Pipe-jacked canopy performance at Bond Street, London, UK

As part of the Bond Street underground railway station upgrade works in London, UK over 500 m of tunnels are being built in a dense urban environment. Tunnelling-induced settlements presented a significant risk to the functionality of a sensitive set of escalators within the existing station. As a means of reducing the risk to their operation, a pipe canopy, consisting of 12 steel tubes 325 mm in diameter, was installed directly above the tunnel crown. Measurements of the forces required to drive the pipe canopy tubes are discussed in relation to pipe-jacking theory, with guidance being provided for the future use of this method in London Clay. Instrumentation installed within the pipe canopy is then used to describe its response to tunnelling. Tunnelling-induced settlements of the escalator are also presented and used to describe the complex soil-structure interaction mechanism. Comparisons of the pipe response with predictions made using a semi-empirical elastic continuum method demonstrate that relatively simple calculations can be used to quantify conservatively the likely reduction in settlements caused by a pipe canopy. Based on the observations, key design parameters are identified and a method for selecting values for these parameters is proposed.