A new photogrammetric system has been developed to monitor tunnel deformations with high precision. The system fundamentally applies digital image correlation techniques to process images. It is digital in nature and is compatible with the industry's vision of moving towards the smart and digital monitoring of existing assets. It has been installed successfully within the London Underground railway and a number of other tunnels to demonstrate the potential of remote deformation monitoring of such assets using off-the-shelf cameras. The system is able to measure deformations with precisions higher than 0·1 mm, enabling the monitoring of sensitive deformations such as imposed curvatures. This paper highlights the application of photogrammetric systems in monitoring the deformation and condition of assets in real time and highlights the advantages that such systems bring. A case study has been chosen to demonstrate these. The case study is the monitoring of an escalator barrel in Moorgate station in London that was influenced by a sprayed-concrete-lining excavation directly underneath for the new Elizabeth line.
Displacements imposed upon existing cast-iron tunnel linings attributed to new underground construction often cause changes in their alignment. The rate of change in alignment, or curvature, is a key parameter in assessing the impact of displacement. Owing to the installation limitations and the precision deficiency of conventional monitoring systems, measuring such curvature deformations, especially locally, has been a challenging task. In this paper, the curvature measurement data of the following two case studies are presented: (a) Crossrail's Liverpool Street station construction affecting the Northern Line (NL) tunnels, and (b) the Bond Street station upgrade project affecting the Royal Mail tunnel (RMT). Both new tunnels ran perpendicular to the existing cast-iron tunnels. They experienced substantial longitudinal curvatures, but the damage was limited. The NL tunnels were monitored using conventional electrolevel-beams for minimum chord lengths of 5 m, while the curvature of the RMT was monitored for chords as short as 2 m using a new photogrammetric monitoring system, called Sattar image tracking. The new system offers a high-precision measurement enabling such lengths of local measurements. This paper describes how curvatures are measured and proposes methods that need to be followed to evaluate the accuracy of curvature measurements.
Heritage masonry structures require detailed monitoring during nearby underground construction, to address the concerns of building owners and mitigate uncertainties relating to the soil–structure system. This paper discusses innovative displacement and strain monitoring of a listed masonry church undergoing ground movements due to nearby tunnelling in London Clay. In addition to conventional surveying and tilt monitoring, laser scan displacement monitoring and fibre optic strain sensing was conducted. Prior to construction, point clouds of the structure were used to map pre-existing geometric distortions in the structure. During construction, detailed monitoring of displacements and strains was conducted across the building facades and the tower. The same techniques were also used to monitor the painted dome of the church where conventional displacement monitoring was not feasible. With a focus on the new engineering insight provided by the innovative monitoring techniques, the impact of the underground construction activities on the church is discussed in this paper. The proposed sensing techniques provide new data on the influence of pre-existing damage and tunnel advance on soil–structure interaction.
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