Earthbag rainwater tank

We are finishing our cob house in Colombia webpage, and a very important part of it is rainwater harvesting. So we decided to try to build a eartbag tank.

First we build the concrete slab. Then we build 10 layers of bags. It can hold 5000 litres. Next steps: cement plaster inside, cob plaster outside. Roofing... we don't know what kind of roof it will be. An almost flat green roof would be nice.

Greg Amos wrote:We are finishing our cob house in Colombia webpage, and a very important part of it is rainwater harvesting. So we decided to try to build a eartbag tank.

First we build the concrete slab. Then we build 10 layers of bags. It can hold 5000 litres. Next steps: cement plaster inside, cob plaster outside. Roofing... we don't know what kind of roof it will be. An almost flat green roof would be nice.

Good to see your work, but I am a little worried for the integrity of your earthbag water tank.

I've never seen an earth bag water tank before, please let me know if it can handle the lateral pressure of rho*gh where rho=density of water , g=9.8m/s^2, and h= height of your tank in meters.

Here's how much force your tank walls are going to experience at the bottom of the tank (I assume the height of your tank is  1.2meters tall):

(1000kg/m^3)*(9.8m/s^2)*1.2m=  11,760kg/m*s^2  or 11.76kPa which is about 1.7 pounds per square inch

In general, both abode and earthbags have great compression strengths but lousy tensile strengths. Your earthbag water tank walls will not experience any compression forces from the contained water; at every point in your earthbag wall, the pressure forces will all be outward tension pressure.  

The shape of a circle is of no advantage because the circle is not being compressed, it is being exploded outward.

If you put two strands of barbed wire between every layer in the earthbag wall, then I suspect you won't have any trouble. If you didn't put barbed wire in between the layers, then you might think that the downward weight of the earthbag will counter act the outward force of 1.7lbs (11.76kPa): but I will ask you to show me where a horizontal force is in your force diagram pushing against the force of the incompressible fluid (pretty sure the coefficient of friction from the normal force of the earthbags is still less than 1.0).

One way to keep your earthbag water tank wall from failing after you have sealed both the outside and the inside, you can pile dirt around the tank to hold counter balance the force of the water pushing outward.

If your tank was 2 meters tall, I'd really be worried.

Don't trust me, ask a mechanical engineer, a civil engineer or a physicist......but please, please ask before you fill your tank full and let it set for a week.

BTW, I am really glad you're using rainwater. Much better than a well. Did you know in Australia they use cisterns instead of wells? Their cisterns are from about 25,000 to 35,000 gallons which fill during the monsoons. I found by living offgrid, that the absolute survival minimum of water needed per person per year is 1000gallons which is 3785 liters......so if your roof collects the rainwater and you get rain year around you should be okay for a few people.  You can check how much water you can expect to catch this way: for every 3x3meter section of roof you have, 16 milliliters of rain will give you 235liters (for the US for every 10'x10' square of roof you have, 1 inch of rain will produce 62 gallons of rainwater).

Good luck and please let me know if you did use wire and how the tank worked out for you!!!

Hi Orin, thanks for your reply and your concern.

We are no civil engineers either, but we tried to take all precautions to ensure the integrity of the tank.

In the first photo, you might be able to see rods poking out of the slab. The first layer of bags was put on those.

Then between each layer we put barbed wire, 2 lines per layer.

Finally, in order to apply concrete, we need to put chicken wire, which should further increase the tensile strength.

I like your suggestion to pile dirt around the base of the tank, easy and efficient.

By the way, the tank is 1.4 m high (outlet), 2.1 m internal diameter.

We will keep posting photos. Have a great day!

Sweet!!! glad to hear you used wire for the needed tensile strength.......had me seriously worried there