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Thus if we did want to dramatically reduce CO2 emissions

Well, the concrete for the tunnel lining would involve a lot of CO2 emissions.

I also don't know whether the cost could be justified on any rationale.

That's a political issue, not a technical one. If a government sees a benefit, whether its benefits are quantified or not, it can decide to shoulder an investment. But, even though I don't think that the Irish Sea Tunnel would cost more with today's technologies than the Chunnel did with technologies back then, it looks like a tall order.

maximum sea depth of c. 100M

At a length of 90+km, the real challenge is not depth. It is length, and water control. Unless one or more expensive articical islands/giant caissons are built in the sea for intermediate accesses, it would have to be bored from both ends, meaning the transport of dug material away from and tunnel lining towards the TBM over up to 50 km. Building watertight tunnels across water-bearing strata is no problem per se, but you should better know in advance what rocks can be expected in sequence, so a lot of boreholes would have to be dug between Dublin and Holyhead.

bridge option - which could be enclosed to avoid weather issues - although snow on Irish sea is v. rare and slight and I presume wind is not a problem for trains

Enclosed: costs even more, you just lifted the tunnel above the sea, and added pylons. And wind is a problem for any vehicle with significant side wall surface area.

I repeat that bridges aren't a cheaper alternative. The reason Denmark built its two big sea strait crossing links as bridge-tunnel combinations was on one hand to avoid complications with ventillation for the road tunnel part, on the other hand to keep bridge-building experts employed. (And the same reasons apply for the planned Fehmarn Belt crossing.)

bridge pillars doubling as turbine pylons

That's not necessarily a good idea. Vibrations, danger to trains if a blade breaks or sheds ice.

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

by DoDo on Sat Dec 19th, 2009 at 03:29:02 AM EST
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