



 
Hi Joel. The volume of the pipe is the volume of the cylinder 100 ft long with a 4" diameter (2" radius). The volume of the drain is the volume of the "box" that is 100 ft long by 6 inches by 18 inches. The difference between these will be the volume of gravel you need. Thats 129600  15080 = 114520 cubic inches. Since 36 inches = 1 yard, (36 inches)^{3} = (1 yard)^{3}, so 36^{3} cubic inches = 1 cubic yard. By dividing then, we get the number of cubic yards of gravel: 114520 / (36^{3}) = 2.45 cubic yards. Stephen La Rocque.>  


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