I know nothing about 3ds max, but seem to recall reading in this discussion group some time ago that in ver 2006, C3D imports a LandXML surface as a series of "add point" edits.
As such it doesn't recognize the outside boundary. If your corridor has curves, it will add triangulation on the inside of those curves. I just did a corridor surface import into C3D 2006 via XML and saw the same behavior.
I suggest exporting the feature lines (as polylines) that were used to define the corridor surface boundary from the original corridor. Then W-block them out of that drawing. Then insert the w-block into the new drawing and explode it. Then import the corridor surface via LandXML. Then add the inserted polylines to the surface as an outside boundary.
You may have to open the w-block in C3D 2007 and save-as a 2004 format dwg in order to insert it into C3D2006.
There may be gaps in the plines exported from the corridor.
As always, I reserve the right to be wrong, and others may have a more graceful approach.
I hope this helps.
Good Luck.