Mechanical Response and Deformation Characteristics of Sinking Well Backfill Structures in Non-Uniform Bedrock and Fractured Strata

Large caisson foundations in sea-crossing bridges are often constructed in non-uniform bedrock with fractured weak layers, where the mechanical response during sinking, bottom sealing, and core filling is complex and not well understood. Existing studies generally assume uniform and intact bedrock and focus primarily on the service stage, leaving the entire construction process in such complex strata largely unexamined. To address this gap, this study investigates a large caisson foundation using field investigation data and three-dimensional numerical simulation with PLAXIS 3D (version: V20), taking the Xihoumen Rail-cum-Road Sea-crossing Bridge as an engineering case. Sonar-derived bedrock geometry, geological exploration data, and laboratory rock properties are incorporated, and the full construction process is simulated. After sinking, the maximum longitudinal and transverse displacements are 0.6269 mm and 0.6298 mm, respectively, both located on the right side of the middle compartment, while the maximum vertical displacement is 2.918 mm in the suspended part. Stress concentrates at the caisson–bedrock interface, with a maximum vertical stress of 3825 kPa at the cutting edge. During core filling, the vertical displacement increases to 4.65 mm, while the longitudinal and transverse displacements show a fluctuating upward trend, reaching 0.854 mm and 0.87 mm, respectively. As the weak-layer thickness decreases from 0.8 m to 0.2 m, the displacements under the most unfavorable load combination decrease. For the investigated foundation, a weak-layer thickness of 0.6 m or more causes a notable increase in deformation, while the base stress changes little. These results are numerical predictions, and the 0.6 m threshold should be regarded as a project-specific recommendation rather than a universal design criterion. The findings provide a reference for similar projects.