Tunnel Engineering Technology
Rodney - My name is Rodney Craig I worked for Sir William Halcrow and Partners Consulting Engineers for 38 years. All in tunnelles - I arrived on day 1 and put in a tunnel and I left on my last day sitting in a tunnel.
Chris - Is there anything that you can't tunnel through?
Rodney - We're much improved compared with 100 or 50 years ago. The reason that in London, all the underground is north of the Thames is because north of the Thames, there is London clay which is a good medium for driving tunnels. In south of the Thames, it is predominantly in water bearing ground. In the 1900s and up to probably the 1960s or '70s, it was quite difficult to build tunnels under the water table because you had to put compressed air on to hold back the water and because you have to compress up to your air pressure and then decompress afterwards. So, in other words, you would only have 4 hours working because the 2 hours beforehand, 2 hours afterwards were compression and decompression. But now, we have tunnel boring machines which can activate in that type of ground and therefore there isn't a problem.
Chris - So, you just deploy the machine and it's unmanned in the sense there's no one actually physically down there at the digging face, and it's all automated.
Rodney - Basically, there are now two types of tunnel boring machines which you can use. One is called the hydromachine and the other is called the earth pressure balance machine. They have different principles, but they both pressurise the face. So, you hold back the water and you hold back the ground. The largest tunnel boring machine which has just completed work in Italy is 15.66 metres in diameter.
Chris - Gosh! That's a lot of rock, isn't it? How much material are they moving an hour, those sort of machines?
Rodney - They were going along at quite a high rate of progress. They were probably doing up to 10 metres per day.
Chris - What about when we want to build tunnels in places where there are geological issues and Japan are masters at this? They've got a geological hot spot there. They've got lots of earthquakes happening regularly. How do you build tunnels that are capable of taking that sort of problem? Because concrete is really good in compression, but as soon as you put stress on it, it fails catastrophically? How do they get around that?
Rodney - Generally, the safest place to be in an earthquake is inside a tunnel because basically, your tunnel moves with the rock. Hence, there isn't really a problem as far as the tunnel itself is concerned. Probably the worst conditions is probably not in Japan but if you are building a tunnel in sand under the water table, you do find that the sand can liquefy. It loses all its strength.
Chris - What about navigating? Because if one looks at what was achieved historically by engineers like Bazalgette, the guy who plumbed in the London sewers, they dug from two different directions and were they out by - you know, it was a matter of a centimetre or 2, wasn't it? That these two tunnels coming over enormous distances and they met in the middle. How did they do it?
Rodney - Basically, you start off from a shaft and you have two plumb bobs which are strings with a piece of metal at the bottom which you put on the inside of the shaft, at the back and the front on the actual alignment that you wish to drive your tunnel out. And then you extend that line by resurveying time and time, and time again. You lose all the errors and you were right with the end, within your 1 inch from your predicted position.
Chris - What happens if you want to go around corners?
Rodney - Well then you have to survey what angle you're doing and so forth and so, it comes in the calculations. But ideally, with the tunnel, you try to avoid having vertical and horizontal curves at the same time because that then does become more difficult.
Chris - What about if, like up in Manchester, there's a very long canal to get historically, products onto canals under a mountain so that the horses didn't have to carry stuff over a mountain over the Pennines. It took them a huge amount of time to build, but they then ended up with water flowing though it beautifully. How do you get the water flowing through them too so that you don't end up accidentally going uphill or downhill by the wrong amount?
Rodney - Well, as long as you go horizontally, you will always be able to get the water flowing so easily. There was a case that I visited a tunnel in Israel which was built in the 9th century BC where they had their wells outside the city. They drove it from both ends and they got their levels wrong. There was a five foot difference between one tunnel and the other when they met. And so, they had to excavate down for another 5 feet to actually get the horizontal.