Underground tanks are a convenient construction option. They can offer options for rain water storage without interfering with your surface property plan. At the same time, tanks designed for underwater installation can bear heavy loads without collapsing. They might be polyethylene, or they may be concrete reinforced with steel plates or wires.
Either way, because the intention is to install them beneath the surface of the ground, they can withstand the pressure of the soil above. Many can even support a driveway, allowing you to park your car ‘on top’ of the tank without damaging it. This gives you added flexibility, because even if you have a small lot you can maximise your space by clever tank positioning.
There are many factors to keep in mind as you choose the best space for your tank. Is it primarily going to be a rain water tank or will you supplement the rainy season with water trucks and water from your mains supply? That really depends on which part of Australia you live in, and what your water supply is like. In some places, there’s no access to municipal utilities, so you have to work with rain water, boreholes, nearby rivers/lakes, and trucks.
Call the council
In other places, you can use council water to fill your tank, then access that water when the government supply is halted. This can be helpful in areas where water is supplied on a roster. If water comes in every other day, or maybe once a week, you want to fill as much water storage space as you can, to tide you over on the days when nothing is coming out of the taps. Similarly, in places where it rains once or twice a year, harvest when you can.
If you’re using a combination of rain water and government-supplied water, you can install a water pumping system with a rain switch. This automatically re-routes you from rain water to municipal water and back, depending on the levels of water in the tank. When the rain water tank is full, you can use it as your main resource, but when it runs low, you can swop to government sources, at least until it rains and the tank refills itself.
For homes that are using underground rain water storage depots, the tank itself may not be sufficient. High volumes of rainfall can often cause flooding and high-level erosion, so in addition to your underground tank, you might need a detention tank. It captures the excess rainfall and seeps it into the municipal storm water system, so that it can get into the sewers gradually instead of pouring in all at once and overwhelming the system.
Consider what lies above it
So then the question becomes where exactly should you position the tank on your property? First you need to check in with your local council. Find out if they have any regulations related to the position of your tank. They may want it a set distance from the fence or from the road, because it might interfere with local infrastructure. You might also have power lines and sewer lines passing under your property, and digging for a tank could affect them.
Tank installation instructions recommend a maximum of 75cm of earth above your underground tank. Also, regarding your storm water tank, it’s not just to prevent the sewer system for overflowing. It can – in given situations – serve the same purpose as fire hydrants in the US. Your stormwater detention site can be used as an emergency supply if your neighbourhood is prone to bush fires. It’s a public service, but it’s a survival mechanism too.
When you excavate and underground tank, you need to leave a foot or two of space on all sides of the tank. This space willeventually be refilled, but during installation, it helps to secure your tank in place. If you position your tank too close to the house you won’t have those two feet or leeway, and you might not even have the space for pipes and plumbing.
Pumping and piping
Remember, this is an in-ground tank, so you need some kind of gauge to see the levels of water in the tank without actually looking in the tank (because you can’t), and you need a pump to push water out of the ground and onto the house. Your distance from the house needs to facilitate that. Remember – also – that your house has a foundation of concrete, sand and stone, and that potential leaks in your tank could affect that foundation. Make sure there’s enough ‘breathing space’ between the house and the tank to prevent that.
That said, many rain water pipes are supplied by the gutters and downpipes in your house, so you want to be close enough to benefit from that. Otherwise, you’ll need to lay some extra piping to connect the house to the tank, which means additional expenses. Many builders suggest installing your tank under the driveway, or under your swimming pool if you have one. Finally, think about tank size. A tank big enough to saturate your house foundation should probably be ‘planted’ a safe distance from the house, just in case.