Case studies of circular shaft construction in London

Abstract

At present, there are few well-documented case studies of circular shaft construction, making it difficult for designers to estimate reliable ground movements arising from such construction. This paper describes field observations of ground surface settlement assembled during the construction of 27 circular shafts built for three major tunnelling projects in London: Crossrail, National Grid's London Power Tunnels project and Transport for London's Northern line extension. Two categories of shaft construction were identified: support before excavation (SBE) and excavation before support (EBS). For the SBE category, the shaft was first supported by pre-installed walls followed by excavation of the soil between the pre-installed walls. For the EBS category, the ground was progressively excavated in sections followed by construction of the shaft lining. Interpretation of the field observations showed the importance of the shaft construction method on ground movements. Settlements were much more significant for EBS shaft construction than for SBE shaft excavation, although settlement arising from the installation of pre-installed walls or dewatering operations should not be overlooked. Normalised charts are presented to help the industry make estimates of settlements due to circular shaft construction in London, with due consideration for different shaft geometries and construction methods.