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start out on its journey from Royal Oak towards Farringdon. This will be followed by the launch of further TBMs to construct the remaining tunnels for the new Crossrail service.

The Royal Oak Portal site has now been handed over to the tunnel l ing contractor for Cont ract C300 – Royal Oak Por tal to Farringdon: a joint venture comprising BAM Nutall Ltd, Ferrovial Agroman (UK) Ltd and Kier Constructon Ltd.

Constructing the portal

The Royal Oak Portal works comprise a massive 285 m long ramp structure that takes the Crossrail tracks from ground-level down into the underground tunnels. The portal has been built within a narrow corridor at Royal Oak, constrained by the A40 Westway to the north and the Hammersmith & City and Network Rail lines to the south.

The worksite, which is only 21 metres wide, had a protectve barrier to protect the workers and machinery from the adjacent live railway, to protect the railway from the risk of plant falling onto it, and to help lessen any noise and light impact on local residents.

A total of 25,000 m³ was removed to construct the tunnel portal. The excavated material being reused on other London constructon sites including Downes Barns Golf Course in Hillingdon.

To enable the tracks to progress from ground level to the tunnels, it was necessary to build a retained cutng. This was built using sheet piles driven into the ground on each side of the tracks, and excavatng in-between. As the tracks go deeper stll, diaphragm walls are used - reinforced concrete walls constructed using a Lieberr piling rig that cut a series of overlapping slots in the ground. The slots are flled with bentonite to stop the sides collapsing, steel reinforcement cages are lowered into positon through the bentonite and then the slot is flled with concrete from the botom upwards. The bentonite is pushed out of the slot, leaving a reinforced concrete wall embedded in the ground.

Fifty four steel and concrete props supported the portal walls during the excavation and some have become permanent support structures for the tunnel entrance. In order to install these props, a purpose built overhead gantry crane was created, capable of lifing 25 tonnes. The gantry crane was considered to be the safest method of heavy lifing when working beside an operatonal railway. By February 2011, the supportng props and piling were in place and excavaton of the ramp began. Once the ramp was excavated a concrete slab was laid to further support the outside walls and to provide the base for the new railway tracks. At t he deepes t end o f t he cutting a concrete ‘headwall’ was constructed, and a lining wall with a pai r of 7.24 metre (nominal ) diameter ‘tunnel eyes’ installed on

Page 12 Contractors World - UK & Ireland Vol 1 No 6

For safety reasons, a special gantry crane was installed to handle the large temporary props at Royal Park works for Crossrail.

With the support walls and props in place, excavaton could begin. Operators had to work within the confnes of width and

height.

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