Harden surfaces and taxiways over rivers...

Hi,This is more a scenery tweaking question than a design one but I guess I'm posting in the right forum...I have a Freeware airport scenery which came as part of a larger package and I've been trying to isolate it (only for my own use of course) to gain better performance etc. I'm nearly there but it seems that the ground under the airport is not properly "hardened" and at two points when taxiways cross canals, the plane falls in! (actually, this also happened before I started trying to isolate stuff)I've tried adding an apron layer to the AFCAD file but that has no effect. Do you think I need to just make a flatten for the whole airport and modify the VTP layer to remove small sections of the canals or will I need to do more?Thanks for any advice,GeoffPS. The airport elevation is below sea level - don;t know if that makes a difference...