Also in: Disaster Response and Recovery
Buried Pipe Flotation and Anti-Flotation Backfill
Flotation check for an empty large-diameter pipe in a flooded trench.
Example
You enter
- Pipe outside diameter (in) 48
- Empty pipe weight (lb/ft) 200
- Resisting backfill weight (lb/ft) 900
- Target factor of safety 1.5
- Water unit weight (pcf) 62.4
You get
- Buoyant uplift 784 lb/ft
- Factor of safety 1.4028
- Backfill to meet the target (FS on the whole uplift; FS on the backfill only, as WSSC C-4) 976.21
Checked against published worked examples from: WSSC.
Details, formula, and sources
Scopeuplift = 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 or tableuplift_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.
Source and editionArchimedes flotation identity by name (uplift = weight of displaced water; FS = resisting weight / uplift); first-principles buoyancy arithmetic.
Free accessBuoyancy is public first-principles physics; the design factor of safety comes from the project specification.
What governsEstimate. AHJ and licensed professional govern.
API field namespipe_od_in, pipe_weight_plf, backfill_weight_plf, target_fs, water_unit_wt_pcf, uplift_plf, fs, required_backfill_plf
- Uplift the full buoyant uplift of the empty pipe (water unit weight x displaced volume per foot)Archimedes' principle
- Resisting weight pipe self-weight plus backfill over the pipe; submerged backfill counts only its buoyant weightdesign engineer
- Target factor of safety 1.5 default; the project specification governsproject specification