The Concrete Backfill Myth
Concrete is mis-understood
You cannot use concrete to make a weak tank strong enough for a high water table site, no matter what the manufacturers suggest. Weak tanks will always be damaged by high groundwater, regardless of a concrete surround.
Concrete backfills are not waterproof - this is why waterproof membranes must be used underneath concrete floors. All that happens is that the groundwater forces its way through the air spaces in the concrete and bends the tank anyway. In our many years of experience, we have seen all manner of tanks be distorted, bent and collapsed due to the pressure of water building up between the tank and the concrete.
Tanks are surrounded with concrete in high groundwater areas to act as weight ballast only. This is to ensure that the tank does not rise if the water table is higher than the water in the tank.
Can concrete be made to be waterproof?
Concrete can be made waterproof using:
- A wet concrete mix
- Steel reinforcement to stop the concrete cracking
- Waterproofing additives
- Vibration to remove all air gaps
However, the above is not possible for a tank installation because:
- Only a dry / semi dry concrete must be used for all tank installations. (Wet concrete is far too heavy and bends or cracks the tank.)
- Semi dry concrete will always have small voids like a pumice stone
- Water can easily penetrate these voids and therefore exert full pressure on the tank, as if the concrete was not there.
- Vibrating pokers cannot be used during tank installations for fear of cracking or damaging the tank and would not remove the small air spaces in dry or semi-dry concrete anyway.
Is there any other way of waterproofing the hole?
Some manufacturers recommend lining the hole with plastic to keep the groundwater out, but the water still can pour through the gaps where the inlet and outlet pipe sticks through, if it rises higher than those levels.
Most manufacturers recommend that you employ a civil engineer to design the install for high groundwater sites, but do you really want to pay another £1000 for the privilege? It is only a way of invalidating their tank warranty in the small print if you decide to ignore the advice.
If either of the above are found in the installation manual, the tank alone is not strong enough - BEWARE!