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I'm not an engineer, but from the geologist perspective I'd not be keen to tunnel through salt strata - salt deposits deform quickly...
by Bjinse on Tue Jun 14th, 2016 at 06:45:36 AM EST
[ Parent ]
I found the following article on Irish Sea seabed geology:

Geology of the seabed and shallow subsurface: the Irish Sea - NERC Open Research Archive

...Seabed sediments are subdivided into regions of soft mud- (clay and silt) rich sediment in the eastern and western Irish Sea and a central gravel belt comprising coarse sand and gravel. Small areas of bedrock outcrop at seabed are also recognised.

...Very stiff diamicts (glacial `boulder clays' or tills) are present across most of the report area of variable thickness.

...The predominant bedrock lithologies in the report area are Triassic and Carboniferous sandstone and mudstone. Geotechnical properties of Triassic rocks are comparable and potentially predictable. Carboniferous rock show high lateral and vertical variability. There are a number of igneous intrusions in the report area and rock properties near to the location of these igneous bodies may differ due to alteration of the host rock during intrusion.

What does this mean for a tunnel? For example, are any of these water-impermeable?

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Tue Jun 14th, 2016 at 02:02:23 PM EST
[ Parent ]
As a rule of thumb, most metamorphic and igneous rocks  as well as densely packed mud- and sandstones are poorly permeable - for as long they aren't fractured.

I didn't know before, but I see that the Chunnel was drilled largely through chalks from the Cretaceous, and the heavily fractured chalks on the French side caused a challenge to contain the influx of high pressurized water. That certainly does not sound ideal, though I guess the technology then was already available to sort that out.

As for the geology of the bedrock in the Irish sea, it indeed looks like it's older - mostly Triassic, Permian and Carboniferous sand- and mudstones, with some younger igneous intrusions. I'll repeat that I'm not an engineer, but from a first perspective, that sounds nearly ideal compared to drilling through chalks. Chances are high that the rocks will only be lowly permeable.

Furthermore, if you follow the link and look at Figure 2, it looks like the lithologies are continuous underneath the sea. That also sounds a lot more ideal to me than tunneling through a heavily fractured and compacted mess, like the Gotthard Base, which sounds like it was practically packed with geologic challenges.

So at the back of an envelope, chances and cost risks actually look pretty decent for a tunnel underneath the Irish sea.

by Bjinse on Wed Jun 15th, 2016 at 06:22:21 AM EST
[ Parent ]
Thanks for the overview!

Regarding the Chunnel, IIRC the problem was that from prior research, the chalk was expected to be continuous and non-fractured on the French side and fractured at the English end, but in practice, the opposite was found to be true.

Regarding the GBT, this was exciting to follow when they built it. From the engineering viepoint, the two biggest challenges expected in advance both proved easier to master:

  • A water-bearing, weak (crushed rock), but very deep ( > high pressure) zone in the south. Before the rest of the tunnel, a test drill was advanced into this area. Amazingly, just a hundred or two metres above the base tunnel's level, the zone became transformed rock which was water-impermeable and easier to drill.
  • In the north, there was a zone of compressive rock ( the tunnel closes up), also very deep in the mountain. At the time the GBT was planned, there was no tried method for dealing with compressive rock with such high pressures, so the solution (steel segments which lock as the rock around them contracts) was a bit of a gamble (some experts thought it won't work). It worked without a hitch.

Instead of these expected challenging zones, the most problematic zone was unexpected asbestos-bearing and/or longitudinally sheared rocks right at the site of the most complex structure, the southern emergency station; as well as right behind the launch cavern of the tunnel boring machines near the southern portal. This caused more than a year of delay.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Wed Jun 15th, 2016 at 08:19:36 AM EST
[ Parent ]
sounds good, all we need is a compelling reason to do it.

I'd wager it'd be far better value for money than a trident replacement

keep to the Fen Causeway

by Helen (lareinagal at yahoo dot co dot uk) on Wed Jun 15th, 2016 at 02:24:13 PM EST
[ Parent ]
I think there are two related factors which could make this feasible:

  1. If Brexit doesn't happen and the EU takes a strategic view that anything like a UK Ireland tunnel which will increase economic integration would be a good thing.

  2. If low cost finance is made available via the EIB or whatever.  A project like this is probably only feasible if the interest rate on finance is close to the rate of inflation - i.e. near zero real interest costs.

Presumably there are long term energy and climate change savings associated with a tunnel compared to air and sea transport which could be used to justify the preferential interest rate regime.

My only other concern would be the risk of bottlenecks in the London area if you are trying to route passengers and freight from Ireland to continental Europe.

Index of Frank's Diaries

by Frank Schnittger (mail Frankschnittger at hot male dotty communists) on Wed Jun 15th, 2016 at 07:06:23 PM EST
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