Buried Pipe Flotation and Anti-Flotation Backfill
Flotation check for an empty large-diameter pipe in a flooded trench: uplift = water unit weight x pi/4 x OD^2; FS = (pipe + backfill weight) / uplift. A 48 in empty pipe sees 784 lb/ft of uplift, so 200 lb/ft of pipe plus 900 lb/ft of backfill gives only FS 1.40 - short of 1.5, needing ~976 lb/ft of backfill to hold it down. Submerged backfill counts only its buoyant weight; the design engineer governs.
Formula and source
uplift_plf = water_unit_wt_pcf x (PI/4) x (od_in/12)^2; FS = (pipe_weight_plf + backfill_weight_plf) / uplift_plf; required_backfill_plf = target_fs x uplift_plf - pipe_weight_plf.
Archimedes flotation identity by name (uplift = weight of displaced water; FS = resisting weight / uplift); first-principles buoyancy arithmetic.
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