Crossrail - A Major London Engineering Project

Beneath the historic streets of London, Europe’s largest civil engineering project, the Crossrail line, is starting a new phase, as excavation begins on its 21 km long tunnel under the capital.

When complete, millions of train passengers will use it each year to avoid the hurly burly of the traffic above, as they travel between commuter areas to the East and West and major transport and business hubs like Heathrow Airport, Paddington, the City, Canary Wharf and Liverpool Street Station.

Fortunately for Londoners, the construction of Crossrail will be a far less disruptive event than the construction of the Underground proved to be in the 19th Century, when the preferred engineering method for such a project was cut-and-cover - the excavation of a trench cut deep into the roads, which was then roofed over to create a tunnel beneath.

Nowadays, there’s no need to disrupt major thoroughfares thanks to the timely arrival of eight giant tunnel boring machines (TBMs), which between them will take on the task of cutting through the London Clay.

Each TBM weighs some 980 tonnes and consists of a front-end cylinder containing a rotating cutting head, nearly 7 metres in diameter. Behind this is a unit which positions precast concrete lining segments against the freshly-cut tunnel surface and then fixes them into place, filling the gap between lining and tunnel wall with cement grout to give a final tunnel diameter of 6.4m.

Seven curved sections and a keystone make up a completed section of lining, each of which is either right or left handed to allow the tunnel to curve as necessary. Where the tunnel needs to go in a straight line, the ‘curve’ is simply removed by alternating left and right linings.

The linings themselves are made from a mix of cement, sand and polypropylene fibres. These fibres are there to stop explosive spalling, or the vaporisation of moisture in the event of a fire, which would shatter the concrete’s surface. Steel strips add flexibility and strength, holding the material together should it begin to crack.

Excavated spoil is carried backwards from the cutting head on a screw conveyor to the rear of the TBM, where it’s deposited onto a much longer conveyor system that takes it up to the surface.

To move forward the TBM uses a series of hydraulic rams to push against the tunnel lining it’s just put in, moving along at a rate of 100 metres per week. Adjusting the relative pressure of each ram steers the TBM forward with an accuracy that’s essential given London’s subterranean past, which itself creates something of a complex challenge for Crossrail’s project engineers.

Not only will the new tunnel come within just one metre of existing tunnels serving 11 Tube lines, the oldest sections of which are 149 years old, but there are a myriad cellars, basements, sewers, water mains and other structures to be avoided too. This makes directing a TBM akin to threading a needle in 3D and up to 67.4m underground - no mean task when a TBM needs a minimum radius of 250m to turn.

So if you’re walking along London’s streets over the next few years, spare a thought for what’s happening right beneath your feet.

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