If so, does this mean that installing the concrete slabs, instead of ceramic tiles say, that the installation might have actually weakened the structural integrity of the tunnel?
I've heard that they're supposed to form a ventilation duct in case of a fire in the tunnel. They don't serve as a structural part of the tunnel.
But the suspended ceiling is held in place by bolts driven into the tunnel roof. The specs require the holes to be drilled to a particular depth. But since the rebar mesh is so dense, drillers hit rebar 50% of the time.
Somehow they got permission to drill through the rebar, thus compromising the strength of the concrete in the tunnel roof. This is a grave mistake, and permission to do this should never have been granted without review by the designers.
I've been in tunnels with those kinds of ventilation systems. I can't recall ever seeing with concrete slabs under the plenum. I've seen ceramic tiling, though I don't know what the tiles were glued to. They're probably a pain in the heinie to replace, if you even bother, but even if a few of those land on a car, they're not going to crush anyone. Why were the slabs used to cover the plenum in the first place, do you recall hearing/reading?