Longitudinal sliding event during excavation of Feng-Qi Station of Hangzhou Metro Line 1: Postfailure investigation

This paper introduces thirteen kinds of typical excavation failures widely encountered in practice in China along with corresponding failure mechanisms and damages to adjacent structures and facilities. Emphasis is placed on investigation of longitudinal slip.failure during excavation of subway station, which frequently occurs but rarely receives serious attention. This investigation is based.on a sliding event in Hangzhou, China, in which a soft earthen slope about 13 m high inside an uncompleted subway station underwent.massive slip failure. Slide debris travelling on sloped terrain caused the detachment of braced steel pipe struts and breakage of interior.columns and two concrete struts atop them. Fortunately, an efficient contingency plan was executed right after the event; thus, potential.catastrophic collapse of the entire earth supporting system, a domino effect of the massive sliding, was avoided. Postfailure investigations.in terms of ground survey, visual inspection, and numerical analyses show that for this three-step earthen slope with an overall slope gradient.(including benches) of 1 vertical (V) to 3 horizontal (H) (1V∶3H), the sliding took place at the uppermost 6-m slope and there was no sign of.sliding in the lower 7 m of slopes (1V∶2H) atop firm to stiff clayey strata. The steep local slope gradient (1V∶1.75H) of the uppermost slope.overlying weak muddy clay was the predominant factor triggering the sliding; probabilistic stability analyses indicate a high slip failure.probability of 88.92%. Degradation of soil strength resulting from soil deformation was mitigated by negative excess pore water pressure.associated with excavation (unloading); consequently, the uppermost slope did not show signs of potential sliding during the excavation..Following postexcavation dissipation of negative excess pore water pressure accompanied by decreasing of soil shear strength, the uppermost.slope underwent an abrupt sliding failure 7 days later. Apart from its recognized effectiveness in limiting lateral wall displacement, quick soil.removal was helpful for keeping the soft earthen slope temporarily stable during excavation. To ensure stability of soft earthen slopes underlain by weak subgrades, a slope gradient no steeper than 1V:3H is essential. Based on sensitivity analysis results, optimal slope angles.corresponding to strength parameters of soft subsoils are proposed for future excavations. To prevent slip failure of soft earthen slopes,.one cost-effective solution is to install recyclable steel sheet pile vertically penetrating through a potential slide at the deepest point of.the estimated critical slip surface. D

本文介绍了在中国实践中广泛遇到的13种典型的开挖事故，以及相应的破坏机制和对相邻结构及设施造成的损害。重点研究了地铁站开挖过程中经常发生但很少受到重视的纵向滑移破坏。这项研究基于中国杭州的一次滑移事件，在该事件中，一个未完工的地铁站内约13米高的软土边坡发生了大规模滑移破坏。在倾斜地形上移动的滑坡碎屑导致支撑钢管支柱脱落，内部立柱以及其上方的两根混凝土支柱断裂。幸运的是，事件发生后立即执行了有效的应急计划，从而避免了整个土体支撑系统因大规模滑坡产生的多米诺骨牌效应而可能发生的灾难性坍塌。通过地面勘测、目视检查和数值分析等事后调查表明，对于这个总坡度（包括台阶）为1∶3（垂直∶水平）的三步土坡，滑动发生在最上面6米的坡段，而在下部7米位于坚硬至硬塑黏土地层之上的坡段（1∶2）没有滑动迹象。覆盖在软弱淤泥质黏土上的最上部坡段局部陡坡坡度（1∶1.75）是引发滑动的主要因素；概率稳定性分析表明滑动破坏的概率高达88.92%。与开挖（卸载）相关的负超孔隙水压力缓解了因土体变形导致的土体强度降低；因此，在开挖过程中最上部坡段没有显示出潜在滑动的迹象。随着开挖后负超孔隙水压力的消散以及土体抗剪强度的降低，7天后最上部坡段突然发生滑动破坏。除了在限制侧壁位移方面公认的有效性外，快速挖土有助于在开挖过程中使软土边坡暂时保持稳定。为确保位于软弱地基上的软土边坡的稳定性，坡度不陡于1∶3是至关重要的。基于敏感性分析结果，针对软土地基的强度参数提出了未来开挖的最佳坡角。为防止软土边坡的滑移破坏，一种经济有效的解决方案是在估计的临界滑动面最深点垂直安装可回收的钢板桩穿透潜在的滑动面。