Design of Undergroud Water Tank?

On a forum such as CR4, you should be searching for general information, not specific designs.

Having said that, it would be best to minimize the depth and use a width of 3.5m (the space available). Using two tanks side by side, you would have 3.5 x 3.5 m for each tank or approximately 3.5 x 7.0m overall. Assuming a wall of 200mm, the effective inside dimension of each tank would be 3.1m. The required capacity of 20m³ would dictate a depth of 2.1m.

Design of the roof slab, floor slab and walls would be relatively straightforward.

Because the tanks are abutting, it would be necessary to provide a sealant on the wall between the two tanks. By the same token, a sealant is required around the outside of the drinking water tank to prevent contamination from groundwater outside.

Suggest the potable water tank be lined on the inside to prevent concrete leaching. A "down and dirty" solution might be to just "bag" the inside and use the concrete to support the sanitary bag. Flavour of the drinking water is a consideration here also.

Ralph

The minimum quantity of material is best achieved by making them spherical!

However, spherical is not always practical, and local site conditions will determine the correct design. Not knowing the value of concrete and steel bar locally, the forum is not in the best position to help.

I would suggest you retain a licensed Civil Engineer for the design. One with some knowledge of the soil and groundwater conditions in your areas, and any potential loading considerations. They actually get paid to do such things as you are requesting.

I will like to draw your attension to some other aspect of the underground tank: Do not leave any open space between earth around and the tank.

Here let me tell you a incidence happened in case of a huge undergound tank of Industrial water tank, near Mumbai, India. Huge underground concrete tank (Master Balancing Reservoir) was constructed. There was space between earth around and the tank walls. Construction was complete, but, it was not yet commissioned, thus it was empty. Rainy season started, and water got filled up around the tank. there was no drain path available. Thus, the whole tank of the size of foot ball gound started floating in the water. It raised by almost 1 meter in the water pulling up the foundation piles.

Engineers hurriedly drained the water and the tank was grounded again.

So never keep empty space around without drain or never keep the tank empty

gsuhas:

Design laws does not allow a concrete dam the size of a football ground to float, save 1 meter above water, except in Mumbai perhaps. CR4 cannot afford these type of stupid jokes...we need constructive ideas please!

We may never know where you reside, but here in South Florida we all have become aware that one should never leave an in ground swimming pool empty when there is a chance of rain. Same problem with underground fuel tanks. When new fuel tanks are installed there is a minimum required amount of gravel that must be surrounding the tank to provide drainage and prevent tanks from floating.

inexpensive plastic tanks that can be burried are available.

You might look at precast concrete septic tanks (new ones of course :-) ). They might be cheaper and easier than site-built ones. The larger ones come in sizes close to what you want, or you can gang two or more together to get the total capacity you need. They have the benefit that the design and construction have been taken care of.

how about using a bolted storage tank instead of a concrete ?

Do a Google search for Cistern designs. Down here thats what everybody uses.

Is it possible to combine the two tanks into a single larger tank, that has individual pick up pipes located ad different levels, so as to never allow the drinking water usage to empty the water needed for fire protection. This is common practice on recreation vehicles with a generator that uses the same fuel as the main engine.

Large diameter pipes (of course designed for buried services) are sometimes used for tanks e.g. see http://www.kubota.co.jp/english/c-data/pdf_en/16-21.pdf .