# Forum Home Renovation Water Saving Garden Ideas  Underground water tanks

## bluprint

Has anyone had any experience with installing an underground water tank. I'm off to Reece tomorrow morning to get some more details on their Graf tanks. 
I've got a problem with storm water drains on my property... long story with my builder  :Mad: , however, an underground water tank will solve 2 problems and will be cheaper than alternative storm water drain solutions.  
I know I'm looking at $2,500 just for the tank without the required extras + excavation and connections. My only upside is that all the down pipes currently go to one place. 
I'd be interested to hear other people's experience.

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## Bloss

I have several lots of friends who have used underground tanks and all are happy. Two have used the plastic tanks and one has installed the tank that is simply a membrane filled with milk crate type boxes then covered. Others have used concrete tanks both pre=formed and poured in-situ. As I said they are all happy - the only issue is getting the right pump(s). 
At one friend's place they have installed a 90,000 litre concrete tank the top of which is around 1m underground and they use it for their ground source heating & cooling - the water stays at around 10-14 (near the ACT) and there are loops that run into the tank and pout again and via a heat pump exchanger. Capital cost is high, but running costs are very low and very efficient - around 5-6 times more heating or cooling out than electricity going in. 
They use the tank water for general purposes, but have level switches so that the water level does not drop below their loops.

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## intertd6

The real problems with empty underground tanks (plastic or concrete) are water table levels, they tend to want to pop up out of the ground if they dont have pressure relief valves in them plus a no fines concrete base with good surrounding drainage material
Regards inter

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## Bloss

> The real problems with empty underground tanks (plastic or concrete) are water table levels, they tend to want to pop up out of the ground if they dont have pressure relief valves in them plus a no fines concrete base with good surrounding drainage material
> Regards inter

  Only a problem if not installed according to destructions - and rare because few people ignore these. In any case water tables are rarely as high as 2-3m below ground level and tanks are not generally set lower than that. Of course you need to know the soil type and water flows in your specific location, but ground water gouging and similar issues are more common if not sufficient attention is paid, but still rare.

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## intertd6

I will re-phrase that "empty tank in ground surrounded by water will float" unless pressure relief valves fitted, tank ballusted or anchored to resist buoyancy uplift, exterior drainage adequate to not allow water levels to reach tank buoyancy levels
Regards inter

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## bluprint

Hi Guys 
After pricing up the tanks with installation and excavation I've decided that it's a bit out of my league on price. 
If you're considering going down this route, you'll need at least $10-15K for 9600 litres. 
Thanks for the replies.

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## chuth77

Or, up here in Brisbane you can get 32,000 litres for $4200 plus excavation....

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## quercus

I can recommend Hendricks concrete in pakenham. I got a 5200 litre underground water tank, delivered and craned off the truck for $1500. Cheapest tank around, and they are nice to deal with . I did a fair bit of research, and there was another company who did a 10 000  litre underground tank, but it was more than double the price. In a way I wish I had bought a bigger one, since I easily could have filled it, but the excavation costs would have been pretty high.  http://www.hendrikxconcrete.com.au/drawings.php http://www.hendrikxconcrete.com.au/index.php 
It weighs about 4.2 tons, and although it displaces a fair volume of soil, I find it very hard to believe it would float. As a matter of fact it has been completely flooded several times all rouind the tank, and it certainly hasnt moved. I wouldnt be picking it as a material to make a boat out of. 
It cost, roughly
Tank, $1500
excavator -5 tonner and truck $800
Soil disposal fees - about 20 m3 as clean fill. $200
gravel for backfilling - about $100
Submersible pump that sits in tank - $350   
Plus plumbing 
It is a bloody good set up. 
The only trap is that the excavation is bigger than  you imagine. It may be surprising how much soil comes out of a hole for a tank that big. The hole had to be 2.8 deep. That is a pretty deep hole, and since the machine basically digs the hole square, you generate a lot of fill. If it is very lumpy or clayey, you may need to backfill with gravel  
I am putting mine under a driveway, so need good backfill. You may be able to get away with just putting back the soil. 
If you need more info, or want me  to put up some photos, let me know. There arent that many people who have put biggish underground tanks in, so a lot if it is trial and error.

