An innovative Verification Process speeds construction of Crossrail’s Moorgate shaft
Summary
Liverpool Street station was on the critical path for the completion of Crossrail’s central tunnelled section; however the start of the station’s 42m deep Moorgate shaft was delayed by the prolonged time required to extract the foundation piles of the building that had previously occupied the site. As this delay would have led to knock on delays in completion of the shaft and hence the transit of the running tunnel TBM through the shaft, a carefully defined and controlled observation based Verification Process that makes innovative use of real-time site monitoring data and state-of-the-art 3D numerical analysis of ground-structure interaction, ground movements and movement of adjacent tunnels was developed.
The paper describes the application of the Verification Process: iterative analysis using FLAC software; a new constitutive non-linear soil model for clays; field observation during the works and implementation during construction. The outcome was that as the shaft was being excavated the Verification Process progressively demonstrated that each of three levels of propping could be omitted, and the number of excavation stages reduced by two. All of this was extremely good news for Crossrail as it shortened the shaft construction programme by 14 weeks which meant there was no delay for the TBM drive for the westbound running tunnel.