Crossrail tunnelling under London

How do the tunelling machines digging a new underground train tunnel in London actually work?
15 October 2013

Interview with 

Will Jobling, Crossrail


To find out how tunnelling has moved on into the modern eraCrossrail Logo, Ginny Smith met up with the Deputy Construction Manager for Crossrail, Will Jobling, who is digging a 26-mile long tunnel under London from the western to the eastern suburbs.

Ginny -   So today, you've brought me to a construction site at Canary Wharf.  So what exactly is going on here?

Will -   Canary Wharf Station is one of the main stations on the Crossrail line which runs east to west across London, connecting Heathrow Airport and commuter areas to the west of London with the central business districts of London and out into the Essex commuter areas to the east of London.  Six stories down below the docks of Canary Wharf, the station is well underway and earlier this year, two of our tunnel boring machines, Elizabeth and Victoria passed through the station on their way into the centre of London.

Ginny -   I like that you've named your tunnel boring machines.  Are they all female?

Will -   By tradition, tunnel boring machines are always named after ladies and they're generally named in pairs as well.  So, Victoria and Elizabeth, name after two monarchs.  Two of the other tunnel boring machines that we have which is starting from near the Olympic stadium at Stratford are named Jessica and Ellie after Jessica Ennis and Ellie Simmonds, two of the Olympic heroes.

Ginny -   Brilliant!  Can you tell me a little bit about how these machines actually work?

Will -   The essence of a tunnel boring machine is the same as this tunnelling has always been carried out since Brunel's time.  There is a metal shield at the front of the machine which has a circular rotating cutter head on it, that cutting head swings around and moves forward at the rate of about 20 mm a minute.  It scrapes off the surface in front of the machine, then a giant Archimedes screw from the bottom of the machine takes the material out onto a conveyor belt. It then passes all the way back through the machine, along the tunnel and out to the surface.  On the eastern tunnels, it then drops straight into a barge.  It's taken out down the river Thames and ends up in a new bird sanctuary that's being built off the cost of Suffolk.

Ginny -   So, you mentioned Archimedes screw.  How exactly does that work?

Will -   Essentially, it turns around and the material that's been scraped off the face of the tunnel false to the bottom and is picked up and then essentially sucked back out from the front of the machine.  It allows the pressure at the front of the machine to be maintained.

Ginny -   So, you've got this machine and it's burrowing into the Earth.  What happens behind the machine?  What turns a hole in the ground into something we could actually put trains through?

Will -   Well, once the front of the machine has pushed forward 1.6 meters, there is essentially a gap created inside the metal shield.  What we then do is we take the 7 segments and key stone of each tunnel ring and place them into a position using a massive hydraulic erector arm.

Ginny -   So, is that made of bricks?

Will -   No, the rings are made of concrete segments which are 1.6 metres across, about 30 centimetres in thickness and they're made using a re-enforced concrete with steel fibres actually mixed in with the concrete to give it strength.

Ginny -   How do you stop it from falling in on itself when you dug the bit of the tunnel out?

Will -   There's two different ways of doing it and it depends on what type of machine you have, which in turn depends on what the ground conditions are that you're going through.

Most of London is built on clay.  In London clay, the Earth has a certain amount of stickiness, shall we say.  So, as you're scraping it off, the front of the machine where the ground is unsupported, if you just left it there, it would just slide in and fall in on you.  So, what we do is we keep pressure at the front of the machine.  That's done in part by having compressed air in there, partly by having some water in there, just to force the earth back from falling in.  We also regulate it by the speed by which the Archimedes screw turns.  So that what we're pumping in to hold, the face up is equivalent to what we're taking out by the screw.

The Crossrail machines that are tunnelling under the Thames are slightly different.  They're slurry machines and that's because underneath the Thames is mostly chalk.  What they do there is they have a slurry machine where the full face is actually sealed water tight and the chalk is basically dissolved in a slurry as it's scraped off.  It comes out through pipes, rather than the conveyor belts which were used on a clay machine.

Ginny -   How quickly can these things get through the Earth?

Will -   At best, we can do in the region of 200 to 250 meters in a week.  On average, we like to go between 100 and 150 meters in a week.  So, that's about the same as 1 ½ football pitches.

Ginny -   So, there's an awful lot going on underneath London.  I mean, we've got sewer systems, we've got the current tube lines and there must be loads of other things that I'm not even aware of.  How do you make sure you avoid all of those when you're digging?

Will -   Well, it's all been planned and mapped out to very high levels of precision.  For example, the western tunnels recently went within 80 centimetres of the operational northern line near Tottenham Court Road.  That's about the length of a hockey stick and to be able to do that, you have to have a great confidence in where you are.  That's not to say that there aren't surprises down underneath London.  There's plenty of things we do know about.  There's plenty of things that we don't and we recently, we hit some piles under the Canary Wharf docks which nobody knew were there.


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