It’s important to understand that when you need to model a vertical face in a TIN surface, you cannot build it truly plumb. Land Development software cannot navigate TIN lines to points sharing the same X-Y coordinate so depending on the software, the triangulation could project to the top point or the lower point.
When we are trying to work walls into our surfaces, we usually choose breaklines since walls are linear objects, i.e. edges. If the two breaklines are on top of each other, with vertices at the same location, AutoCAD Civil 3D will throw one out even if the surface is configured to average the elevation where breaklines intersect.
If the two breaklines representing the top and bottom of feature are offset from one another even infinitesimally, the TIN will build incorporating the wall feature. There are a few ways to do that:
1. Use the Wall breakline feature in the surface. When using this option, we do not need two breaklines. As soon as the OK button is pressed, we are prompted to select the breakline representing either the top or bottom of the wall feature. Once selected, we are prompted for an offset side, i.e. the side we are about to define where the vertical deflection occurs. Once done, we can give a vertical deflection value for each vertex or enter a constant value. The results will give us a wall. The surface will show TIN lines that are parallel to the original breakline by a tiny distance.
2. Another method is to create a feature line representing the top or bottom of wall. Then using the Stepped Offset command from the Edit Geometry panel of the Modify ribbon, we can create a parallel feature line with a given horizontal offset and then the option of specifying the vertical deflection for each vertex of a constant value. Then both feature lines can be added as standard breaklines to the surface.
3. An alternative to the stepped offset method is to use grading criteria with the feature line. The advantage is that the grading objects will hold the horizontal and vertical relationship between the original feature line and the feature line created by the criteria. Grading criteria does not offer the option of specifying varying elevations for each vertex, but it works well for constant slope projections and can transition into other criteria. A criteria can be configured to the criteria set as shown below, and can be applied to the feature line using the Grading Creation Tools.
For more information about grading and building surfaces, you should check out our class on Grading Design in Civil 3D.
– Cyndy Davenport
