Rohit Puri Posted on 1:00 pm

Can You Install an Underground Water Tank on a Hill?

With more Australians taking to water harvesting and storage, water tanks have become an important fixture in any household. There are two types of water tanks which are named depending on where they are installed; above ground and underground water tanks. Underground tanks are preferred by people because they allow you to utilize some of the space above them, something you cannot do with an underground tank.
When installing an underground tank, location is important. A flat surface is preferred, since it cuts down on the cost of levelling the excavation. It is however, completely possible to install an underground water tank on uneven ground, including on a hill. The main concern would be to make sure you follow the right steps. Here is a breakdown.

The excavation

When picking the site, the first step is to inspect the soil. The importance of this is to help you make sure you are digging in the right spot, where the soil will not give in. You are also looking for a place that has above average sub-soil drainage, while retaining enough soil integrity to ensure that the tank does not get washed away. Good soil integrity and good drainage also reduces the instances for a mudslide.
Once you have picked the correct place, the next step is to actually dig the hole that will house the tank. The excavation is a tricky affair, since the whole tank has to be immersed into the ground, while the bottom remains flat. For that to happen, the measurements have to be exact, depending on the size of your tank. The bottom of the hole has to be made even, and well compacted. While digging, take into account just how deep you need to go, since one of the walls will undoubtedly be shorter than the other. This will be dependent on the incline of the hill and the size of the tank, since it needs to be inside the hole. You should also leave some space all around the tank for wiggle room while at the same time compacting the backfill around the lower half of where the tank will lie.
underground water tank
After digging, the next step is to create stable bedding for the base of the tank; the compacting alone is not enough. Add a mixture of sand and gravel, to a depth of about 15 centimetres. You should then compact and shape this bedding to suit the underside of the tank. It will help you avoid uneven ground which could puncture the tank from the underside, under the weight of the water.

The Placement

When placing the underground water tank, make sure you check and note down important tank information. This includes the serial number for the warranty card, the shipping damage, and of course the orientation of the inlet and outlet ports. Be sure to be gentle with the tank when placing it, since any severe impact could crack the tank, which is not covered under most warranties. You should also use a pipe level across access to measure the levels of the inlet and outlet. It is important to note that poly tanks especially, are prone to floatation and distortion at the base if they are pumped out during a high water table. This is why you should always keep the tank partially filled especially during the rainy winter months when it is extremely wet.
After placing the tank, backfilling should be done when the tank is either totally empty, or at no more than 30% filled. The trick to a successful backfilling is to make sure the soil is compacted especially around the lower half of the tank to provide support. Avoid using machines to compact the soil close to the tank. When backfilling, use either clean fill or granular soil and at avoid clay soil at all costs. Compacting should also be done under the inlet and outlet pipes in order to offer support and prevent any potential pipe fractures that might occur as the backfill sets over time. If the tank has been exposed to the sun, it should be allowed to cool before the backfilling is completed.
Finally, pipes should be installed only when you are sure that the backfill will not shift during the process.

Other considerations

Make sure you follow the tank manufacturer’s guidelines with regards to depth. For most poly tanks, the recommended soil cover is up to two feet while super tanks are rated for four feet of soil. Make sure that the location you have chosen has low traffic. Another consideration you should make is to avoid areas with a high water table. For such locations, use reinforced tanks. Finally, make sure you deflect surface run-off after installing the tank. This is because it can damage the backfill, especially while it is still fresh.
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