How do I prevent my concrete tank from floating

**Please note Versatile Tanks DO NOT repair water tanks**

Is there a rule or standard for installation of underground tanks ?

The Australian National Water Commission has complied a Rainwater Tank design booklet to assist in answering these often raised queries. These procedures are to be followed if the rainwater tank or any vessel placed underground that is designed to withhold  or store liquid. Any Hydraulic Engineer can insist that  their specific procedure or method be used and that over rides the Rainwater HB2008 handbook.

Versatile Tanks has an engineered design method for installation for areas that may be prone or suspect able to hydro issues. This is supplied upon ordering our tanks. We ask the client if their engineer has raised concerns in this area. It is very easily rectified.

Exert from the standards in laymen’s terms


Buried tanks may have a tendency to move (float) when empty, especially if the ground is water charged. To avoid floating, the total weight of the tank, overburden (the backfill above the tank) or an engineered design must provide resistance to counterbalance the buoyancy of the tank (hydrostatic lift). The ballast needs to be greater than the volume of the tank (e.g. a 5000 L tank requires approximately 5000 kg (5 tonne) of ballast when empty).

A consulting engineer may be used to design a more efficient solution based on the specific site conditions and backfill material. These calculations are only required if not specified by the manufacturer, or if installation be as per manufacturer’s instructions. If tanks are installed underground, check manufacturer’s detailed installation instructions.

The following provides a simplified method to calculate ballast.

The installation site assumptions include the following:

(a) The groundwater level at the finished surface level (FSL).

(b) There are no side frictional forces as the water-charged ground acts as a


(c) Physical constants that can be use in calculations:

(i) Weight (mass) of water = 1000 kg/m3 (1000 L = 1000 kg)

(m3 = cubic metres).

(ii) Weight (mass) of concrete if used (in air) = 2400 kg/m3

(iii) Weight (mass) of alternative backfill = Dependent on backfill material and moisture content (e.g. saturated crushed rock is 2120 kg/m3).

(iv) Weight (mass) of tank = as per manufacturer’s specifications.

Sufficient mass or resistance is needed to counterbalance the buoyancy of the tank void:

(i) Determine the buoyant force exerted on the tank. The buoyant force is the

weight of the displaced water or weight of the water when the tank is full (Vt)

(i.e. the volume of tank, units = L » kg).

(ii) From the manufacturers technical information determine the weight (mass) of the tank (Wt) (units = kg).

(iii) Determine the approximate weight of the overburden required (A1 × A2 × D0 × 2120, units = kg. where A1, A2 and D0 are measured in

metres). The overburden plus the weight of the empty tank (Wt) must be equal or greater than Vt. If this can not be achieved, a structural engineer will be for the design.

Many tank designs incorporate ways to counteract hydrostatic lift through the design and installation process (refer to manufacturer’s or structural engineer’s design)

Feel free to call Versatile Tanks should you require further information on installation of our tanks. Our number is 1300 783 344

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