Cast-iron tunnels’ tolerance to imposed longitudinal settlement curvature

Abstract

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.