Well buoyancy (anti-flotation) calculator
Forces [kN]
Buoyancy W (↑): 18,11 kN
Self-weight Gₛ (↓): 34,17 kN
Stability
R = Gs + T = 34,17 kN
R/W = 1,89 (≥ 1,1)
Verification
| Check | Calculated | Allowable | Degree of utilisation | Status |
|---|---|---|---|---|
| Flotation stability | R/W = 1,89 | ≥ 1,1 | 58,3% |
Calculation details
Geometry
Dzew = 1,24 m, Dwew = 1 m
H = 3 m, submerged height hw = 1,5 m
Forces
W = 18,114 kN
Gs = 34,17 kN
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How to use the well buoyancy calculator
The calculator checks whether an empty, closed well (chamber, manhole) buried below the groundwater table will float under hydrostatic uplift, and sizes the ballast needed.
The check is performed for the worst case: an empty, watertight well with the groundwater at its design (high) level.
Uplift mechanism
An empty, watertight well behaves like a submerged float: groundwater displaces the external solid volume below the table. If uplift exceeds self-weight, the well floats.
Stability condition: R ≥ SF·W, where W = γ_w · V_submerged and R is the sum of stabilising forces.
The governing case is an empty well at the highest credible water level (often at terrain).
Forces
Uplift W (destabilising)
Archimedes uplift acting on the well volume below the water table. γ_w = 10 kN/m³.
Self-weight Gâ‚› (stabilising)
Weight of the well structure (walls + base slab) plus any additional dead load (cover, cone).
Soil resistance T (optional)
Shaft friction mobilised between the backfill and the well wall. Off by default — manhole backfill is narrow and recompacted, so friction is unreliable. With a widened base, shaft friction and the soil-on-flange prism are alternative mechanisms — the smaller one governs, never the sum.
Two verification methods
Global safety factor (screening)
Stability margin (Gₛ+T)/W compared with a factor SF (default 1.1). A quick preliminary check. Ballast uses concrete with effective unit weight γ′ ≈ 14 kN/m³ (24 − 10).
Eurocode 7 (PN-EN 1997, UPL)
Uplift limit state: 1.0·W_k ≤ 0.9·G_k + T_d, reported as utilisation Eₔ/Rₔ ≤ 1 (not a ratio against SF). Ballast uses concrete with effective unit weight γ′ ≈ 11.6 kN/m³ (0.9·24 − 1.0·10).
Ballasting
When the stabilising forces do not overcome uplift with the required margin, the calculator sizes the volume of a concrete ballast chamber Vb and converts it to a depth Hb for an assumed anchor-slab diameter. Only the ballast volume is sized — not its structural connection to the well.
Out of scope
The calculator checks only global flotation stability. It deliberately omits:
- the structural well-to-slab / ballast-chamber connection,
- base slab reinforcement and punching shear,
- forces from connected pipes,
- structural settlement,
- seepage, base failure (HYD) and heave.
ATV-DVWK-A 127 governs buried-pipe statics, not manhole uplift — it is deliberately not the basis here.
Standards & sources
- PN-EN 1997-1 (Eurocode 7) — UPL (uplift) limit state.
- PN-EN 1610 — construction and testing of drains, sewers and manholes.