Pipe Buoyancy
Uplift on an empty pipe and the resistance available.
An empty pipe in saturated ground displaces water and is buoyant. The critical case is normally during construction, before backfill and surface load are in place, and it is easy to overlook because the in-service pipe is full.
Resistance comes from the pipe's own weight, the weight of embedment and backfill above it, and any surface surcharge. Where resistance is insufficient, the options are anchorage, weighted blocks, thicker embedment cover, or dewatering during construction.
Key design considerations
- Construct the check for the empty-pipe, saturated-trench condition.
- Resistance from pipe weight, embedment, backfill and any surface surcharge.
- Lightweight pipe in a wide trench has the least resistance.
- Anchors and weights change the load path and need their own design.
- Consider a rising water table and a flooded trench during a storm.
- For maintenance holes, the base area and structure weight govern, not the pipe diameter.
Equations
U = γ_w · V_displaced
Uplift force, with γ_w the unit weight of water and V_displaced the external volume of the pipe.
FS = W_resisting / U
Factor of safety against flotation, with W_resisting the sum of pipe self weight and the weight of soil above.
Design checks
- Uplift and resistance computed at the governing section.
- Factor of safety compared with the adopted requirement.
- Mitigation designed where the factor is insufficient.
Standards and authority references
- Authority requirements for flotation
- Geotechnical report, groundwater level and soil unit weights
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Hand calculations are a screening check; soil friction and arching are conservatively ignored in most simple methods.
- Required factors of safety vary with the authority and with whether the case is temporary or permanent.
Related calculators
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.