Stones and gravel vary a lot in weight and volume. The shape of the stone controls how it goes together which determines how much stone goes into a cubic foot. The best thing for you to do is ask the supplier how many pounds of the stone you will be ordering, are in a cubic foot. I would

__guesstimate__that amount to be about 120 pounds per cubic foot. The volume of the hole is 8 x (3.14 x 9) = 226 c.f. X 120 pounds/c.f. 27,120 lbs. Check my math but it would be about 14 tons