Structure Buoyancy / Flotation Check

Factor of safety against flotation for an empty basin, wet well, tank, or vault with groundwater present — the check to run before draining a structure or turning off the dewatering. Runs entirely offline; inputs persist in this browser via localStorage.

Use limits: This is a screening check on the classic worst case — structure empty, groundwater high. It ignores side-wall friction and any structural anchorage (piles, tie-downs), which is conservative, but it also trusts your groundwater elevation, which is the input most likely to be wrong. If this tool says the FS is thin, the answer is the project geotech/structural engineer and the approved dewatering plan — not a slightly more optimistic groundwater guess.

Structure Geometry (outside dimensions)

Shape

Structure height, base of slab to top (ft)

Outside length (ft)

Outside width (ft)

Groundwater height above BOTTOM of base slab (ft)

The worst credible case: seasonal high, or the level when dewatering shuts off — not today's piezometer reading on a dry week.

Resisting Weight

Dead weight

Wall thickness (in)

Base slab thickness (in)

Top slab thickness (in) — 0 if open-top

Concrete unit weight (pcf)

Soil cover over top slab (ft) — buried structures

Soil unit weight for cover (pcf)

If the cover soil is below groundwater, use the buoyant unit weight (total − 62.4).

Other permanent resisting weight (lb) — equipment, base slab ledge soil, etc.

Only count what is verifiably permanent and in place at the moment the structure is empty. Contents don't count — the check assumes empty.

Required factor of safety

From the project specs or the structural general notes — it varies.

—

Footprint area	—
Displaced volume (to groundwater level)	—
Uplift force	—
Concrete dead weight	—
Soil cover weight	—
Other resisting weight	—
Total resisting weight	—
Factor of safety	—
Max groundwater height for required FS	—

How the Numbers Are Derived

The math

Uplift (Archimedes, in feet and pounds):

U = A_footprint × h_gw × 62.4

where h_gw is groundwater height above the bottom of the base slab, capped at the structure height. Resisting weight is concrete dead load (walls as the outside-minus-inside prism, plus base and top slabs at the footprint) plus soil cover plus any entered permanent weight:

FS = W_resist / U

The "max groundwater" line inverts this: the groundwater height at which FS drops to the required value — a useful trigger level for the dewatering plan ("keep the water below elevation X or the pumps stay on").

What this deliberately leaves out (and why)

Side friction and adhesion: real but unreliable, especially in soft or recently disturbed backfill — most flotation checks neglect it, and so does this tool. Anything it adds is silent extra margin.
Soil wedge on base slab projections: if the base slab extends beyond the walls, the soil block above the ledge resists uplift (at buoyant unit weight below groundwater). Add it under "other resisting weight" if the geometry exists and the backfill is actually in place.
Anchors and piles: tension piles or rock anchors change the whole problem — that's a designed system, not a field check.
Contents: the check assumes the structure is empty because that is the case you are creating the moment you drain it. A basin that is safe full and unsafe empty is exactly the situation this tool exists to catch.

Field scenarios where this bites

Draining an existing basin for rehab while the plant's other basins stay full — the local groundwater around a working plant is often higher than the borings from the original construction suggest.
Turning off well points over a weekend "because the excavation looked dry."
New wet wells and vaults after backfill but before the equipment, top slab, or pavement that the designer counted on is in place — check the interim condition, not just the final one.
Heavy rain filling a granular backfill zone around a structure faster than the surrounding native drains — a temporary perched groundwater condition.

References

ACI 350 — Code Requirements for Environmental Engineering Concrete Structures (flotation provisions and load factors). https://www.concrete.org/

Project geotechnical report — design groundwater elevations (the governing input).

Approved dewatering plan — the document this check should send you back to when the FS is thin.