You need a second surface to get a volume. Create the surface by using just the points that form the base of the borrow area to create a datum surface. Then, bring up the volumes dashboard and make a volume surface based on the datum surface as your base and the complete surface as your comparision.
What exactly is your surface showing - the pit after it has been completed? Is a 50' grid sufficient to define the pit correctly e.g. breaklines for top and bottom of slopes etc??
You will need an original ground topo as well in that case
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