Patrick Reynolds, Freelance Reporter

With all three main tunnelling contracts and most portal contracts now awarded, and while procurement continues for the main works at the underground stations, Crossrail has released a schedule for the TBM boring and major construction works, reports Patrick Reynolds.

Fig 1. Layout of planned tunnel drives

Staging of TBM and station excavations on Crossrail is a story of east and west, with the border line through Farringdon station in central London.

To the east is the bulk or about 70% of the project's 41.5km of twin tube running tunnels. There are also four underground stations east of Farringdon with three in the more straightforward excavation process to the west (Fig 1).

established A further staging difference between east and west was established during the long contractor procurement phase. In the east the four underground stations at Liverpool Street, Whitechapel, Canary Wharf and Woolwich will be constructed before arrival of the running tunnel TBMs. In the west, the winning contractor of the running tunnels and the early SCL works for the stations, will drive the TBMs through the stations zones and break out to create the station caverns at Paddington, Bond Street and Tottenham Court Road.

With vast amounts of material to excavated from beneath the hearth of London, a new tunnelling strategy by Crossrail allows more spoil to be moved by rail or river rather than by road, easing congestion otherwise caused by construction traffic(Fig 2). Early SCL work at Bond Street, for example, will create a cross passage to switch all excavated material, including from the station, to a single running tunnel feed back to Royal Oak (Fig 3).

Further aspects and elements of the new tunnelling strategy are borne of efforts towards more efficiency as well as external requirements of the Government to economise in an era of greater austerity. While there is some shifting in the TBM programming, the revised schedule also sees moving some station works towards the latter parts of the scheme. A rescheduled start of rail services pushed back by a year to 2018, allows "a more efficient construction timetable", said Crossrail.

The change of plan was announced in the UK Government's Comprehensive Spending Review, last October (2010), after pledging support of the project but emphasising that cost savings would be needed for the public and privately funded rail scheme.

With the previous schedule anticipating award of the main tunnel contracts before mid-2010, the national election that brought in the new Government, had those procurement decisions taking longer.

The first awards were therefore made more than half a year later than initially anticipated. The top acceptable cost for the project, accounted by the Government in the spending review, is now £14.5 billion (about US$23.6 billion), which is almost 9% below the original £15.9 billion budget. About £1 billion of the savings identified so far are accrued by engineering led changes in the construction plan and revision of contract cost schedules.

Fig 4. Plan for TBM drives in the west tunnels

Driving plans
Changes in tunnelling strategy for the west tunnels contract come from a revision of the TBM launch and completion schedule. While still starting in the west at Royal Oak portal, the first TBMs are to set off in the second quarter (Q2) of 2012, or about half a year later than previously scheduled. The 6.16km EPBM drives eastward to Farringdon are to take up to a year and a quarter to complete with the station excavations progressing in their wake.

Shortly after, two TBMs will launch on the longest drives of the east tunnels contract, the 8.3km tunnels westward from Limmo Peninsula to Farringdon. Boring longer distances than those in the west, and requiring the Whitechapel and Liverpool Street stations to have platform areas opened before they can pass through, these EPBMs are due to complete after two years work, in Q3-2014, a year later than their opposite numbers.

Fig 5. Plan for TBM drives for the east tunnels

The remaining two of the six EPBMs to complete the running tunnels under central London, are to complete the shorter sections in the east tunnels contract. First, a 2.72km long drive off the Limmo-Farringdon section to be constructed in about a year before the TBMs are removed, transported, reassembled and relaunched to bore the 930m shorter drives that will take the main tunnels from Limmo towards Victoria Dock.

Further east, the one slurry machine to be used on the project will have started on the river crossing Thames Tunnel.

All TBMs on the east side of the project will then be in operation towards having all running tunnel work complete by Q3-2014.

Starting slightly later than previously proposed (Q4-2012) but finishing in still in Q2-2014 is less time given to the Thames Tunnel and while having only one shield and back-up system, there will be two cutterheads.

This is designed to fit around the programme for the station box at Woolwich. The first TBM assembly will bore one of the tubes to the box where the cutterhead will be detached and left. The retracted shield will then be connected to the second cutterhead for the second tube to be finished completely, being walked through the excavated station box. A final retraction will reconnect the shield to the first cutterhead for finish of the final drive. Originally two shields were foreseen with one back-up train but this has been reduced to making available two cutterheads.

Required TBMs
Confirmations of TBM orders to selected manufacturers are expected to be announced in late April. The six EPBMs to drive through clay, sand and gravels beneath central London have a 7.1m diameter and will erect rings of 300mm thick PP fibre reinforced segments. The slurry shield for the 2.6km Thames Tunnel will drive mostly through chalk. Sites for the manufacture of the precast segments have yet to be finalised. Segments will be transported to the four TBM launch sites at Royal Oak, Limmo, Stepney Green and Plumstead.

Station works
All TBM work is programmed to be completed by mid-2014. This will be followed by finishing works at the portals and continuing works at the stations. While excavation of ticket halls and entrances at many of the mined stations will get underway later this year, the civil works will not be completed until 2017-18, which is much later than previously envisaged. Procurement is underway for excavation of the stations with bidder shortlists preparing proposals at present for the Paddington and Farringdon stations.

The stations are large and will be constructed by a combination of methods, including box structures and SCL. Excavation of these stations is not only to progress beneath one of the world's busiest, congested and most expensive urban environments, work must also be phased-in and integrated with extensive upgrading works by London Underground at the interchange stations. In many cases the completion of the civil works will be the best part of a decade after enabling works started. While the stations are part of an extended construction programme they are also vital elements in a transport scheme that is focused on being less expensive than originally planned.

References

Crossrail contract awarded tally climbs - TunnelTalk, April 2011
Crossrail awards tunnelling contracts - TunnelTalk, Dec 2010
Preparing the ground for Crossrail - TunnelTalk, Aug 2010
Crossrail moves ahead with enabling works - TunnelTalk, March 2010
Monitoring contract for Crossrail - TunnelTalk, June 2010
London Underground upgrades and expansions - TunnelTalk, Dec 2009

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