IFC export and IFCinfra export

[Buzz0m]

Hi,
I have an issue where I'm trying to export TIN surface models (made from featurelines and polylines) to IFC with IFCEXPORT. I have a number of almost identical surfaces at different locations. The surfaces are modelling a shaft, and hence they have a "hole" at the bottom. The surface models consists of a top edge (featureline, elevation of FL is defined by existing ground model elevation) and a bottom edge (polyline, elevation at specific elevation). The polyline is used as a hide boundary to create the hole.

All surfaces are being shown in Civil3D just as intended.

The issue is that the SOME of the surfaces are exported as solids with a volume when they should be just TINs, i.e. objects without a volume. I'm expecting geometries which are rectangular pipes, but I get rectangular bars. Most surfaces export fine and the result is as intended.

1. I've tried adding elevation points to both the upper and lower breakline definitions but issue still persists. 
2. I've tested IFCINfraexport and the result is in respect to the geometry correct but it is not a satisfactory solution due to other reasons.
3. I've checked that the issue is not with the slope of the triangles, since there are shafts that are much higher than the problematic surfaces. I've also modified the breaklines/fl significantly to check if it is somehow geometry related.

Now that I've written all this I think I found a work around and identified the issue. I'll post the solution/workaround in the comments.

[Buzz0m]

It seem that if all the triangles have exactly the same slope (they are on one plane) and form a box like structure IFCEXPORT will interpret this as a solid box and create a solid object instead of creating the triangle network that is intended. The reason why some of my surfaces behaved like this is because the existing ground data was sparse and the upper edge resided within one triangle of the EG Surface.

The export also disregards the boundaries (the hole in my case). This issue seems to come only at certain simple geometries (all the triangles of each side are on a single plane).

Anybody could test this by basically modelling two rectangles, giving them different elevations and then creating a surface based on them. Then run IFCEXPORT and check if the result is a surface or a solid.

My solution is to add a elevation point inbetween vertices of the upper edge and giving it a slightly different elevation. This leads to one of the sides of my shaft to not be totally flat ->   no solid + hole.

If anybody at autodesk sees this (I know, wishful thinking) please comment to indicate, that the issue has been noted. Perhaps it is a know bug?

To everybody else: is there a setting that can be set to prevent this behavior?