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## Bloss

> I 
> If you need more info, or want me  to put up some photos, let me know. There arent that many people who have put biggish underground tanks in, so a lot if it is trial and error.

  Gotta watch the age of the threads - this is a January post so fair chance they've resolved their issue!  :Smilie:  But the info might be helpful to others . . .  :2thumbsup:  BTW - there are plenty of very large underground tanks installed - of 100Kl and more (not 10Kl). This is not new - just a bit newer in residential areas, although I know of a suburban house now nearly 20 years old that has 90,000 litre concrete tank under it.  :2thumbsup:

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## intertd6

[quote=quercus;757362] 
It weighs about 4.2 tons, and although it displaces a fair volume of soil, I find it very hard to believe it would float. As a matter of fact it has been completely flooded several times all rouind the tank, and it certainly hasnt moved. I wouldnt be picking it as a material to make a boat out of. 
with your tank having a capacity of 5200 L  if it got below 1000L in the tank then it would become buoyant & could float, if completly empty it would have a lifting force of 1 tonne fully submerged
regards inter

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## Bloss

[quote=intertd6;757407]  

> It weighs about 4.2 tons, and although it displaces a fair volume of soil, I find it very hard to believe it would float. As a matter of fact it has been completely flooded several times all rouind the tank, and it certainly hasnt moved. I wouldnt be picking it as a material to make a boat out of. 
> with your tank having a capacity of 5200 L  if it got below 1000L in the tank then it would become buoyant & could float, if completly empty it would have a lifting force of 1 tonne fully submerged
> regards inter

   :What he said:  It's not the weight of the tank that matters it is the weight relative to the area displaced (if water can fully surround the tank - an important point as that is a highly variable factor). So as Interd6 says a 5200l tank is ~5200kgs of water so once the internal level gets below the weight of the tank ie: <1000l then you have balance - keep going and you have potential for lift. But the forces involved are more complex and are not just from the bottom - but from the sides too. There also are countervailing forces from the friction between the soil and the tank walls, and the capacity of the soil type to hold water is another factor that reduces the likelihood of pop up or displacement in most places in Australia. 
So the theory of the problem is fine, but in actual practice this is not a big issue for tanks or pools in Australia and hydrostatic valving systems are used in areas where it is known problem.  :2thumbsup:

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## Uncle Bob

> I wouldnt be picking it as a material to make a boat out of.

  As a side note, plenty of boats have been made out of concrete.

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## journeyman Mick

> As a side note, plenty of boats have been made out of concrete.

  A disproportionate number of which have disappeared without trace at sea. Theory is that whales will often come up to check out yachts as they're usually quiet, no motors. They come up, have a back scratch and crack the hull which sinks so quickly the occupants don't have time to bail out. That's the theory, anyway. 
Mick

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## Bleedin Thumb

Another potential problem with underground tanks is pressure from the surrounding soils if you have expansive soils. Thats why underground tanks cost more - they have much thicker walls. 
Normal tanks can usually be buried up to a third of their depths anyway...if you dont have highly reactive soils you can push this to about 2/3rds.....I have even buried them completely without any problems15 yrs on....not that the manufacturer would recommend it.

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## Bloss

> A disproportionate number of which have disappeared without trace at sea. Theory is that whales will often come up to check out yachts as they're usually quiet, no motors. They come up, have a back scratch and crack the hull which sinks so quickly the occupants don't have time to bail out. That's the theory, anyway. 
> Mick

  Totally off-topic, but I reckon that's an urban myth. Ferro-cement hulls have been around for along long time and are extremely strong. There is no evidence that they are any more prone to damage or loss at sea than any other hull type. 
For reliable information about them see: http://www.ferrocement.org/

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## rrobor

Ok my system was half explained here. I bought plastic crates from a dutch guy near Ballarat, they were used to ship bulbs in  and from memory were 30 X 20 X 60 for a dollar a crate.  My soil is pure clay so watering is a problem at the best of times. Anyways I dug trenches a meter deep and stacked them 2 high and covered them over with weed mat and filled the trenches in. I knocked a hole in the bottom of the storm water pipe so my trenches get all the water from my roofs and if ever filled it just flows away as storm water. My trees did OK last year, I have a silver birch which withstood 47 degrees and I dont water at all.

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