When working with Civil 3D surfaces, it’s a good idea to display the triangulation as “shaded” once in a while. Using a shaded visual style, you can easily identify any holes that may exist in the surface model. Sometimes holes are created as the result of an error in editing. (Deleting interior triangle lines is a common cause.) Likewise, holes can be introduced when multiple surfaces having non-matching boundaries are pasted together.
A surface hole represents a “void” in the data. These voids can cause errors in earthwork calculations, broken contours, and gaps in render materials. No matter how you ended up with a hole (or holes) in your surface model, it’s important to close them up such that your surface accurately reflects the existing or proposed conditions.
If you are interested, I have created a quick video that demonstrates some strategies for patching holes in Civil 3D surface models.