London’s Elizabeth line: creating caverns and tunnels

Five of the Elizabeth line's central London stations – Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel – were built in this way. At up to 250m long, the platforms are more than twice the length of current Underground platforms.

Sequential excavation and SCL were appropriate for these tunnels because they are relatively short. The platform tunnels have connecting passages, leading to the escalators, lifts and station concourse, so there were many junctions and complex, changeable geometry that required an agile construction methodology.

The technique involves excavating a small ‘pilot’ heading that is supported temporarily with sprayed concrete as it advances to a length of anywhere from 3m-10m. This heading is then enlarged, with sprayed concrete again applied to provide support. The process is repeated until the final tunnel diameter and profile have been achieved. The key to the technique is to create a smooth, curved profile, enabling the sprayed concrete lining to act in compression like an eggshell, supporting the surrounding ground. When the SCL tunnel is fully excavated, waterproofing and a secondary concrete lining are installed.

For the Elizabeth line caverns the primary lining was made up of two SCL layers, the first a 75mm thick ‘sealing’ layer and the second being up to 275mm thick, creating overall thickness of 300mm to 350mm, or greater at tunnel junctions. The waterproof membrane was sprayed directly onto the primary lining. A further 250 to 300mm of sprayed concrete formed the secondary lining, followed by a fire protection layer.

We were SCL consultant on the project and worked with contractors to identify ways to save materials and time. Efficiencies focused mainly on the use of sprayed waterproof membranes, the amount and type of steel reinforcement, and how to integrate the initial sprayed concrete shell into the permanent structure, designing the primary and secondary linings as a composite shell.

Alternative mining techniques would have required more excavation and extensive temporary support of the tunnel walls and crown, making for a congested workspace and requiring complex phasing of works. The conventional waterproofing solution would have been a sheet membrane; and ordinarily the secondary lining would be in situ cast reinforced concrete – as was the case at Farringdon Station – and designed without accounting for the structure support provided by the primary lining.

The SCL design delivered substantial time and cost savings.

It also enabled the creation of softer corners between the platforms and passenger access tunnels, giving Crossrail stations a light, open and flowing character, with safety and circulation benefits, unique in London. Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel stations have spacious circulation routes. Their glass-fibre reinforced concrete panel cladding follows the flowing geometry of the SCL. Travelling down an escalator to platform level, along a cross-passage and onto a train, passengers can see where they’re going.

Creating the comparatively open geometry of the Elizabeth line stations was made possible by recent advances in computer modelling and analysis, combined with computer-controlled excavation and SCL equipment and laser surveying, enabling precise control over the lining geometry, which was required to achieve structural strength in such large spaces.

SCL was also used to connect shafts with the running tunnels.

For the Elizabeth line, the SCL process involved applying a sealing layer of up to 75mm of sprayed concrete immediately after excavating an area, followed by a second, thicker coat. Together the two layers form the primary lining. A regulating layer of concrete is added to the primary lining to provide a smooth surface before a waterproof coating is applied. A further 250 to 300mm of sprayed concrete forms a secondary lining, followed by a fire protection layer. The success of SCL on the Elizabeth line has helped to redefine its potential.

When SCL was introduced in London in the 1990s it was regarded only as a means of providing temporary support. Now it’s seen as a permanent solution, which compared with other tunnelling systems takes less time, and saves materials, with no compromise on structural capacity and durability.

Mike Savill

Tunnel practice leader for UK and Europe