Observed Behaviors of an Ultra-Deep Excavation with an Innovative Pre-Support System in Shanghai Soft Deposits

Time-dependent deformation of excavation in soft deposits is an important issue concerning the security of underground engineering. To mitigate the construction duration and alleviate the influence of time effects, an innovative sliding pre-support system (SPS) has been proposed, which enables effective support within 4 to 6 hours following excavation. In this study, the SPS was applied in a 43 m deep excavation. To have a better understanding of the impact, an exhaustive field monitoring program was conducted. Additionally, the monitoring data of wall deflection and ground settlement was compared with that of another excavation without the new pre-support. It was found that: The implementation of the SPS resulted in a 40% reduction in the construction period; The normalized maximum deflection of pre-supported excavation was less than 0.19%H and the settlement remained under 0.10%H (H is the depth of excavation), which were substantially smaller than cases observed in soft clay; Furthermore, analyses indicated that deflection of diaphragm wall showed a rapid growth when excavating through the micro-aquifer and aquifer.