By Justin Doughty
There are a few simple tips you can use when you encounter holes or gaps in a Surface in Civil 3D.
Start by making sure to use the triangulation surface style, so the points and lines are visible. Also make sure that boundaries are turned on.
Then rather than adding supplementing points around the outside edge, actually add 1 point down one side of the triangle (use midpoint snap). This generally fills the hole.
Usually that point can then be removed afterwards and the holes will still remain patched. Also use add line between points you think should be joined to tidy up. Sometimes something as simple as swap edge will work on existing triangulation lines to resolve issues.