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Getting to the core

The central tunnelled section of Crossrail will involve an intricate path from Paddington to the Docklands. It will have to avoid Tube stations, sewers, power lines and other obstacles and cause minimum disruption for London’s commuters.

Maybe it is because Crossrail has been a long time in the planning that Cross London Rail Links engineering director David Anderson seems able to look at this immense, once-in-a-lifetime project and not be overwhelmed by its challenges. He regards his task for the coming year as straightforward. "The main thing is to make sure we get the right outcome from the detailed design work," he says.

Click here for a typical central area Crossrail station

But there has been much taking place behind the scenes to get to this point, particularly since the Crossrail Bill received Royal Assent in July. Much of the focus up until then, and since, has centred on the feat of tunnelling below the heart of London and around its arteries of Tube stations, services and other underground obstacles. Although not wishing to play down the challenges too much, Anderson says the central section of the route has been safeguarded since 1990, which made dealing with much of what lies beneath the city easier – fortunately the route’s alignment has changed little since then.

It is also fortunate that the safeguarding of the route stayed in place despite the project’s failure to win approval 14 years ago, which could have meant the scheme contained in today’s Crossrail Act would not be possible.

When it is opened in 2017, Crossrail will run from Maidenhead in Berkshire and Heathrow airport in the west, through a central tunnelled section via stations at Paddington, Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel. It will head east to Abbey Wood via the Isle of Dogs as well as stretching much further east of London to Shenfield in Essex. All of this adds up to a £15.9bn price tag and 42km of bored tunnels – typically 15m to 30m below the busy streets of London.

Click here for Crossrail's route under London

Two major drives with tunnel boring machines (TBMs) are planned, which will create the bulk of the Crossrail tunnels. One will head east from Royal Oak, just west of Paddington and another will work westward from the Limmo Peninsula – a site currently serving the new Docklands Light Railway (DLR) extension to London’s City airport. The TBMs will meet at Farringdon where their journey ends.

Originally the tunnelling strategy proposed an intermediate site for launching the TBMs at Hanbury Street, near Spitalfields Market in east London. However, a 2006 review led to a revision of the strategy that now means there will be longer tunnel drives. The change has removed the need for a shaft at nearby Pedley Street and a conveyor to a site at Mile End Park to handle the excavated material. In addition, the shaft at Hanbury Street has since been reduced in size to become a smaller ventilation and intervention aperture.

Click here for a Crossrail station section

A great deal of attention has been given to the issue of spoil. An estimated 8M.m3 is expected to be generated by the tunnelling work and excavation of stations and ticket halls. To mitigate the negative associations of taking it away by frequent and numerous lorries that would clog up the capital’s roads, the spoil will be moved along the tunnels as much as possible to sites at Westbourne Park near Paddington and Limmo Peninsula near Canning Town, which benefits from being set up for the current DLR extension. In addition, there are plans afoot to take away spoil using a combination of barge and rail. At the moment this method of transport is expected to cope with about half the spoil, but Anderson says it is possible it might be able to take an even bigger proportion.

Given the scale of the project, Anderson says the tunnels will have a larger diameter than those in the London Underground, but smaller than the High Speed 1 (HS1). This is because it will use a rolling stock gauge as used on the national rail network today. Anderson brings substantial experience of major projects to the scheme. He has been engineering director for two years and for four years before that he worked as head of planning for the scheme. Prior to that he worked for BAA for 14 years – with 10 spent working on Terminal 5. "But Crossrail is bigger than anything else, and it is the scale that’s striking," he says. "Yet the environmental impact of Crossrail is quite benign, considering."

Along with his own experience, Anderson points out that the scheme is benefiting from the lessons learned at that other mega-tunnelling project, HS1. At this stage, there is still a lot to do before tunnelling can begin. Already the team has spent a long time looking at ways of mitigating and dealing with a variety of obstacles. For a start there are the seven London Underground lines that the safeguarded central tunnel section will weave above and below. In addition, there are numerous underground services and deep piled foundations that reach down close to the Crossrail tunnels. Finalising proposals for protecting them is one of the engineering team’s main tasks.

Tottenham Court Road, where the foundations are very deep, will prove a particularly tricky spot. Anderson describes it as one of the most complex locations with the Northern line running close to the Crossrail route. Once again, safeguarding the route has provided a buffer against some of these issues because any planning applications for piled structures along the planned route have been vetted by Crossrail.

There are listed buildings to cope with too. "When we get to Paddington station we will get quite close to a grade-I listed building but we’ve got a series of protocols in place with local authorities to deal with that," says Anderson. Perhaps to prove his point that Crossrail’s impact will perhaps be less than might be expected with such a large project, only one listed building will be fully demolished to make way for the new railway. "We haven’t got the same impact on heritage that the work at St Pancras for CTRL had," says Anderson.

Another big part of the challenge will come from trying to build the new line, stations and ticket halls, while limiting the impact on existing stations. "We want to minimise the amount of time they will have to shut. But the scale of the work, the stations are large, and the scale of the activities, stands out," he says. "One of the biggest challenges will be to fully integrate Crossrail stations with the National Rail and London Underground stations." The level of work varies along the route however, and along the central section all the Crossrail platforms and station will be new.

Main station upgrades are planned at Bond Street, Tottenham Court Road, Liverpool Street, Farringdon and Whitechapel, while two box stations will be needed at Paddington, which is quite near to ground level and at the Isle of Dogs, which is close to the bottom of the docks.

Another of the impressive features about the central section is that some of its stations will interface with other existing stations. These are at Liverpool Street, where the gap between the existing station there and the Tube station at Moorgate will be bridged, and at Farringdon station which will combine with Barbican station.

At Farringdon, Thameslink is already pressing ahead with a development, so Anderson says the schedule of Crossrail works will partly integrate with current work. Again, to simplify matters, the original more challenging plan to create a crossover at Farringdon has been eliminated. But logistics will remain challenging at Tottenham Court Road, which will go through six stages of traffic management during construction. Here, again, Anderson speaks of similarities with the feat of managing traffic around St Pancras, but without such wide roads. It will take careful planning to navigate traffic and the red buses that snake around the already snug area of Tottenham Court Road, day and night.

Before work can really begin on the tunnels, Crossrail engineers need detailed information on ground conditions. A dedicated team of Crossrail geologists and engineers, along with consultants, is working to analyse the ground along the entire length of the proposed route. The majority of these investigations involve drilling boreholes at regular intervals into the ground beneath the city and even under the Thames.

Investigations have unveiled river terrace deposits, alluvium, London Clay, Lambeth Group, Thanet Sands and Chalk created by a variety of environments such as rivers, estuaries and shallow seas. "The western half of the tunnelled route is generally in the London Clay. However, at Farringdon station the ground becomes more variable," says Anderson. "But we can take special measures there because it will be the end point for the TBMs."

In addition to boreholes, the team is also using geophysics – a nonintrusive technique using acoustic reflections to pick up underground structures and layering. All of this information will help inform the detailed design stages, including the choice of TBMs for each tunnel, tunnel lining design as well as ways of minimising above-ground impact of tunnelling.

Turning his attention back to what happens next, Anderson says it is all about the procurement to appoint framework designers and delivery partners. And then it is on to the big part. Main construction is set to start in 2010 at the TBM mileposts of Royal Oak and Limmo. It seems a long way off , but then it will be full steam ahead to ready the new line for its fi rst customers in 2017.

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