It's not just the digging. It's the fit-out. Channel Tunnel suffered significant delays and additional costs for instance because the brackets supporting cable

It's not just the digging. It's the fit-out. Channel Tunnel suffered significant delays and additional costs for instance because the brackets supporting cable and pipe runs needed to be taken down after everything had been installed because of a latent design defect and replaced. Imagine how long it takes to pull 20+km of cable off the wall. Also, compared to other construction sites, tunnels are a different beast, you often need to transport huge amounts of materials, people and equipment long distances to get to your work-front (and back again). It might be a straight line, but it's still far. As much as I love the Channel Tunnel (my father was an engineer on it and I grew up during the construction) the actual outrun cost benefit ratio just isn't there. Extra ferries probably would have been more economical.