ABSTRACT
This publication presents observed settlement of an existing cast iron tunnel that was influenced by the excavation of a new tunnel directly beneath and parallel to its axis for more than 100 m. The existing tunnel is 2.7 m in diameter and the new tunnel is up to 11 m in diameter. The clearance between the two was around 2 m. The new excavation took place in two stages; a 6 m diameter pilot tunnel, and a later enlarged to approximately 11 m in diameter. The longitudinal deformation of the existing tunnel due to these two stages was monitored using a newly developed ‘digital image correlation’ technique (CSattAR) and fibre optic sensing cables. This paper focuses on the CSattAR data which show that at subsurface level, the magnitude of the immediate longitudinal ground movement ahead of the tunnelling face in London Clay is likely to be less than empirical assessment methods suggest.
