KEITH BOWERS | INTERVIEW
them – all of that is much the same whether your tunnels are 5 metres or 15. “But the large diameter does start to give
us some real opportunities to do things a little bit differently. It is because the traffic envelope you need to run cables and pipes and MSVs through to service your TBM is not 15 metres diameter so it only takes up half of your tunnel instead of three- quarters. You need a certain proportion of that cross-sectional area to run your MSVs and all the rest, but you don’t need the full size so we can segregate the space within it quite early and use that spare space for other useful things.” One of those things is building the
cross-passages. “Historically we have built cross passages
concurrently while we’ve been driving the TBMs in the main bore, but we’ve had to build them with fairly traditional mining techniques by a fairly small gang with fairly small plant. For Lower Thames, there is space for maximising the mechanisation on these things. “Effectively you have space for a rather
standard for a long time. A modern metro tunnel will be bigger: we’d probably go for about 5m diameter and we use that extra space to do things like putting in walkways. So if you plot diameter against year of construction there is a steady upward trend.” A trend which LTC exemplifies in
spades. “We are roughly 15 metres internal diameter, 16.5 on the outside. In round figures that is substantially bigger certainly than any other TBM tunnels you’ll find anywhere near. There are a handful now around the world at that sort of size. China has two or three on the go. There is the Alaskan Way in Seattle. But 16 metre tunnels are still relatively unusual and they do provide some different opportunities in construction.” Opportunities rather than problems?
He thinks so. “The actual TBM operation scales up with diameter pretty well. The operations at the front end - shoving the shield forward, building the rings, bringing in the segments and grouting
smaller TBM which you drive from one bore to the other. Because it is only a short drive – the bores are about 16 metres apart – the TBM can do its job on umbilicals, and you can arrange the backup longitudinally along the side of the main bore. Even so it amounts to a fair bit of kit but if your parent tunnel is 15 metres internal diameter you can devote half of that space to the backup for your mini TBM and you can still have your MSVs for the main bore moving without restriction through the other half. “Possibly a bigger opportunity is in
fitting out the other systems in the scope of the contract, the equipment that make the tunnel finally useful: the road surface, the lighting and ventilation and communications and control systems and so on. Perhaps unusually, all of that sits within the same contract as the actual driving of the tunnel – and that means that you can integrate all those things within the one contractor’s programme, which obviously simplifies matters.
THE BUSINESS CASE
FOR THE LOWER THAMES CROSSING HAS A NUMBER OF POSITIVE IMPACTS...
Above Developing a machine to remove damaged expanded linings and replace with SGI segments, on Jubilee Line 2015
Above LTC southern tunnel approach “So what we would like is to do as
much of the early systems fit out, and possibly even some of the testing and commissioning of those systems, while we are still driving the main bores. Usually you wait until the TBMs have finished, then you lay the road works, then you put in the electrical, mechanical and control systems. But with 15 meters to play with we can do an awful lot of that while the TBMs are still working. “The way to achieve it is by building
the road deck structure close behind the TBM. The road deck provides a very robust segregation between what is on top and what is underneath. In our case what is underneath is tall enough to drive a double-decker bus through, so we are talking a really big space. Our plan is to put a lot of that equipment underneath the road and install it while the TBM is still running, perhaps a couple of kilometres ahead. We have created a suitable sterile space for a lot of work, which is a more traditional programme would have had to wait for the TBM to finish. “Along the length of the tunnel there
are a number of plant rooms below the deck which are logical break points;
the
idea is that we build the road deck up to the first of them and then put a bulkhead on the end of the lower deck, and so we will have created a completely separate environment, sealed off for the dust and dirt and liquids and additives that go with the TBM and that you wouldn’t
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