基于改进破坏接近度的千将坪岸坡失稳机制分析

Analysis of failure mechanism of Qianjiangping slope based on improved failure approach index

千将坪滑坡是三峡水库蓄水后库区发生的首个特大型岩质顺层滑坡。以往大量的研究认为,水库蓄水和降雨是诱发该滑坡发生的2个主要因素,但关于这2个诱发因素对岸坡失稳影响程度大小的问题仍存在2种不同的观点:(1)水库蓄水的影响大于降雨;(2)降雨的影响大于水库蓄水。为了弄清该问题,选取千将坪滑坡前缘缓倾角非连续结构面岩桥附近的岩块,开展库水浸泡对岩块强度和变形影响的三轴压缩试验;建立千将坪岸坡的有限元模型,基于流固耦合理论,采用ABAQUS软件分别对该岸坡在三峡水库蓄水、降雨及两者共同作用下岸坡的地下水渗流场和应力场进行计算;基于试验测得的岩块应力–应变关系,改进破坏接近度指标,并将其用于定量分析评价水库蓄水、降雨及两者共同作用对千将坪岸坡稳定性的影响程度。研究结果表明:受水库蓄水的影响,岩块经历库水浸泡后其弹性模量、抗剪强度会显著降低;潜在滑动面(指岸坡中后部的层间错动带及前缘的缓倾角非连续结构面)的危险程度在水库蓄水、降雨及两者共同作用的3种情况下依次增大,降雨对千将坪岸坡稳定性的影响大于水库蓄水,并且仅在水库蓄水和降雨共同作用下潜在滑动面才贯通。岸坡前缘缓倾角非连续结构面中岩桥在水库蓄水后,受库水浸泡影响强度逐渐降低,加之降雨增加了促滑段(中后部层间错动带)的下滑力,最终岩桥剪断,千将坪岸坡发生失稳。

Qianjiangping landslide is the first large scale rock slide in the area of Three Gorges Reservoir（TGR） after impoundment. Most of the researchers agreed in the previous studies that reservoir impoundment and rainfall were two main factors triggering the landslide. However,there were different views regarding the degree of influence of two factors on the failure of the landslide,one considered that the influence of reservoir impoundment was greater than that of rainfall,the other considered that the influence of rainfall was greater than that of reservoir impoundment. In order to clarify this issue,the rock near the rock bridge that was part of the slip zone after sliding was selected. The triaxial compression tests on the influence of reservoir water immersion on strength and deformation of rock mass were carried out. The seepage field of groundwater and stresses in the slope under three conditions（reservoir impoundment,rainfall,and combined effect of reservoir impoundment and rainfall） were calculated with the software ABAQUS. Failure approach index was improved based on the constitutive relation of the rock obtained experimentally,and the improved failure approach index was applied to analyze the failure characteristics of the landslide under each condition. Under the influence of reservoir impoundment,the elastic modulus and shear strength of rock decrease remarkably when the rock is soaked. The influence of rainfall is greater than that of reservoir impoundment on the failure of the landslide,and the influence of combined effect of reservoir impoundment and rainfall is greater than that of rainfall. The sliding surface runs through under the combined effect of reservoir impoundment and rainfall. The study results suggest that the water immersion after reservoir impoundment reduced the strength of discontinuous rock bridges in the gently dipping discontinuous structural plane at the toe of the slope,the rainfall increased the sliding force of the slope,finally,the rock bridges fractured,and the failure of the slope occurred.