Immersed tunnels are underwater structures consisting of prefabricated elements that are floated to the construction site and then immersed in place. These elements are typically composed of segments, with all structural connections between the segments and elements formed by shear joints or the shear keys. These shear connections are essential for restricting movement within the tunnel, ensuring both waterproofing and structural safety. However differential settlement, a common issue in such structures can introduce significant shear forces within these joints, posing a threat to the tunnel’s structural integrity. The performance of these shear joints is heavily influenced by the complex interactions between the tunnel and the foundation. The foundation and subsoil stiffness variability plays a critical role in differential settlement between adjacent tunnel elements, thereby directly impacting the performance of the shear keys.
The existing design methodology for the shear joint design used in Fehmarn Belt project, rely on techniques like Gaussian random field, Monte Carlo sampling, and joint statistics in the post processing
to model the influence of the spatial variability. While these methods provide detailed insights into the effects of spatial variability, their complexity, and the time intensive nature of their application could effect the timely delivery for large scale projects where efficiency is paramount. Thus, the main objective of this thesis is to develop a simplified approach to Soil-Structure Interaction (SSI) analysis that remains effective without the cumbersome detail of the existing design methodology. The purpose of this simplified approach is to offer a reliable alternative to the existing SSI methodology used in the Fehmarn Belt project.
The approach presented in this thesis simplifies the modelling of the combined stiffness variability of the subsoil and gravel bed beneath the tunnel. This approach utilizes a multi-linear Winkler Spring model. The soil stiffness parameters are derived from the CPT-NEN approach, and the combined stiffness variability of the subsoil and gravel bed is calculated using the spring-in-series equation. The soil
deformations, simulated in PLAXIS 2D, are translated into springs using the multi-linear Winkler Spring approach. This simplification forms the basis for the subsequent analysis performed in SCIA Engineer
software, where joint shear forces are calculated under various stiffness variability schemes and load levels.
The thesis concludes by comparing the shear response of the simplified approach with those obtained from the complex soil-structure interaction design in the Fehmarn Belt project, validating the reliability of the used approach in terms of safety. The comparison demonstrates that within acceptable tolerances, the simplified approach produces reliable results for the considered design conditions, taking into account the scope defined in the thesis.