Distributed fibre optic sensing of a deep excavation adjacent to pre-existing tunnels

This research study investigated diaphragm wall (D-wall) behaviour due to deep excavation at Paddington tunnel station site in London Clay. The Paddington site was the only train station in the Crossrail project constructed using a top-down excavation, and it provided the opportunity to evaluate the effect of a pre-existing tunnel on D-wall behaviour using distributed fibre optic sensing (DFOS) for the first time. Distributed fibre optic cables were embedded in the D-wall panels to monitor the changes in strain conditions during three key stages of construction; tunnel passage, concourse excavation and base excavation. After station construction, relevant finite element (FE) analysis was conducted to evaluate the D-wall performance during excavation, and the computed results were compared against the field measurements recorded by embedded DFOS in the D-wall as well as the ground inclinometers. The DFOS measurements depicted the D-wall behaviour in agreement with the conventional inclinometer method and FE results, demonstrating its feasibility in monitoring underground earth retaining infrastructure. A comparison between the D-wall behaviour with pre-existing tunnels at Paddington site and that of the same wall without tunnels allows providing some guidance for the design and construction of retaining structures adjacent to pre-existing tunnels.