For the backfill of a 30×3 feet retaining wall, you will need a volume of 90 ft³ for a weight of 7,563 lb. To find these results:
Assume you are using the same dimensions of the retaining wall for the backfill.
Compute the volume of the backfill by multiplying height (3 ft), length (30 ft), and width (1 ft):
V = 3 × 30 × 1 = 90 ft³
Calculate the weight of the backfill gravel by multiplying the volume by the density (84.03 lb/ft³):
W = 90 ft³ × 84.03 lb/ft³ = 7,563 lb
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