OutdoorKing Repair Forum Forums General Discussion Forum Off Topics Underground tank

We are putting a 1000 litre tank in the ground at a mates place and I'm wondering if anybody has any idea what type of fill is best to put around the tank to hold it in the ground. The tank is shaped like a hand grenade so we have to be careful with what we put in there because the shape might help force the tank up and that is something I couldn't face again because this tank has caused us massive issued with it popping up and breaking the pipework because of the amount of rain we have had since the hole was originally started back in May. This week is the only time we have had where we can see we have a number of days in a row where it is not going to rain. We got the tank back in the ground yesterday and got it piped up. We have hammered 4 star pickets down into the subsoil a meter and 2 chains with turnbuckles over the tank to hold it in place while we put whatever fill around the tank. We have considered concrete but it might try forcing the tank up. Googling it they suggest using cement powder and crusher dust compact just 75 to 100 mm at a time. I am not sure what the mean by this because the mini mix is just going to dump this in the hole. Any thoughts welcome.

We have decided to go with a meter of concrete and hopefully it doesn't crush the sides of the tank

Hi Norm,


I have heard Americans saying you use pea gravel to fill in around the tank , cement powder ,I think they are referring to recycled
concrete powder (recycled broken down concrete)

Crusher dust is broken down stone and I think it's also washed.

Obviously the 75 to 100 mm at a time is when back filling the tank you back fill in say 75 mm then compact it down then
fill in another 75 mm and compact it down and so forth.

I know when putting a tank under ground in clay it's one of the worst things for the tank floating.

There are lots of rules for putting tanks under ground ,I don't remember them without looking them up but some are like
the diameter of the tank, say 5 foot has to have 5 foot of back fill on top of the tank and a car can't drive past the tank
unless it's 5 foot away.

The recommendation for crusher dust etc may be to provide excellent drainage due to its light weight and porous structure. It can be used as a general filling material for low areas within landscaping projects: These low spots regularly collect water which leads to problems including root rot, wet soil and insects around the home. ( stops the water building up in one spot by letting the water drain through the crusher dust)

There are other rules like a concrete base or concrete blocks are a certain thickness to strap the tank to
so it doesn't lift , something along the lines of the size of the tank will displace a certain amount of water that you can
calculate what weight is needed to hold the tank down.

I guess you could use a Buoyancy Calculator ,( B = ρ × V × g ) to work out the weight to hold the tank down.

I've used plastic Drainage Channels in the drive that were in clay and I had to cut bricks in half length-ways
and put the cut brick in the bottom of the drainage channel to stop them floating.


1000 litre tank ,I think I would have put 10 star pickets in with 5 chains.

I'm assuming this tank is the correct type of tank to put under ground ,as I've heard some poly tanks will crack.

https://teampoly.com.au/2018/06/15/how-to-install-underground-poly-water-tanks/

https://blog.enduraplas.com/water-s...nstall-an-underground-water-storage-tank

https://www.localsearch.com.au/guides/landscape-supplies/what-is-crusher-dust

Cheers
Max.

Hi Max,
Thanks for that good info there. At this stage we will put a meter of concrete around the bottom and probably half way up the tank, then once it sets probably put another meter around it. It is a tank for inground installation so that is not a problem and there is no hope of getting any drainage from under the tank as it is in clay all the way and it is over 6 feet deep. With a 1000 litres of water in it it just bobs around like cork. Hopefully with 2 meters of concrete holding it down it should be ok. The chains we have put on it are mainly to hold it in position while we pour the concrete in, fingers crossed
Another thing I have thought about because of the amount of surface water in this yard, which is about 1 acre and dead flat which for months he is unable to mow with his zero turn rideon because it gets bogged easily, is to put in a pit close to the tank so we can pump any water collecting there into the stormwater. That idea is not going down well with my mate but we pumped thousands of litres out of this are over winter this year. Before winter comes again we want to put in a couple of hundred meters of trenches and fill them with scoria. He is getting a lot of water coming in from both properties either side of him. Always a problem if you buy on the floor of a valley, water is always a problem during winter.

Couple of pics for you Max so you can see the problems we have been having. Latest thought is we have 27 bricks there that weigh probably 20kg each so we will lower them inside the tank to help hold it down when we pour the concrete around the tank. Tank is also full of water probably 700kg as well

Looks like a fair amount of work Norm to fix this tank from floating..

"we want to put in a couple of hundred meters of trenches and fill them with scoria."

I have scoria under my drive here with a slotted drainage pipe in the scoria then it connects to the storm water drain
but I also have 3 Drain boxes and a drainage channel in front of the roller door.

The drainage boxes and the drainage channel work a lot better than the scoria with the slotted pipe.

The only place the water would build up here is where the scoria is ,as the water doesn't get away quick enough
so in that spot I have an open concrete drain channel now that takes the water to a drain box.

If you have a lot of water run off then you could put some drain channels around the outside of the water tank and
if there is a lot of water run off from next door you may need to divert the water into a drain ,you can use a drainage
channel or those concrete half channels that take the water to a drainage pit.

I wouldn't have thought you would need to pump the water, most times you just dig the trench so the water flows down hill but if
that's not possible I guess you need to pump the water.

The other way to have a tank out of the way is if your house isn't on a concrete slab, a rubber bladder tank can be used under the house .

Once you get the concrete poured over and around the tank hopefully that will be enough to weigh the tank down Norm.

Cheers
Max.

This is not to get the tank out of sight Max, there is 2 20,000 litre tanks behind it, this tank is so we can pump water up into those tanks. The bloke who built the place thought he was being smart but he completely botched the way the stormwater was done. Downpipes off the house remained full all the time as they went underground across to the tanks and rose up and into the tanks. All fine until you get a cracked pipe underground and it constantly leaks into the ground. They also had 2 x 90mm into the tanks with 1 x 90mm going to the drain, this was never going to work when the tanks were full or the rain was heavy.. With this tank in the ground it will be set up with a float switch to pump water into the tanks when needed or turned off to allow it to go down the drain. Hopefully we are starting to make some progress

Concrete poured this morning and hopefully this is the end of tank in the ground dramas that have gone on since May last year. Now just to get the bricks out of the tank which could be interesting

Hi Norm

Hopefully that should be enough weight to hold the tank down.

I know a 1000 litres of water is 1 ton ,so if this tank had no air trapped in the top and was full it couldn't float.

1 ton of cement I think is 415.54 liters ,so that's a bit more concrete than two / 44 gallon drums to hold down
an empty tank.


Besides the weight problem on top of the tank previously not holding the tank down ,I don't think this tank can be filling
at 100 percent as it doesn't look like there is a bleeder valve (vent valve) at the top and the filler hose is not
at the top so I think air will be trapped at the top of the tank which is why this tank was floating when full.

As we know a full bottle of water sinks in water but air trapped in a bottle will have some buoyancy .

Cheers
Max.

Hi Max,
I cubic meter of concrete weighs 2.5 tonnes and that is wedged into the sides so it can never lift it. The tank has the 2x 90mm inlets and 2x 90mm outlets and one of those outlets is an overflow/vent into the tank. The tank can only fill to probably 80% where the outlet pipes come out of the tank. The issue may come when the bore pump in the tank kicks in and pumps the water half way down in the inground tank. On top of the concrete we are going to place the bricks and that is going to be another 500kg on top and then we will backfill over the top. Somebody will be cursing us one day.