The Moorgate shaft

The Moorgate ticket hall connects to the western ends of the Liverpool Street Station platforms via one of the deepest shafts in London – 42m deep. It contains two escalator banks, emergency escape stairs, passenger and firefighting lifts, ducts for station and tunnel ventilation plus smoke extraction, and plant rooms. It also connects to London Underground’s Northern line.

The shaft is bounded by London Underground’s sub-surface lines to the north and Northern line to the east, plus about 50 buildings, some of them listed. With the shaft’s east wall less than 5m from the northbound tunnel of the Northern line tunnel, it was essential to avoid ground movement. The target was to limit horizontal wall movement to no more than 45mm, but for the east wall this was set at a maximum of 30mm.

Joint venture contractor Bam Nuttall-Kier Construction were awarded the construction contract in March 2011 and started on site in July. The shaft was designed as a diaphragm wall structure, an irregular polygon in plan measuring approximately 35m x 35m. Excavation was top down, with seven ring beams stiffening the walls to resist ground pressure. Plans were to install additional temporary steel props, plus two temporary reinforced concrete cross-walls to further safeguard against movement. Props would be removed when the base slab was completed, and the reinforced concrete shaft lining and internal walls constructed.

Staying on the critical path

The shaft was on the critical path for completion of Crossrail’s central tunnelled section. It had to be fully excavated in time for arrival of the westbound tunnel boring machine (TBM). But extracting the foundations of a six storey 1970s building previously on the site took much longer than anticipated, delaying the start of diaphragm wall construction by 11 months. This made it all but impossible for contractor Bam Nuttall/Kier Construction JV to complete the shaft in time, without a major rethink of the construction methodology.

A new verification process

To meet the deadline, we worked with the contractor and client to redesign the solution, using new parameters for characterising the ground and modelling movement. Our redesign drew on the very latest analyses of other major excavations we had recently worked on, including the basements of Heathrow Terminal 5 and the Crossrail SCL caverns. We built a non-linear model of the ground using state-of-the-art software, FLAC 3D. We also employed a means of support pioneered at Heathrow T5: the propping action of temporary concrete slabs or ‘blinding layers’ was used to stiffen the shaft walls while ring beams were cast.

We combined these innovations with an observation-based approach to construction. This involved checking observed structural movement against predictions. If movement was within predetermined parameters, work could proceed. And if movements exceeded the parameters, mitigating steps had to be taken.

Observations included real-time monitoring of movement in the diaphragm wall and adjacent structures, measurement of pore water pressure in the excavated material, and heave at the base of the excavation.

This approach enabled the contractor to carry out excavation without needing to install temporary props. Not only did it save the considerable time required to install each prop, but it also meant that excavators could work, spoil could be removed, and reinforcement and concrete for the ring beams and base slab supplied, without needing to manoeuvre around a steelwork grillage, saving still more time.

Getting to the point

Three verification points were set at depths of 19m, 26m and 30.5m. At each, the FLAC 3D model was updated to incorporate the as-built details and used to predict ground behaviour for the next phase of excavation and revise the ring beam design to suit.

Observations showed that it was not only safe to carry out excavation without the need for temporary props, but to combine some excavation stages, enabling progress to be accelerated.

The cumulative time savings meant that the contractor clawed back the 11 months of time lost battling with site preparation. And better: the 1800m3 concrete pour for the base slab was completed 14 weeks ahead of the original deadline. Ready for the TBM’s drive westward to Farringdon, with time to spare.

Winner: Ground Engineering Awards 2015 – Editor’s Award