Observed Performance of Long-Zoned Excavation with Suspended Waterproof Curtain in Yangtze River Floodplain

Abstract

A 1,130-m-long, 12.9- to 62.0-m-wide, and 7.8- to 12.2-m-deep braced excavation supported by stiffened deep cement mixing (SDCM) walls in Nanjing Yangtze River floodplain, China, was constructed using zoned excavation technology. Because such excavation was carried out in soft clays underlain by a very thick deposit of fine silty sand to a depth over 40 m, suspended waterproof curtains (SWCs) were chosen toward groundwater control, where the bottom of SDCM walls did not enter the deeper confining bed. Although SWCs had attained a remarkable economic result, it was unable to effectively cut off hydraulic connections between two sides of the wall. Thus, more attention needs to be paid to the excavation safety and induced deformation during excavation. This paper presented a case study to observe the performance of such long-zoned excavation, where the variations of wall deflection, groundwater level, and ground surface settlement were monitored in real-time. The effects of SWCs on the excavation performance were also explored. The monitoring results indicated that SDCM walls with relatively lower stiffness only had a small wall deflection. The zoned construction procedure and ground improvement measures were demonstrated to be effective in controlling wall deflection. However, significant drawdown of groundwater level as high as 7 m was observed outside the excavation under predominant groundwater seepage, resulting in an extremely obvious ground surface settlement as high as 150 mm. The occurrence of ground settlement was more influenced by seepage effect than unloading effect induced by excavation. The maximum changes of groundwater level and ground settlement in this case were far beyond the limitation of the industry standards. The SWC should be used prudently in Yangtze River floodplain, with more concerns into the surrounding environment.