川藏铁路廊道湖相地层斜坡断错变形机理分析及其稳定性评价:以松宗镇湖相剖面为例

Analysis of Deformational Mechanism and Stability Evaluation of Lacustrine Deposit within the Corridor of Sichuan-Tibet Railway:An Example from an Outcrop at Songzong Town

青藏高原东南部高山峡谷地区广泛发育湖相地层,该套地层常可见到无根的、类似正断层的断错变形,易于与活动断层的识别相混淆.同时这些断错变形易导致湖相斜坡破坏失稳造成一定的经济损失和人员伤亡.为深入揭示湖相斜坡的稳定性及其变形机制,以川藏铁路廊道藏东南松宗镇西侧某湖相斜坡为例,开展了详细的野外调查及测试工作,优化了极限分析上限定理力学分析模型,并通过强度折减法计算了不同互层层厚比、粘聚力、内摩擦角条件下斜坡安全系数.研究结果表明湖相地层产生断错变形并破坏失稳的主要因素与砂、泥质软硬互层结构特征有关,其中相对软弱地层的内摩擦角可能是最主要的影响因素.另外,湖相斜坡在重力作用下就可发生失稳变形,地震动作用下能进一步加剧变形,并主要表现为类似正断层的旋转-平移滑动破坏样式.因此,湖相地层区域无根断层的发育易于造成活动断层识别的不确定性,这给类似环境的地震危险性及湖相斜坡稳定性评价工作带来了一定挑战.

The lacustrine strata are widely developed in the valleys of the southeastern region of the Tibetan Plateau. Rootless faults with a shape of normal fault offset have been identified in the lacustrine deposits, which usually results in uncertainty in studying active tectonics. Meanwhile, the deformation in the deposits often leads to the destruction and instability of lacustrine slopes, which may cause economic losses and casualties. In order to reveal the stability and deformational mechanism of the lacustrine slopes in the region, we conducted detailed investigations at an outcrop west of Songzong Town in the Sichuan-Tibet Railway corridor. We used shear testing, updated the model of upper bound theorem in limit analysis, and estimated safety factor of the slope by the strength reduction method under different conditions such as interlayer thickness ratio, cohesion and internal friction angle. We suggest that the failures of lacustrine slope are primarily related to the structural characteristics of the interbedded soft sandy and hard clay layers, particularly affected by the internal friction angle of the soft sandy layer. Moreover,normal-fault-shaped offset deformation in the slope could be produced by gravity and be more dominant under seismic loading,the failure in the slope shows a rotational-translational sliding pattern. Therefore, the development of unrooted faults in the lacustrine strata is likely to cause uncertainty in identification of active faults, which brings extra challenges for the evaluation of seismic hazards and the stability of lacustrine slopes in similar environments.