Abstract
A series of centrifuge model tests in clay was carried out to investigate the response of an existing tunnel at different clear distances to new tunnelling. A three-dimensional (3D) staged tunnelling model was adopted to simulate a wide range of tail void volume losses for the new tunnel construction while monitoring detailed 3D soil surface settlements and tunnelling-induced strains in the existing tunnel lining. This paper also presents a detailed case study of a similar scenario in the London Underground redevelopment of Bond Street station; various state-of-the-art instrumentation methods, including fibre optic Brillouin optical time domain reflectometry, instrumented tunnel bolts and photogrammetry, were deployed to monitor the response of the existing Royal Mail tunnel due to the new tunnelling works close beneath. The combination of field and centrifuge modelling data provides important new insights into the deformation mechanisms encountered in such complex tunnelling scenarios.
Highlights
Tunnelling underneath an existing tunnel in close proximity is a common recurring problem in densely populated cities with underground transportation networks
Boonyarak (2014) carried out centrifuge testing of a perpendicular undercrossing in sand; this study modelled tail void volume loss in series of advancements at a fixed volume loss of 2·0% at two different clear distances
Taking this into account along with the alternative subgrade modulus analogue proposed by Klar et al (2005), overestimations of maximum bending moments were reduced by approximately 5–7 times for a volume loss of up to 4% as opposed to up to 20 times if the pipelines were forced to conform to greenfield settlement profiles
Summary
Tunnelling underneath an existing tunnel in close proximity is a common recurring problem in densely populated cities with underground transportation networks. Vorster et al (2005) proposed that this method should be used in conjunction with an appropriate shear stiffness degradation to arrive at a representative soil stiffness value Taking this into account along with the alternative subgrade modulus analogue proposed by Klar et al (2005), overestimations of maximum bending moments were reduced by approximately 5–7 times for a volume loss of up to 4% as opposed to up to 20 times if the pipelines were forced to conform to greenfield settlement profiles. While these numerical approaches have been shown to be very useful for pipeline assessments, none of these methods have been validated for tunnels.
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