What is buoyancy uplift (flotation)?
Buoyancy uplift — in Dutch opdrijven — is the phenomenon of an underground box wholly or partly below the water table rising because it floats on the groundwater. It happens as soon as the weight of the box is less than the weight of the water it displaces.
This is Archimedes’ principle applied to a basement. An empty concrete box 5 by 10 metres standing 2 metres into the groundwater displaces 100 cubic metres of water — roughly 100 tonnes of upward force. If the box weighs less than that, it comes up.
Where it occurs
- Basements and lower ground floors in houses, particularly during construction when the slab is in place but the superstructure is not.
- Car parks and tunnels — large, relatively light volumes below the water table.
- Swimming pools — a pool pumped dry in wet ground is a classic loss case; it can literally rise out of the garden.
- Excavations and basement boxes during the works — the risk is greatest at the moment dewatering is switched off.
- Septic tanks, soakaway crates and rainwater tanks — lightweight plastic elements are especially vulnerable.
How to prevent it
- Add weight — a thicker base slab, heavier walls, or backfill placed over the structure.
- Tension piles — piles permanently connected to slab and walls that hold the box down.
- Flanges or projecting ledges — outstands that mobilise the soil above them and so provide additional downward load.
- Keep dewatering running until the superstructure provides enough weight; stopping dewatering too early is one of the most common causes of damage.
- Anchor tanks and crates with a concrete ballast slab or straps, following the manufacturer’s instructions.
The calculation always includes a margin of safety; a margin of at least around 10 per cent above the point of equilibrium is usual. Have this calculation done by a structural engineer and base it on the highest groundwater level to be expected, not on today’s level.
Related terms
- Excavation
- Dewatering
- Groundwater level
- Basement
- Tension pile
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