Longitudinal response of an existing cast iron tunnel subjected to parallel piggy-back tunnelling

Tunnelling close beneath an existing tunnel is a common occurrence in an urban environment. Alignment of these tunnels range from a perpendicular undercrossing to perfectly parallel. The latter is much less common, thus much less understood. Conventional assessment methods impose greenfield soil displacements directly on the existing tunnel as a worst case scenario, often leading to high estimates of longitudinal radius of curvature which are beyond the allowable design limit. Subsequently, costly and time consuming mitigation measures would have to be designed and mobilised. The scarcity of high quality case studies of parallel piggy-back tunnelling impedes the continuous effort to understand and improve the efficiency of such impact assessments; this paper presents a detailed case study which focuses on the longitudinal response of an existing 3m diameter, a century old cast iron Royal Mail tunnel that is affected by the construction of an approximately 12m diameter platform tunnel in a parallel piggy-back scenario just 2m directly beneath. Various state-of-the-art instrumentation systems including distributed fibre optic strain sensing, photogrammetry and vibrating wire instrumented bolts and electronic displacement transducers were deployed over a 40m section of the existing Royal Mail tunnel to monitor its response. This enabled the full response of the existing tunnel to be captured in a high level of detail in terms of strain and displacements as the tunnelling face progressed towards and beyond the zone of monitoring for both the pilot and tunnel enlargement phases