Keith Sibley, Crossrail area director west, said: “Tunnelling for Crossrail has now commenced. Phyllis is now beginning to slowly disappear below ground and will steadily ramp up to the planned average tunnelling rate of around 100 metres a week.”

Following the official unveiling of 1,000-tonne Phyllis, Crossrail’s first TBM travelled 400m to Royal Oak Portal including under Hampden Street footbridge which was jacked up to allow the 7.1m diameter machine underneath.

The arrival of the TBM at the portal in mid-March allowed for remaining major works to get underway to support tunnelling.

A major piece of work has been the construction of the launch structure which allows the TBM to propel itself forward. The total force required for the TBM to move forward is equivalent to the force needed to lift over 2,900 London taxis. A steel seal has been fitted around the portal entrance to support the ground during the early stages of tunnelling.

Work has also been underway to install the major conveyor system behind the TBM to remove more than one million tonnes of excavated material from the cutter head to the portal entrance. A total of 24 kilometres of conveyer belt will be used to construct the western tunnels.

A further tunnelling machine, Ada, will subsequently be launched from Royal Oak. Once the first machine has progressed sufficiently, the second tunnel boring machine will be brought down to the portal headwall.

When the second TBM has reached Paddington, both machines will then progress through the Paddington station box to Bond Street and then to Farringdon. The tunnels between Royal Oak and Farringdon should be completed in late 2013.

Eight tunnel boring machine will be used to construct 21km of Crossrail’s  twin bore tunnels running between Royal Oak in west London and Pudding Mill Lane and Plumstead in east London.

First spray concrete lined tunnels completed under Finsbury Circus

Crossrail’s first two sprayed concrete tunnels have been constructed under Finsbury Circus in the City of London. The works are part of construction for Crossrail Liverpool Street station. The 4.5m diameter tunnels have been built from the main access shaft within the Finsbury Circus worksite.

The two tunnels are temporary structures and will be used for compensation grouting, one of the ways that Crossrail will control any ground movements that could result from tunnelling activity.

Small diameter tubes will be drilled and installed from the temporary tunnels allowing the contractor to inject grout into the ground. This will stabilise the ground and limit surface settlement.

A sprayed concrete lining (SCL) technique has been used to build these tunnels. This technique involves rapidly spraying the excavated ground with concrete to stabilise it and form the permanent tunnel lining. The first SCL tunnel measures 80m in length and the second (running east from the worksite) is 100m long.

The SCL technique will be used to build 12 km of station platform tunnels, passages, access and grout tunnels along the Crossrail route where smaller, relatively short tunnels are needed.

Unlike bored tunnels, SCL allows variation in the tunnel shape and diameter and 7.5km of SCL tunnels will become permanent features of the project.

Colin Niccolls, Crossrail’s Whitechapel & Liverpool Street platform tunnels project manager, said: “Crossrail will primarily use two different types of tunnelling technique to build the new rail tunnels and stations. While tunnel boring machines will create the new rail tunnels, work has commenced on creating the station tunnels using mining techniques to excavate the ground which is then sprayed with shotcrete to form the tunnel linings.”

Once Crossrail works are completed at Moorgate, scheduled for 2017, Finsbury Circus will be re-instated to its pre-construction condition.

Got a story? Email news@theconstructionindex.co.uk