坝基碎屑岩三轴蠕变特性及长期强度试验研究

Experimental study of triaxial creep behavior and long-term strength of clastic rock in dam foundation

基于物理和常规变形特性分析,认为某水电站坝基碎屑岩力学特性异常复杂,故采用岩石三轴伺服仪开展了系列的蠕变力学特性试验研究。首先,探讨了轴向、侧向及体积蠕变特性和速率变化规律;其次,分析了蠕变对偏应力-应变特性影响并开展了破坏岩样微细观电镜扫描试验;最后,在采用等时曲线簇法确定长期强度的基础上,以实际延性扩容为依据,认为岩石侧向体积扩容速率大于轴向压缩速率的临界点为快速蠕变破坏的标志,提出确定长期强度的新方法。结果表明,碎屑岩表现出显著的蠕变特性,蠕变曲线主要分为初始衰减和稳态蠕变两阶段,且最后一级应力施加后呈现加速蠕变现象;应力增加导致蠕变变形加剧,最终破坏表现出轴向压缩变形过大、体积延性扩容明显、稳态蠕变速率较大等特点,且蠕变速率与应力符合指数函数关系。岩样长期强度与三向稳态蠕变速率交点和侧向体积扩容应力阀值基本一致,为常规强度的54%~80%。试验结果旨在为相关岩石工程长期稳定分析及蠕变模型构建提供可靠的依据。

Based on the analysis of physical and normal deformation characteristics, it is considered that the mechanical properties of clastic rock in dam foundation in hydropower station are extremely complex. Therefore, triaxial creep tests are conducted to investigate the creep behaviors of clastic rock by using an automatic triaxial servo-instrument. Firstly, the axial, lateral and volume creep properties and strain rate are discussed. Secondly, the effects of creep properties on the stress-strain curve are analyzed, and then the scanning electron microscope （SEM） tests are performed on the fractured rock specimens. Finally, the long-term strength is determined by isochronous curves. Based on the volumetric expansion of specimen, a new method is proposed to determine long-term strength by assuming that the fast creep failure occurs at the critical point where lateral volumetric expansion rate exceeds the axial compression rate. The results show that the clastic rock has a significant creep property, and the creep curve generally have two stages including transient creep and steady creep. However, the creep accelerates after applying the last level of stress. Thus the creep behavior aggravates until the specimen finally shows the characteristics of large axial compression, obvious volumetric expansion and large steady creep rate. In addition, the relationship between creep rate and the deviatoric stress can be described by an exponential function. The long-term strength of clastic rock, which is 54% to 80% of conventional strength, is basically the same as the intersection of the steady-state creep rate and the stress threshold of volumetric dilation. Therefore, the experimental results in this study provide dependable reference for the establishment of rock creep model and analysis of long-term stability.