From the Beyond Design blog post, “Why does my model look sectioned?”
If you are experiencing clipping issues in Navisworks – especially with large models – you may have a spurious object far away from the main geometry. Your best option is to delete the extraneous element in the source model and reload it. If your composite model is just that large (airport, neighborhood, city…) then you might need to adjust the Clippng Planes under the Culling tab in File Options.
Read the complete article on the Beyond Design blog for more info.