In Ground Water Tank

[Ronaweigh]

I am in the process of renovating a townhouse in Pattaya a complete strip out and rebuild I want to put the water tank under the ground in the court yard. The builder has dug the hole when I first looked it had just finished raining after a storm so had water in the bottom of the hole, I went back yesterday and there is still water in the hole. I asked the builder where is the water coming from he said the townhouse is in a land depression and it is the run off from the rain.

Now alarm bells are starting to ring in my mind if I go ahead and put the tank in I can envisage it rising up next wet season about november next year. When I expressed my concerns to the builder yesterday he said no problem we just buy a differant tank look like balloon, then i asked how he was going to put a concrete slab in the bottom of the hole to support the tank with the water in the hole his answer was now can not now don't need concrete slab because use differant tank.

Can any one give advice on which way to best solve this problem also I want to buy a mitsubishi water pump what is the best size for a two storey town house with rain shower.

[trogers]

Any shape of plastic tank will float when near empty on the inside and submerged in soggy earth.

Proper way to install the tank is to first drive in two or four (depending on size of the tank) 8-9feet long concrete piles place near the four corners to support a 20cm thick concrete plinth, hack off concrete at the top 30cm of the piles and tie the exposed steel bars to the bottom layer of steel bars of the concrete plinth.

The plastic water tank should be strap down to this plinth.

Edited November 17, 2009 by trogers

[scorecard]

Any shape of plastic tank will float when near empty on the inside and submerged in soggy earth.

Proper way to install the tank is to first drive in two or four (depending on size of the tank) 8-9feet long concrete piles place near the four corners to support a 20cm thick concrete plinth, hack off concrete at the top 30cm of the piles and tie the exposed steel bars to the bottom layer of steel bars of the concrete plinth.

The plastic water tank should be strap down to this plinth.

Or forget the idea of a plastic tank and build the tank from cement.

My builder did exactly this because he also had concerns about a plastic tank floating up and in fact he charged me less for the cement tank, compared to the price of a good quality plastic tank.

[trogers]

Any shape of plastic tank will float when near empty on the inside and submerged in soggy earth.

Proper way to install the tank is to first drive in two or four (depending on size of the tank) 8-9feet long concrete piles place near the four corners to support a 20cm thick concrete plinth, hack off concrete at the top 30cm of the piles and tie the exposed steel bars to the bottom layer of steel bars of the concrete plinth.

The plastic water tank should be strap down to this plinth.

Or forget the idea of a plastic tank and build the tank from cement.

My builder did exactly this because he also had concerns about a plastic tank floating up and in fact he charged me less for the cement tank, compared to the price of a good quality plastic tank.

He is staying in a townhouse and I suspect the tank will be under the car porch. A cement tank is liable to crack and leak.

[KarenBravo]

He is staying in a townhouse and I suspect the tank will be under the car porch. A cement tank is liable to crack and leak.

I don't agree. I have an underwater water storage tank that is made from the 1.5 mtr diametre concrete rings. It is 5 mtrs deep and I have had no problems.

The plastic storage tanks are not designed for underground use as the outlet for cleaning out build-up of sludge, dirt etc. is at the bottom of the tank. You will never be able to clean it.

With concrete rings you can open the top and clean it out every few years.

The water that is always at the bottom of your whole is probably the water table.

Edited November 17, 2009 by KarenBravo

[Ronaweigh]

I just went back and had another look at the hole and my wife spoke to the workers at the site they told her when they dug the hole it was a lot deeper and it is all sand now the sand has half filled the hole the water starts about two metres down.The townhouse is in Pattaya I didn't realise that two kilometres from the beach would be all sand.

Karenbravo I like the idea about the cement rings I assume you mean the ones used for wells in the village.

I have a meeting with the builder tomorrow morning my thoughts are if he can't make a water tank from concrete then will have to have the ugly water tanks every one else have taking up to much room thought about putting it on the roof but have a pitched roof.

[Artisi]

Be warned, whatever tank you install underground, steel, plastic or concrete if subject to a high water table during any wet period will want to float if it is not correctly installed any time you pump the tank to a low level when the weight of the tank plus water is less than the up-thrust from bouyancy. Remember the up-thrust from bouyancy keeps even the largest ocean going ships afloat.

[trogers]

I just went back and had another look at the hole and my wife spoke to the workers at the site they told her when they dug the hole it was a lot deeper and it is all sand now the sand has half filled the hole the water starts about two metres down.The townhouse is in Pattaya I didn't realise that two kilometres from the beach would be all sand.

Karenbravo I like the idea about the cement rings I assume you mean the ones used for wells in the village.

I have a meeting with the builder tomorrow morning my thoughts are if he can't make a water tank from concrete then will have to have the ugly water tanks every one else have taking up to much room thought about putting it on the roof but have a pitched roof.

Be careful about using the concrete rings. You cannot dig too deep in sandy soil without erosion and collapse.And concrete rings, if not sealed water tight, will mean penetration of salt (sea) water into your fresh water supply.

Here is an example of a plastic underground water tank. Both inlet and outlet pipes are near the top, not at the bottom like those placed on ground. There is an access cover at the top for cleaning the tank.

http://www.dos.co.th/product/productdetail.php?pid=15

[KarenBravo]

Be careful about using the concrete rings. You cannot dig too deep in sandy soil without erosion and collapse.And concrete rings, if not sealed water tight, will mean penetration of salt (sea) water into your fresh water supply.

Good point, but, it doesn't affect me. The water table is approximately 7 metres down at its highest during the wet season (I'm on a small hill).

