BIZOMAT MAKEOS · Hydraulics
Buried or Submerged Tank Flotation Check (Uplift and Ballast)
Will this empty tank float out of the ground when the water table rises, and how much concrete ballast holds it down?
Flotation check of an empty buried or submerged tank: buoyant uplift on the displaced volume (as the buoyancy tool), resistance from the tank’s own weight, the submerged weight of the soil over it and the submerged weight of concrete ballast or an anchor slab, the safety factor against flotation, and the concrete mass needed to reach your required factor.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Volume displaced (tank outside volume below water) | m³ | Outside volume of the tank (and any attached structure) that can be below the water table. |
| Water density | kg/m³ | Density of the groundwater or flood water. |
| Tank mass (empty) | kg | Mass of the empty tank. |
| Plan area of soil carried on the tank | m² | Plan area of soil that sits on the tank and must lift with it. |
| Soil cover depth | m | Depth of soil cover over the tank. |
| Submerged unit weight of the soil | kN/m³ | Unit weight of the soil when below the water table (about 8–10 kN/m³; your value from the geotechnical report). |
| Concrete ballast or anchor slab mass | kg | Mass of concrete ballast, collar or anchor slab tied to the tank. |
| Concrete density | kg/m³ | Density of the concrete. |
| Safety factor required | Safety factor against flotation you require (often 1.1–1.5; follow your code). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Volume displaced (tank outside volume below wat…
- What it is
- The outside volume of the tank, and any attached structure, that can sit below the water table. It sets the buoyant uplift.
- Units and format
- Enter a number in m³: cubic metres. The form accepts 0.000001 m³ to 100000 m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Uplift rises in direct proportion.
- Get it right
- Use outside dimensions including wall thickness, not the tank’s storage capacity.
What this means: Water density
- What it is
- The density of the groundwater or flood water around the tank.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 900 kg/m³ to 1300 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Tank mass (empty)
- What it is
- The mass of the empty tank with its fittings, the part of the hold-down resistance that comes from the tank itself.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Plan area of soil carried on the tank
- What it is
- The plan area of soil directly above the tank that would have to lift with it if it floated.
- Units and format
- Enter a number in m²: square metres. The form accepts 0 m² to 100000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Count only soil directly over the tank; soil wedges and side friction are not included.
What this means: Soil cover depth
- What it is
- The depth of soil from the top of the tank up to the ground surface.
- Units and format
- Enter a number in m: metres. The form accepts 0 m to 50 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.6 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The soil resistance rises in direct proportion.
What this means: Submerged unit weight of the soil
- What it is
- The weight per cubic metre of the cover soil when saturated below the water table, with buoyancy taken off; from the geotechnical report.
- Units and format
- Enter a number in kN/m³: kN/m³. The form accepts 0 kN/m³ to 15 kN/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 9 kN/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A unit weight in kN/m³, not a density in kg/m³, and the submerged value, not the dry one.
What this means: Concrete ballast or anchor slab mass
- What it is
- The mass of concrete ballast, collar or anchor slab tied to the tank to hold it down.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12000 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The tool reduces it for its own buoyancy, because the concrete is also under water.
What this means: Concrete density
- What it is
- The density of the ballast concrete, used to work out how much of its weight buoyancy takes away.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 1500 kg/m³ to 4000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2400 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Safety factor required
- What it is
- The factor by which the hold-down resistance must exceed the uplift, from your design code or the approving authority.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 3. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Uplift F_b = ρ_w g V (as the buoyancy tool).
- Resistance = tank weight + soil γ′ A h + ballast weight × (1 − ρ_w ÷ ρ_c) (the ballast is itself submerged).
- Safety factor = resistance ÷ uplift; ballast needed = (SF × F_b − tank − soil) ÷ [g (1 − ρ_w ÷ ρ_c)].
- Example: 10 m³ gives 98.067 kN of uplift; with 12000 kg of concrete the factor is 1.2905, and 11,305 kg would be needed.
Limitations
- Empty tank with the water table at the ground surface (worst case).
- Soil friction on the tank sides is ignored (conservative).
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design hold-down for a buried fuel or water tank
- Check a pit or sump for uplift
- Size a concrete anchor slab
Who it is for
- Civil engineers
- Plant engineers
- Design engineers
What you get
- Buoyant uplift
- Resisting weights and safety factor
- Ballast needed for the required factor
Interpreting the result
- Concrete only weighs about 58% of its dry weight under water: ballast must allow for its own buoyancy.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3027. Topic reference; no source tables or recommended settings are reproduced.
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Open the Buried or Submerged Tank Flotation Check (Uplift and Ballast) calculator