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) ($1\mathrm{V}:3\mathrm{H}$), the sliding took place at the uppermost 6-m slope and there was no sign of sliding in the lower 7 m of slopes ($1\mathrm{V}:2\mathrm{H}$) atop firm to stiff clayey strata. The steep local slope gradient ($1\mathrm{V}:1.75\mathrm{H}$) 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.