I thought about this a bit. Based on some back of the napkin calculations and guesses, I feel like one 12m diameter megabore (e.g. Barcelona Line 9/10) would use a significantly greater amount of concrete than 6m diameter twin bores.Interesting article. I'd like to see an environmental audit of the various options. Sure megabores are cool. But do we end up expending twice as much energy and greenhouse gases from the concrete? Canadians have more or less abandoned small cars for big SUVs and trucks. Same trend.
The circumference of two 6m circles is exactly the same as the circumference of one 12m circle, so assuming the thickness of the walls is the same, the volume of concrete for the outer shell is pretty much identical (actually it is slightly more for the two 6m diameter bores due to the greater curvature). Maybe a bit more concrete would be used in the 12m bore if the walls have to be thicker.
Platform where tracks are laid:
In a double bore, a horizontal platform is typically built in the tunnel, and the space underneath is left empty. In a twin bore, they usually just pour concrete into the bottom of the tunnel to create a flat surface, filling the entire space underneath the tracks. Guessing a little, I'd think both of these would be similar amounts of concrete.
Stations, crossovers, tail tracks, pocket tracks, emergency exits, etc:
Here's the big one - since all of these pretty much fit inside the megabore but require additional outside structures for twin bores, I think it's reasonable to assume that twin bores would require more concrete for all of these structures.
Overall I can't imagine the amount of concrete is too different. If anyone has better information or actual numbers though, feel free to correct my guesstimates .
Besides, any transit (other than buses on deserted suburban bus routes) is far better in terms of GHGs than cars, so any transit is good in that sense.