Assuring the Required Remaining Lifetime of a Railway Viaduct Heavily Deformed during Tunnel Execution

Abstract

Within the Spoorzone Delft project, a two-phased tunnel execution process is being carried out to create a new four-track railway tunnel. This tunnel will replace the existing two-track railway viaduct that splits the heart of the city into two halves. In order to maintain railway traffic at any moment, the two-track eastern tunnel tube is to be established first. Once the railway traffic uses this tunnel, the existing viaduct can be demolished and the execution of the western tunnel tube—once again comprising two tracks—can be started. Owing to unforeseen circumstances, the viaduct was heavily deformed during the execution of work on the eastern tunnel tube. To ensure the remaining lifetime of the viaduct until the time the railway traffic can be diverted through the eastern tube, a series of actions had to be taken. With the immediate implementation of a train traffic speed reduction combined with primary strengthening works, the contractor borrowed time to investigate further necessary measures. Causes of the deformations were defined. Additional strengthening of the structure was performed, which led to the withdrawal of the temporary speed reduction. Finally, the existing execution methods were adapted and applicable mitigating measures were installed in case additional deformations would occur. The existing monitoring was upgraded on the basis of an assessment of the structural condition of the viaduct. The paper focuses on the whole design and investigation process that took place, combining structural as well as geotechnical aspects, which resulted in a well-defined execution sequence and an elaborate monitoring program with narrow bandwidths between upper and lower deformation boundaries.