Volume = pi *r^2 * h
h( height, or depth in this case ) = volume/pi * r^2
Some conversions may be needed here.
A contractor is going to pour a footing and is told that each footing must be 8 cubic yards and the diameter is 60-inches. To what depth must the pit be dug to hold the 8 cubic yards of concrete in a 60-inch diameter hole?