More homeowners are installing their stormwater tanks underground to save space and improve the aesthetics of their properties. In the industrial sector too, we are seeing a trend toward underground tanks. There is no doubt about their increasing popularity in all sectors. Things are less clear-cut regarding the best material, and that is the question we get asked.
The question architects and engineers keep asking
Is it plastic or concrete? Architects, engineers, builders and homeowners frequently put this to us, and it is a fair question to ask, because the tank is going in the ground and nobody wants to revisit that decision later. As you will read, we believe plastic is the correct answer in more ways than one. We are a plastics manufacturer, so we would say that. What follows is the evidence we base it on, drawn from what happened to tanks in this country, so you can weigh it yourself rather than take our word for it.
What the earthquakes showed
Traditional thinking might lead you to conclude that concrete is strong and durable, making it the ideal material for underground tanks. Unfortunately that is not always the case, and all the evidence we need is in our own backyard. It became apparent during the Christchurch and Kaikoura earthquakes that rigid structures like concrete tanks did not perform well during the tremors. Their lack of flexibility counted against them, and that is why Promax replaced many badly cracked concrete water tanks after the earthquakes. On the other hand, we have had no recorded failures of our plastic tanks after these seismic events.
Flexibility is the point, not softness
This was no great surprise to us. We design our polyethylene water tanks to be remarkably tough while retaining enough flexibility to withstand significant ground movement. Those two properties sound opposed and are not. Strength decides how much load a tank carries. Flexibility decides what happens when the ground it sits in moves and the tank has to move with it. A rigid structure has to resist that movement with brittle material, and cracking is how it loses. Our work on storage solutions in seismic zones goes further into what this means when you are specifying for a shaky site.
How the underground tanks are engineered
Flexibility is one of the many benefits of Promax plastic water tanks, including our corrugated underground models. Geotechnical experts design and engineer our underground tanks, which are then Finite Element Analysis tested. That combination matters for a buried tank, because the loads are not just the water inside. They come from soil and groundwater pressing in from every direction at depth. Promax corrugated underground tanks are light and easy to install, which reduces handling costs, and they withstand ground and water pressure at depth, meaning you can trust them under driveways, footpaths, car parks and forecourts. Our article on hydrostatic crush pressure explains the force involved.
The practical advantages
Here are a few other things that make our plastic underground stormwater tanks a better investment:
- Fast delivery: Promax holds stock of all standard sizes to improve delivery times.
- Maintenance: a 600 mm diameter utility hole and lid for easy inspection.
- Durability: one-piece rotational moulding for most sizes provides extra durability.
- Adaptability: it is much easier to add fittings to plastic tanks.
- Affordability: plastic tanks are very cost-effective compared to concrete.
Adaptability is the one that tends to be underrated at design stage. A tank you can add a fitting to is a tank that can absorb a late change on site without a specialist crew, and late changes on site are not rare.
Sizes and diameters
To keep things simple, we standardise Promax Tanks in incremental capacities from 1,000 litres up to 30,000 litres, which suit virtually all applications. Depending on invert levels and site space availability, they are available in standard diameters of 750 mm, 900 mm, 1200 mm and 1900 mm. That diameter range exists because underground siting is usually a geometry problem before it is a volume problem. The invert level of your pipework sets how deep you can go, and the space available sets how wide. Between the two, one of those diameters will generally let you reach the capacity you need. The underground specification tool will help you narrow it down.
Detention, retention, or both
Before you settle on a size, it is worth being certain which job the tank is doing, because the answer changes the volume you need. A detention tank holds stormwater back and releases it at a controlled rate. A retention tank keeps a portion of it for reuse. They are not interchangeable, and councils do not treat them as such. Our explanation of the difference between a detention tank and a retention tank is worth reading before you commit to a capacity or a diameter.
So which is best?
Our underground stormwater tanks come in the size you need, for the setting you want, and with the strength you require. That is why we firmly believe plastic is the best material for your underground stormwater tank. The strongest part of the case is not a specification, it is the record: rigid concrete tanks cracked in two major New Zealand earthquakes and we replaced them, while our plastic tanks have no recorded failures from those same events. Contact us for further information and advice, or call Promax on 0800 77 66 29.