Customers sometimes ask, “Can I put the above ground tank underground?”.

Above ground tanks are designed only to deal with the normal pressure of gravity and the corresponding outward pressure of water stored inside the tank (internal hydrostatic pressure), so they are never to be used underground. However, some manufacturers are prepared to have the bases of their tanks dug into the ground to a very limited degree and subject to an independent engineering assessment.

Hydrostatic pressure is an issue for all tanks. It can be simply defined as the pressure coming from the weight of the water and its value as determined by the height of the liquid container and the density of the liquid. The tank walls have to be constructed to resist this pressure under all conditions and at all levels of water storage. Basically the taller the tank, the higher the pressure on tank walls, one reason why wide diameter squat round tanks are so strong. Slimline tanks, being taller, need special design features to help resist the pressure of the water inside the tanks.

Underground tanks are very suitable for locations which are short on space but rainwater tanks used underground have to be strong enough to withstand both the outward pressure from the water stored inside the tank (internal hydrostatic pressure), above ground pressure (exerted by the surrounding soil, driveway or slab for example) and hydrostatic lift caused by upwards pressure of groundwater if it is present. Underground tanks therefore need a special design to cater for these pressures. As pure water at 4oC has a Specific Gravity (S.G.) of 1, the tank requires a higher S.G., for example 2.1.

To use a Polyethylene water tank underground the tank has to be moulded in a certain way with strong ribs and interior voids to make it possible for it to be used under the extra loads it experiences.

We can use a pressure measurement device to measure the volume of water in an underground tank. The pressure sensor is placed at the bottom of the tank and measures the hydrostatic pressure which is directly proportional to the water level, for example 100mbar~1 metre of water. A cylindrical tank 1 metre in diameter with a hydrostatic pressure of 50mbar has a constant cross-section so to calculate the total volume of water in the tank, multiply the water level by the area in square metres which gives a result of approximately 0.39 cubic metres or 390 litres.

Mbars or megabars are measurements of pressure used by most engineers over the world. They are sometimes preferred to kPas, as 1 megabar = 100 kPas.

Hydrostatic lift also has to be taken into consideration for underground tanks. This is the tendency for a tank to “pop out of the ground” when empty. Therefore the manufacturer has to ensure that the tank is well-designed and/or there is sufficient ballast or anchoring to prevent this possibility.

Newly manufactured tanks have to pass a hydrostatic test to ensure that there are no leaks or deformation of the tank when filled with water.