Would the deep excavation in front of Chatellerault not have cut into where the tunnel would have been running? I can't remember the exact local history, but I do know there was some sort of excavation work that occured, and of course the hamiltons had expolited the area for coal workings.
I remember doing the tour of the mausoleum when I was a kid, and they guy said it was prone to flooding, which was why the Hamiltons bodies were exhumed and moved elsewhere, so I guess the tunnels must have been submerged as well. The tour's really good, not sure if they still run it but well worth a look if it is. Chatellerault a good place to visit too, especially on a nice day, and of course on bonfire night for the fireworks display at Strathy park.
Hmm, funny that, they sell the land for coal mining and both the palace and the mausoleum both had subsidance problems, thats why the palace was knocked down.