The civil model used in this example shows an interchange with several roads and bridges.  The model was originally an open mesh with thin elements, and many gaps and inconsistent normals that needed to be repaired.  

A) Thin elements, such as walls and support beams for bridges need to be thickened in order to print well without breaking.

B) Separate mesh elements often have gaps between them which can be difficult to fill.  The Fill Mesh Voids tool works well for small gaps in a single element, and the Bridge Mesh tool works to connect separate meshes. 

C) The Mesh Audit tool can be used to check for gaps and inconsistent normals in the mesh that would cause errors when printing. 

D) The Thicken Mesh tool can create uneven geometry. Thin mesh elements usually need to be remodeled by extruding their profile along a path.

 

The picture below shows the printed civil model.