大。坝基柱状节理岩体时效松弛是由内部节理张开导致的岩体强度参数的时效劣化引起。该研究对认识柱状节理岩体卸荷力学行为及优化坝基柱状节理岩体开挖保护方案具有重要意义。

Under the condition of excavation and unloading, the induced relaxation and cracks of columnar jointed basalts（CJB） were the main challenges for the stability of the dam foundation of Baihetan hydropower station. To analyse the loosening characteristics of CJB, the temporal-spatial evolution of the loosening depths and degrees of class Ⅲ1 and class Ⅲ2 of CJB were studied by the single-hole acoustic wave test and borehole camera inspection which were conducted in the CJB test section of the left bank dam foundation of Baihetan hydropower station. The reason of loosening characteristics was then analysed in detail. The results showed that the loosening depth was mainly caused by excavation. The loosening degree of CJB adjacent excavation surface grew progressively with time by the negative exponent type, with a semi-annual growth period. The loosening characteristics were closely associated with the rock mass quality. The loosening depth, the time-dependent characteristics and distribution with borehole depths of loosening degree were largely different from class Ⅲ1 to class Ⅲ2. The time-dependent loosening was caused by the ageing degradation of strength parameters of rock mass due to the opening of internal joints. This study is useful to understand the unloading mechanical behaviour of CJB and to optimise the excavation and support of CJB in the dam foundation.