Checking the float switch of an underground tank used as a fire protection water reserve

Checking the float level of an underground tank: When a fire protection water supply system is installed in presence of a water table, this check becomes a crucial step in preventing potentially disastrous consequences.*

Buoyancy Test: Why It’s Essential

In the case of underground water storage tanks installed in presence of groundwater, the buoyancy test is of fundamental importance. This applies to both reinforced concrete structures and tanks made of rolled sheet metal.

The goal is to prevent the pressure from the aquifer from counteracting the force of gravity and causing the structure to rise back to the surface, which could have very serious consequences.

Groundwater pressure and hydrostatic force: what we’re calculating

The aquifer exerts an upward force on the submerged tank. As is well known, hydrostatic force is directly proportional to the submerged volume of the object: the more of it is submerged in the liquid, the greater the hydrostatic force acting on it.

This force acts on any object placed in a fluid; for this reason, the correct approach to the problem always begins with an accurate assessment of the submerged volume.

The first step: determining the submerged volume

To set up the buoyancy test, the first step is to calculate:

• Geometric Volume $V_g$ of the tank (based on external dimensions)
• Submerged Volume $V_i$, that is, the portion that is actually “submerged” relative to the water table

From $V_i$ , the upward thrust is then derived, calculated conservatively by assuming the tank is completely empty.

Example 1: Prefabricated reinforced concrete tank (complete calculation)

Let’s consider a precast reinforced concrete tank with:

• External dimensions (floor plan): 2.5 × 2.6 m
• height: 5 m
• water table: 0.5 m below ground level

Calculating Volumes:

• Geometric volume:
  $V_g=2.5\times 2.6\times 5=32.50{\text{m}}^{\mathrm{³}}$
• Submerged volume:
  $V_i=2.5\times 2.6\times (5-\mathrm{0,5})=29.25{\text{m}}^{\mathrm{³}}$

Bottom-up thrust (empty tank):

• $F_g=V_i\times \text{density of water}\times g$
• $F_g=29.25\times \mathrm{1,000}\times 9.81=286942\text{N}$

Weights facing down:

• Tub weight: $\mathrm{24,800}\text{kg}\times 9.81=243288\text{N}$
• Weight of fill soil: $600\text{kg}\times 9.81=5886\text{N}$
• Total weight: $F_{pt}=249174\text{N}$

Check: Let’s look at the criterion with the overall safety factor below.

Example 2: Calendered sheet metal tank (complete calculation)

Now let’s consider a tank made of calendered sheet metal with:

• outer diameter: 3 m
• geometric volume: 80 m³
• (same conditions regarding the presence of groundwater)

Volumes:

• Geometric volume: $V_g=80.00{\text{m}}^{\mathrm{³}}$
• Submerged volume: $V_i=76.00{\text{m}}^{\mathrm{³}}$

Bottom-up thrust (empty tank):

• $F_g=76.00\times 1000\times 9.81=745560\text{N}$

Weights facing down:

• Tub weight: $6200\text{kg}\times 9.81=60822\text{N}$
• Weight of fill soil: $\mathrm{3,000}\text{kg}\times 9.81=29430\text{N}$
• Total weight: $F_{pt}=90252\text{N}$

Here, too, the verification is done by comparing $F_{pt}$ with the momentum amplified by the global security factor.

Non-floatability criterion and overall safety factor (1.22)

The criterion for preventing the object from floating is:

• Requirement:
  $F_{pt}>F_g\times \mathrm{1,22}$

Where 1.22 is the overall safety factor.

Application to Example 1 (Reinforced Concrete Tank)

• $F_g\times 1.22=286942\times 1.22=350070\text{N}$
• Comparison: $249174<350070$

Therefore, the request has not been verified.

Application to Example 2 (Sheet-Metal Tank)

• $F_g\times 1.22=745560\times 1.22=909827\text{N}$
• Comparison: $90252<909827$

In this case as well, the request has not been verified.

How to Prevent Floating: Ballast, Curb, Slab, and Anchors

When this condition is not met, the “downward force” must be increased using specialized structural solutions.

Solution to Example 1: Ballast at the Bottom (Perimeter Curb)

To prevent the tank from floating, ballast must be installed at the bottom of the tank with a weight of at least:

• $350070-249174=100896\text{N}$

One suggested example is to install a curb along the entire perimeter of the pool.

Solution to Example 2: Base Slab + Anchors

For sheet-metal tanks, to prevent them from floating, it is essential to construct a base slab and, using anchors, securely fasten the tank to the slab to form a monolithic structure.

Minimum required weight of the floor slab:

• $909827-90252=819575\text{N}$

In this example, this means a slab with base dimensions of 5 × 14 m and a thickness of approximately 0.45 m, with the tank anchored to it to form a single monolithic structure.

Data to Collect for Your Real-World Case

To set up a float test correctly, you need at least:

• external dimensions (width and height) of the tank/reservoir
• depth of the groundwater table relative to ground level (to calculate the submerged volume)
• weight of the product (in kg)
• Estimated weight of the fill soil (in kg)
• selection of the precautionary condition (in the text: completely empty tank)

Using these elements, it is possible to calculate $V_i$, determine $F_g$ and check the condition $F_{pt}>F_g\times \mathrm{1,22}$.

Conclusion

The buoyancy verification of an underground fire protection water tank, especially in presence of groundwater, requires correctly calculating the submerged volume and comparing the upward buoyancy with the downward weights, applying an overall safety factor of 1.22. If the verification does not hold, countermeasures are taken using ballast/curbs or a slab and anchors, depending on the type of tank.