Abstract: The force mechanisms and influencing factors of unilateral water-adjacent deep foundation pits for immersed tunnels with limited support are investigated. A model test of foundation pits with unilateral water-adjacent cofferdams and limited soil is designed to examine the mechanical characteristics of the supporting structures during foundation pit construction, based on typical immersed tunnel scenarios. Test results are compared with field measurements, and a three-dimensional numerical model is constructed to analyze the spatial force characteristics of the foundation pit, focusing on the deformation patterns and influencing factors of the support system. The results reveal the following: (1) The earth pressure exerted by the water-adjacent foundation pit with a cofferdam and limited soil is significantly less than that generated by infinite soil. Consequently, the bending moment is reduced and the inflection point shifts upward, significantly altering the pile’s mechanical characteristics, which must be considered in pile design. (2) The earth pressure on the limited support side increases nonlinearly with excavation depth. The fixed embedding of the pile tip creates a relatively static soil zone, resulting in greater earth pressure and a distribution pattern that deviates from classical earth pressure theories. (3) The lateral displacement of the retaining piles on the waterside exhibits a linear relationship with the connected water level and an inverse linear relationship with the width of the limited soil, showing slight dependency on the slope ratio of the cofferdam. A critical limited soil width determines whether the top of the foundation pit displaces outward; this critical width is significantly larger than the calculated width according to earth pressure theory. In practical engineering applications, this critical value is recommended to be set at one times the foundation pit depth to maintain active equilibrium on both sides of the pit. (4) Geotextile bags combined with sand cofferdams demonstrate a wrapping reinforcement effect and favorable selfstability. Their horizontal earth pressure cannot be calculated under existing classical theories. The slope ratio design of the cofferdam should not simply follow hydraulic engineering standards; instead, a slope ratio of 1:1.0 to 1:1.5 is recommended for temporary cofferdams.

Key words: immersed tunnel, water-adjacent foundation pit, supporting structure, limited soil, mechanical characteristic, model test, numerical simulation