If you mean 40 cubic yards, then divide 1080 (40 times 27, the number of cubic feet in 40 cubic yards) by (the area of the circular base if the diameter is 10 feet), or by (the area of the circular base if the radius is 10 feet.), or by (the area of the circular base if the circumference is 10 feet).
John
My calculator said it, I believe it, that settles it