Groundwater and Buoyancy Calculator
Hydrostatic uplift, resisting load and factor of safety against flotation.
Inputs
Results
Factor of safety against flotation
1.74
Required 1.10 · uplift 32.9 kN
Resistances include structure self weight and any additional load entered. Soil friction, side adhesion and anchors are excluded unless entered as an additional load.
Calculation details
Engineering equations
U = γ_w · A · h_w
R = self weight + additional resisting load
FS = R / U
Variables
- U
- Hydrostatic uplift force (kN)
- γ_w
- Unit weight of water, 9.81 kN/m³
- A
- Plan footprint area of the structure (m²)
- h_w
- Submerged height from the structure base to groundwater level (m)
- R
- Total resisting force (kN)
- FS
- Factor of safety against flotation
Assumptions
- Hydrostatic uplift acts over the full plan footprint of the structure.
- Submerged height is measured from the structure base to the groundwater level; groundwater above the base only.
- Resistance is structure self weight plus any additional resisting load entered.
- Soil friction, side adhesion and anchors are excluded unless entered as an additional load.
- Groundwater is assumed at rest with a horizontal water table.
Worked example
Maintenance hole, D_i = 1200 mm, wall 150 mm, ground level 25.00 m, base 21.60 m, groundwater 23.50 m, concrete 24 kN/m³
A = 1.77 m², h_w = 1.90 m, U = 9.81 × 1.77 × 1.90 = 32.9 kN
Self weight = 2.39 m³ × 24 = 57.3 kN
FS = 57.3 / 32.9 = 1.74
Results are preliminary engineering estimates based on the stated assumptions. Confirm inputs, assumptions and requirements against the applicable standards and authority requirements before use in design or construction.