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.}

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