Performance of Immersed Tunnels subjected to Settlements

Abstract

In the next decades, Rijkswaterstaat will renovate many existing immersed tunnels so they can meet their intended lifespan in a good condition. Ongoing settlements could be a threat to the tunnel structural performance, as deformation may cause structural damage. Currently, immersed tunnels are subjected to greater and more uneven settlements than considered in the design. The issue of these ongoing settlements is mainly concentrated in the segment joints, which have typically been executed by a spigot and socket structure to prevent differential settlements. Significant deformation of the tunnel structure could result in concrete cracking, which entails the risks of structural connection loss, tunnel misalignment, and leakages (leading to durability reduction and damage to installations). This could impact the reliability of the structure over time, and consequently affect the availability of the road network. Therefore, the key objective of this research is how to assess the structural reliability and tunnel availability, and to investigate in which way the performance of existing immersed tunnel structures could be optimised given the ongoing settlements. Assessing the residual lifespan is beyond the scope, but optimising the tunnel performance during the remaining operational life is the main motivation for this research project. The research methodology consists of a parametric structural model, which determines the tunnel reliability and availability by using a Monte Carlo approach. The Noordtunnel case is used to demonstrate the research methodology, as this tunnel showed excessive settlements and a major renovation will be performed in the coming years. Additionally, the maintenance and renovation strategy for the segment joints in immersed tunnels are examined, to optimise the performance throughout the residual lifespan.