your best bet would be to go into surveyor, and move the building slightly. If it changes high, the raise the building back up with the 'change height' tool.
This sort of thing happens with trees and shrubs on top of a tunnel too. The only way I have found to fix it is, place the building on the rails in surveyor then raise it with the height adjust tool. This works with trees and shrubs but unfortunately some buildings do not allow for height adjustment. If that is the case with the building you are using, sorry I can't help.
Use a dighole to cut a hole for the tunnel, then raise the terrain over the tunnel. Then the building sits on the ground over the tunnel. Problem solved?
I had the asme problem, it helps if you can move the building slightly to get it on a piece of "solid" ground.
BTW where did you get the retaining walls from
Use a dighole to cut a hole for the tunnel, then raise the terrain over the tunnel. Then the building sits on the ground over the tunnel. Problem solved?
Actually not with this tunnel. The tunnel ends automatically create digholes, and shape the terrain, so you can't raise it over the tunnel entrance. But I'll try adding more digholes, and the other suggestions.
Thanks,
Mick Berg.