Underground tanks fit neatly into New Zealand's shrinking residential sections. They also float. Both of those things are true at once, and the second one is what this article is about, because an empty tank on a wet site does not stay where you put it unless something holds it there.
Sections are getting smaller
Many of us remember the quarter-acre section, which equates to about 1,000 square metres. In today's metropolitan centres, houses are frequently built on far less land. New section sizes in Auckland have reduced by a third in the past three decades. They now have a median area of less than 500 square metres, with the average section about 120 square metres smaller than in the rest of New Zealand. That is a halving inside a single working lifetime. Every square metre of a modern section is spoken for, which changes what you can reasonably ask a homeowner to give up to a water tank.
The tank still has to go somewhere
Builders not only have to construct designs that fit into smaller sections, they also have to install water tanks to stay compliant with council regulations. Most of New Zealand's councils now expect homeowners to do their fair share of stormwater mitigation, and detention and retention tanks achieve this. The obligation does not scale down just because the section did. So where does the tank go? Above-ground slimline tanks are one solution, using a narrow footprint along a boundary or wall. Going underground is the other, and it is the one that gives the section back.
Space saving comes with a condition
Underground water storage tanks are a space-saving way to store water on smaller sections while meeting council compliance standards where on-site stormwater mitigation is compulsory. So far, so good. However, some residential sections sit in flood-prone areas, or in locations with high groundwater. Since empty underground tanks are naturally buoyant, how do you keep them in place? This is the question that catches people out, because the tank behaves impeccably when it is full and the risk only shows up when it is empty, which is precisely the state a stormwater tank is designed to return to.
How uplift restraints work
Uplift restraints are the answer, and they are the ultimate in simplicity while performing a critical function on sites with high groundwater or in flood-prone areas. Straps are looped over the top of the tank and attached to restraint boxes on either side. The boxes are then filled with concrete to ensure the tank remains stable, even when hydraulic pressures are high. The concrete is the point: the tank is not being held down by the straps so much as tied to a mass heavy enough not to move. Promax uplift restraints are designed and engineered to stop the tank from floating and to hold it in the ground should groundwater rise to ground level while the tank is empty.
Who asks for them
Some council inspectors are aware of flotation issues in certain areas and ask for restraints. At other times the site engineer will require them to be used. Both routes lead to the same place, but neither is guaranteed to catch every site, and that is worth sitting with. The absence of a request is not a verdict that the site is dry. If you know the ground holds water, or the area floods, the case for restraints does not depend on somebody else raising it first. Our article on hydrostatic crush pressure covers what groundwater does to a buried tank from the outside in.
How many sets you need
For residential size tanks, one set of Promax uplift restraints is typically used for every 1,000 litres of tanking, and this applies to all underground tanks. A 2,000 litre underground tank will require two sets of restraints. A 7,000 litre model will need seven. The ratio is deliberately easy to hold in your head, and it scales with the tank rather than with the site, because the thing being resisted is the tank's own buoyancy. Our underground specification tool will help you work through the rest of the configuration.
Installation anchors are not uplift restraints
There might be some confusion about installation anchors and whether they perform the same function as uplift restraints. They do not. Installation anchors help hold the tank in place during installation. They offer a limited degree of restraint, but they are not designed to keep the tank in place if groundwater rises to ground level while the tank is empty. That is the whole distinction: anchors are for the day of the install, restraints are for the next few decades. Treating one as the other is an easy substitution to make on paper and an expensive one to discover on site.
Make restraints part of the system
Uplift restraints are considered a safe and reliable long-term option in areas where high groundwater is present or where floods are frequent. Even the best underground tanks are buoyant when empty, especially when hydraulic pressures are high. For that reason alone, restraints should be part of the complete tank system rather than an accessory you decide on later. They will keep the tank in place, while the tank itself keeps everything compliant on a residential section of any size. If you are still weighing the siting decision, our comparison of underground against above-ground tanks is a good next read. Contact our technical team to talk through your site.