I agree, call the local quarries and contractors for your best guess.
My many years estimating on the contractor's side give me confidence using a factor of about 150 lbs/cu ft (or about 2.03 tons per cubic yard) -- this is higher than a soil engineer will tell you (that is, a this same...
All I can say is, this looks like trouble, and you'd better have lots of money to handle it.
I have worked as an estimator and project manager for a pipeline construction company for a lot of years, and we ran into a similar situation on a project in the middle of a city street about 15 years...
First, get your surveyor out to check this.
In construction on "Bay Mud", the most common problem is that an added overburden will "squeeze out" a layer from below--example, you think that you have added three feet of fill, but come back the next week and find that the elevation is only one...
