You seem to be assuming that the whole tunnel should be one continuous structure, and that the pressure vessel must occupy the whole volume. A segmented structu

You seem to be assuming that the whole tunnel should be one continuous structure, and that the pressure vessel must occupy the whole volume. A segmented structure, possibly with pressure vessels that are either flexible or of a smaller diameter than the outer walls may be more practical. I'm also not sure whether luncrete is really a practical option, and you certainly won't have clay for bricks. I think you're making a lot of assumptions, and don't have a comprehensive view of how the structure(s) would be designed and built.