Application of observational method at Crossrail Tottenham Court Road station, UK

Excavation of the western ticket hall box at Tottenham Court Road station represented a critical path activity of the Crossrail railway project in London, UK. Base slab construction and preparatory sprayed concrete lining works needed to be completed before the arrival of the two tunnel boring machines. A full observational method (OM) design was implemented to eliminate the lowest level of temporary propping, resulting in a 13 m prop-free excavation at the bottom of the 30 m deep diaphragm wall station box. This paper describes the design and successful implementation of the OM, which eliminated the need to fabricate, install and remove the fifth level of temporary propping. It covers the comprehensive review of the original design input parameters, back analysis of the most probable geotechnical design parameters and comparison of wall deflection and forces in the temporary props from early stages of the construction works. The robust strategy formulated as part of the observational method design to control the subsequent excavation is described: this included trigger criteria, a review process and predetermined contingency measures to ensure safe execution of the excavation works. A genuinely collaborative effort between client, permanent and temporary works designers, independent design checker and the contractor is elaborated. Cost and programme savings achieved by implementation of the observational method approach are outlined.