Groundwater and Flotation
Uplift on the structure, and the resistance that prevents it floating.
Groundwater exerts uplift on the base of a maintenance hole equal to the displaced volume. Where resistance is insufficient, the structure can rise, crack at joints or lift relative to the incoming pipes.
Resistance comes from the structure's own weight, the weight of backfill and cover, and any base extension or anchorage. The critical case is frequently the construction stage, before backfill and surface load are in place.
Key design considerations
- Design groundwater level, taken as the highest credible level rather than one site reading.
- Structure self weight including base slab and walls.
- Weight of soil and pavement above the cover slab, where it can be relied on.
- Base extension, shear keys or tension piles where weight alone is insufficient.
- Construction stage: an empty structure in a flooded excavation.
- The consequence of movement on the incoming pipes and joints.
Equations
U = γ_w · A_base · h_w
Uplift, with h_w the head of water above the base and A_base the base area.
FS = (W_structure + W_soil) / U
Factor of safety against flotation; content weight should be treated conservatively.
Design checks
- Uplift and resistance computed at the deepest structure on the line.
- Factor of safety compared with the adopted requirement.
- Mitigation detailed where the factor is insufficient.
Standards and authority references
- Authority flotation requirements
- Geotechnical report and groundwater data
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Simple methods ignore base friction and passive resistance; they are conservative screening checks.
- Required factors of safety vary with the authority and the load case.
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.