Sometimes when an energetic fault line is cured, the physical aspect will be ameliorated to some extent.
As far as whether dowsing could help prevent damage from a physical fault line, I would suspect not. Do you have any idea what is causing the physical cracking? While dowsing can help with the energetic impact of a physical fault line, I don't think I would count on it curing the physical fault.
What do your dowsing rods say?
All the best!