some thoughts from Dozer
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Underground Concrete Water Tank

image DSC03014.JPGUnderground water tanks are quite popular here. I had the pleasure of watching one being constructed at my friend Martin’s house remodel project. His is quite large, about 6 * 4 meters with a depth of 4 meters. Here is a shot of a corner section during construction. You can see that concrete is used to fill in the cavities in the heavy cement blocks. This along with metal rebar strands adds much to the overall strength of the tank walls.

image DSC03014.JPGMartin told me he had been down this road a time or two before, and normally the inner surface of a concrete underground tank would crack, which would let in surrounding soil and spoil the water. That is why he went with the heavy cement blocks, as shown here. Each one weighs about 13 kg, as compared to about 5 kg for the small size block. Also, he had the workmen use a heavy stucco coat of about 2 cm. on the surface to seal it and add additional strength.

image dsc20301.jpgThe water is harvested from every section of the roof via gutters and galvanized pipes as shown here.

image dsc20302.jpg image dsc20303.jpgFinally water runs down into the tank via a 6 inch pvc pipe.

image dsc20304.jpgA small automatic pump is used to pump the water from the tank system into the house.


  1. ok if water not intended to be potable, have to be aware that polluted surface water may seep into the tank

  2. yes, care must be taken to ensure unit is sealed to prevent surface water entry — as was done in this case.

  3. i realise this is an old article but i’m looking for ideas about building a big (20-40,000 litre) underground water tank.

    cement block seems an easy way to go but i worry about its long term crack resistance. i wonder how this tank is holding up after 4 years? i usually build with reinforced concrete walls, same as a swimming pool but that’s an expensive process, especially if you don’t have a lot of steel form work around.

    what about the shape of the tank, it seems that round is a more self supporting shape than square.

    i was thinking about a round tank with the excavated dirt walls as the outside form. for inside form work, a fairly light weight plywood form that’s only 1.2m (4′) high. the idea is to pour fairly dry concrete to a depth of 1.2m, wait a day or 2, move the form up and pour another meter. 1m deep concrete walls are far easier to deal with (vibrate) than 2m tall so hopefully i’ll get better concrete and less expensive forms.

    any ideas out there? anything i’m overlooking?

    thx steve

  4. I prefer the reinforced concrete wall approach, that is how I currently do underground tanks, although not as large as the one you are planning. As far as I know this underground tank has held up OK though, it used the large 14 cm think cinder blocks. Be sure to use a waterproofer in the cement mix. Also there are specialty products made to skim coat the inside of the tank as an extra waterproofer.

  5. i think 14cm blocks would end up costing as much or more than poured concrete so i think i’m going with concrete. water is becoming a bigger problem all the time on ko phangan and the site i’m building on doesn’t have government water anyhow so i’m looking to make the biggest tanks possible for a reasonable price.

    i always use Sika Plastocrete ( into all cement mix and sometimes use SikaLatex ( in the render coat for better adhesion and waterproofing. i’ve also used Sika -101a for waterproofing before but not sure if its really necessary and it works out pretty expensive for a big area.

    i’ll post pics and notes once i get going.


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