Back-Silting Characteristics of Foundation Trench Excavation in an Ultra-Wide Inland Immersed Tunnel and Its Impacts on Slope Stability: A Case Study of the Tanzhou Waterway in Shunde

During the construction of a large immersed tunnel crossing an ultra-wide inland river, the long drying time after the excavation of the foundation trench and changes in river flow velocity result in the river carrying a large amount of sediment into the foundation trench and the slope, increasing installation difficulties and threatening construction safety. This study investigates the back-silting characteristics and their impacts on foundation trench slope stability during an ultra-wide immersed tunnel excavation at LunGui Road in Foshan City, China. Numerical simulations reveal the spatiotemporal distribution patterns of deposited sediments at the trench bottom and side slopes, with distinct behaviors identified between the flood season and dry season. Siltation predominantly occurs at the trench bottom, with deposition thickness decreasing almost linearly from the bottom to the slope crest. Hydroperiod variations considerably influence the spatiotemporal distribution of back-silting. Then, the Morgenstern–Price method was employed to analyze slope stability under varying back-silting and dredging conditions, quantifying the relationship between safety factor and sediment thickness. Furthermore, the evolution of critical failure surfaces and the safety factor under different dredging strategies was systematically examined. The critical values of back-silting thickness corresponding to different dredging slope ratios are provided. The research findings provide valuable insights for formulating engineering strategies for trench excavation of extra-wide immersed tube tunnels in inland waterways.