How was the underground built?

[China Doll]

Afternoon All,

Hope you're all well.

Now I did want to put this question in to Technology but it seemed mainly electrical in there so i thought this was the next best place so please don't shoot me.

I just wondered how the underground was built? Or more specifically, (since I have seen how they build it courtesy of living in London), how did they do the bits that go under the thames? Does it go right under the river? How did they know how deep it was? It must have been a major feat considering the time the underground was first concieved.

Cheers
China

[Lucy Thomas]

It must have been a really boring job.

[ChuckFickens]

the first of the underground tunnels weren't built as tunnels as such, they closed the road dug huge trenches and then put a roof over them to make a tunnel.

Thats why if you check a proper street map with a (proper graphically correct) underground map overlaid over it you can see a lot of the early tunnels follow the roads

for the latter tunnels they had to stop doing this due to the disruption it was causing above ground with the road closures.

BTW the London sewers are probably a more impressive feat of tunnel engineering.

[China Doll]

I think dangerous rather than boring based on reading the link left by Zac Lucy.

I'm being thick though, I've read the link and looked at the pictures but I can't imagine it in my head. I don't understand how the tunelling shield works.

I'm happy to include sewers Chuck if you can explain how they're made too!

[mushroom25]

http://en.wikipedia.org/wiki/Tunnelling_shield

[China Doll]

I've read that mushroom, thank you. I understand the prinicpal of it, I just don't understand how it works under water or how you even start to make the tunnel using the shield. As I said, I can't picture it in my head.

[Lucy Thomas]

China Doll. Boring, digging tunnels, boring, geddit. Obviously not.

[whiffey]

Boring is more often associated with vertical drilling, e.g. for oil. I can see why people might have got confused between boring and tunneling. So when the POWs dug their way out of Stalag Luft III, they sunk (or bored) the entry shaft, but tunneled the tunnel itself.

[China Doll]

Charming to see you as ever Lucy.

[jno]

The original Brunel tunnel was the world's first under a river; it's now part of the East London Line (closed till the Olympics).

The underground builders thought they were digging well under the river but they had to close the Bakerloo line for months a few years back because the water had washed away a lot of the riverbed above it and it was starting to leak, as I recall.

That's the trouble with London Underground: it was the world's first so they were never able to learn from anyone else's mistakes, they had to make them themselves. New York, for instance, realised that double tracks would be more practical for when you had to repair lines; LU have to shut down at night to allow maintenance, New York doesn't.

The shield's just a shield: you dig more tunnel in front of it, then move it along and line the new bit of tunnel behind it with bricks. It's complicated because the shield comes in sections; but the principle's the same.

[China Doll]

Ah. Cheers Whiffey, now I get it. Apologies Lucy.

[jno]

... so you're saying the Severn Bore actually goes up and down rather than back and forth, whiffey? No wonder it's considered an interesting phenomenon. I myself applied for the post of Severn Bore in 1983 but lost out to an old woman from Newcastle-under-Lyme.

[China Doll]

So the shield moves forwards which progresses the tunnel and they do the bricking up as they go along?

[jno]

yes, as I understand it. I forget what they actually use to move the shield, possibly sprinkling marbles or something underneath it.

[sp1214]

As many of the tube lines are below the water table its a massive daily job to pump out the water every day. Lets hope those maintenance engineers never go on strike.

[China Doll]

I'd quite like to know what they use to move it too jno, hopefully someone can enlighten me at a later date.

Thanks to everyone who answered and bore with me while my brain slowly ticked in to action. I shall be replacing the wheel and hampster that operate said brain next week.

[ChuckFickens]

the early ones where pushed forward by large screw jacks braced against the brickwork they had just built behind it, the more modern ones used hydraulic jacks.

Either way the principle is the same, the shield supports the tunnel roof so they can dig a small way out in front of it, it is then forced forwards by either of the above methods and the brick tunnel lining is built in the gap that leaves behind it, while at the same time a bit more is being dug out infront of the shield.

And so this repeats until you have a tunnel.

[China Doll]

Thank you!

Erm... How does a screw jack move?

[New Judge]

Large screws mounted around the circumference of the shield (which is called a Greathead Shield, by the way, after its inventor, James Greathead) were turned against the end of the tunnel lining already in place, thus forcing the shield forwards.

The tunnels were (and indeed still are) lined with cast iron segments, not bricks. These are bolted together to form a continuous lining.

When tunnelling under water compressed air was used to pressurise the work area and prevent water ingress. This led to a number of workers getting �the Bends� (decompression sickness) similar to that experienced by divers who surface too quickly.

This form of tunnelling is only suitable in areas of soft strata (London is built predominantly on clay). This is one of the principle reasons why no deep level "tubes" were built in south-east London as most of that area is built on chalk, and the early Greathead shields could not cope with that material.

[China Doll]

Thank you very much New Judge.