Edited November 17, 2009 by KarenBravo

[Ground Engineer]

A point to consider when you choose your anti flotation measures on your underground water tank.......concrete weighs twice as much above the groundwater table as below.

To calculate the upthrust from the groundwater on your tank, just calculate the volume of the tank and multiply it by the unit weight of water. For example if the volume of the tank is 3 cubic metres then the upthrust will be approximately 30kN. Therefore weight of concrete needs to be about 33kN (above the groundwater table) to resist flotation. 33kN is about 1.4 cubic metres of contrete ( weight of conc 24kN/m3). As discussed before, if the concrete weight is below ground water table use a submerged weight of 14 kN/m3).

Edited November 23, 2009 by Ground Engineer

[trogers]

A point to consider when you choose your anti flotation measures on your underground water tank.......concrete weighs twice as much above the groundwater table as below.

To calculate the upthrust from the groundwater on your tank, just calculate the volume of the tank and multiply it by the unit weight of water. For example if the volume of the tank is 3 cubic metres then the upthrust will be approximately 30kN. Therefore weight of concrete needs to be about 33kN (above the groundwater table) to resist flotation. 33kN is about 1.4 cubic metres of contrete ( weight of conc 24kN/m3). As discussed before, if the concrete weight is below ground water table use a submerged weight of 14 kN/m3).

Usually we do not use just mass concrete as a counterweight against flotation, but also use frictional piles. That's why we have to expose the top steel rebar and tied them to the rebar of the concrete.

The piles will also help support the laden tank and concrete when the ground is not water logged.

Edited November 23, 2009 by trogers

[Ronaweigh]

I havn't been able to post an update on the in ground tank as meetings and disscusions have been in thai time. As of this afternoon the tank will be much shallower and above the water table and made of concrete.

My inatial understanding was the tank was goining to be full concrete with steel reo but upon going around and looking this afternoon the hole was lined with cinder block bricks. I was a bit alarmed and tried to get an explanation from the workers with my wife translating this went no where so a quick call to the builder and he turned up in ten minutes. He explained that the bricks were to be used as the frame for the concrete pour which will have reo in the concrete and tied to the reo for the concrete slab. I also asked if he was using an additive to make the concrete more water proof he said yes he does it all the time in concrete water tanks. The bricks are also full of concrete.

The estimated tank size will be about 1300 litres, I will be able to get a better idea on the volume when the pour has been done and I can do the measurement of the whole tank.

Trogers they couldn't put piles in as you suggest because the courtyard for the townhouse is 4mtrs by 6mtrs no room for a driver unles you used one on a tracter which is beyond my ability to explain to my wife or builder. I wil try to get some photos before and after the concrete pour.

Also thanks everyone for the helpfull advice

[trogers]

I havn't been able to post an update on the in ground tank as meetings and disscusions have been in thai time. As of this afternoon the tank will be much shallower and above the water table and made of concrete.

My inatial understanding was the tank was goining to be full concrete with steel reo but upon going around and looking this afternoon the hole was lined with cinder block bricks. I was a bit alarmed and tried to get an explanation from the workers with my wife translating this went no where so a quick call to the builder and he turned up in ten minutes. He explained that the bricks were to be used as the frame for the concrete pour which will have reo in the concrete and tied to the reo for the concrete slab. I also asked if he was using an additive to make the concrete more water proof he said yes he does it all the time in concrete water tanks. The bricks are also full of concrete.

The estimated tank size will be about 1300 litres, I will be able to get a better idea on the volume when the pour has been done and I can do the measurement of the whole tank.

Trogers they couldn't put piles in as you suggest because the courtyard for the townhouse is 4mtrs by 6mtrs no room for a driver unles you used one on a tracter which is beyond my ability to explain to my wife or builder. I wil try to get some photos before and after the concrete pour.

Also thanks everyone for the helpfull advice

The ground water in your area is probably tidal as you have sandy soil and located near to the sea. During the excavation, you said that you see water at about 2m down. So I would imagine your contractor is building a concrete tank with the base above this water level, and with internal dimensions of 1m x 1.3m, and about 1m high. The weight of the concrete will hold the tank in place even if ground water were to rise by a metre. Make sure you have at least a 30x30cm access cover at the top for maintenance.

What you have to watch out is the placement of PVC pipes as the joint area between the stiff concrete and the 'soft' pipes is a point of frequent pipe breakage. I would place all PVC pipes vertically down through the concrete top, and then use inverted U-bends to connect them to the horizontals. We call this 'goose' head and will allow some movements to the pipes without breakage.

Make sure you have a ball valve connected for the supply pipe, and an overflow pipe which can also act as an air vent.

Edited November 24, 2009 by trogers

[Rimmer]

I once saw an empty swimming pool that had 'popped' out of the ground because it was not piled in and the ground was waterlogged, it broke it's back and had to be dug out.

Have a look at the nice sandstone color tanks outside Homeworks they don't look so bad for overground if you have the space. Tall and thin maybe?

Would suggest the Mitsubishi WP405 pump.

[trogers]

I once saw an empty swimming pool that had 'popped' out of the ground because it was not piled in and the ground was waterlogged, it broke it's back and had to be dug out.

Have a look at the nice sandstone color tanks outside Homeworks they don't look so bad for overground if you have the space. Tall and thin maybe?

Would suggest the Mitsubishi WP405 pump.

Not only DIY people like us make error of not using piles in swimming pools. A 5-star beach resort in Phuket also made this error and their pool is only a few tens of metres from the beach. In such sandy soil, bore piles is the only option as driven piles cannot be drive deep enough.

The hotel pool was rebuilt since the mid 90s.