Check your contract and see if it says anything about soil/compaction. You may or may not know it, but they often put in a line that discusses compaction. If you "say" you have 91% or better (at least in this area) then they will build to that statement, or state that a soil compaction test be done (at your cost and time). If a permit was pulled, and engineering was provided, it would be predicated on that representation. If you stated that you had solid material, and you did or did not know that you did not, then you "misrepresented" your lot and they will not be found at fault.
I'm not saying this is right; just offering you some inside advice and giving you a place to look before you contact an attorney. Good chance an attorney would not take the case if that was included and initialled.
Good luck!