Optimization of Design and Monitoring of Immersed Tunnels

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Many immersed tunnels have already reached or are about to reach the halfway point of their intended lifespan of 100 years. Around this time, several structural problems were noticed during inspection which have led to performance deterioration and durability issues of the tunnel. Often large- scale maintenance was required to meet the design life of immersed tunnels. To counter these problems and improve the constructability of tunnels several design changes have been developed over the past two decades. Furthermore, the current monitoring plan lacks in terms of frequency, accuracy and often requires tunnel tube closure. This thesis aims to improve the life cycle operation of immersed tunnels based on long-term structural problems encountered during their design life.

To achieve this, common structural problems encountered were initially identified by reviewing literature and maintenance reports of past projects. Excess deformations, joint gap opening/closure, concrete cracking, corrosion and leakage were determined as the five typical structural problems. These problems were then further analysed based on construction technique, geotechnical engineering and design flaw from a mechanical and structural perspective. Once the structural problems were identified, recent design changes were inventoried based on element material, cross-section design, transverse prestressing, foundation treatment, waterproofing and joint formation. The primary objective of this step was to understand the impact of these design changes in mitigating structural problems.

Following the structural problem study, an improved monitoring plan was developed to serve as an early warning system for structural problems. Firstly, the parameters to be monitored were determined based on how they influenced structural problems. These parameters were vertical displacement, transverse displacement, joint width, crack width, shear key force, leakage detection, chloride diusivity, pH in concrete, concrete strain and foundation soil stiffness. Subsequently, threshold limits for each parameter were set to implement quick remedial actions once these limits were crossed. Afterward, an inventory of sensors and monitoring techniques were selected to monitor these parameters based on accuracy, frequency and how helpful they were to contractors in current-day practice. Remediation measures specific to each structural problem were also briefly discussed.