See the video for answers: After putting in a segment, the trench is backfilled with rocky material (over the segment), so that any buoyancy is negated by the

See the video for answers: After putting in a segment, the trench is backfilled with rocky material (over the segment), so that any buoyancy is negated by the fact that it's held down by a large layer of rocks. This layer presumably also prevents most, if not all, leaks from generating instant floods in the tunnel. Lastly, once completed the project will result in 5 parallel, but mostly independent tunnels (two train tunnels, two highway tunnels, and a service tunnel between the highway tunnels). I presume that the tunnels can be isolated and waterproofed seperately.