Building an earthbag cistern

Hello folks,

I have a question regarding building an underground cistern for rain water catchment.

I have the amazing opportunity to build my own house to my own specifications. Rainwater collection will be of utmost importance for obvious reasons. As well as above ground water tanks, I also want to build an underground water tank to store rainwater primarily for drinking/cooking. As such, the tank will be placed as close to below the kitchen as possible.

I plan to use earthbags for the structure of the tank.

My main question is this: Will I have to use a cement plaster for the inside of the tank that will constantly be submerged? Is there a natural and less wasteful plaster I can use? Lime?

How would I be able to reinforce the top of the tank so that it would be suitable for life on top of it?

I do not have to worry about freezing in my climate so no worries there.

Any thoughts/suggestion/ideas welcome.

Hey Mike, a lot depends on the soil conditions/water table of your site. If your site has a high water table, this could be good since the soil will already be saturated. If your water table is low, the soil could be relatively dry and the cistern will leak. Sometimes that leak could stabilize as the clay in the soil gets wet and expands, other times there are fissures in the soil that will only get bigger. A perc test could help give you an idea. The earthbags will leak if there is a place for the water to go on the backside, even if plastered. The taller your tank, the more pressure and possible leaks you will have. Water tanks and liners are pretty dependable and mostly affordable these days. Figure out how much water you'll need/want and design a system accordingly. Remember, if you don't have water, you don't have anything! Not an area to get cheap in. As far as covering the top of your cistern, what kind of floor were you planning for the house?
For my home in Colorado, here is what I'm planning:

Thanks for the input Daniel! As I am in a water scarce country I am going to be storing as much water as possible. The water storage system I am designing is going to be endlessly expandable and will include ponds and maybe even a natural pool if we have a rain event large enough to fill it. And of course I will be using the soil to store the most water.

The water table in this area is pretty low as I am on the top of a ridge right before it plunges to a ravine below. Soil is mostly sandy with a small amount of clay. I am trying to steer clear of the easily available plastic tanks because I'm not so jazzed about the idea of all of our water sitting in plastic tanks all day for many reasons.

The floor above the tank: I'm not too sure yet. Most likely an earthen floor maybe with some tile or something over the top for extra protection in the kitchen which will no doubt get plenty of use. I'm still in the planning phase so I'm trying to gather all the info and ideas to help solidify my ideas.

What do you people with freezing concerns do about rainwater storage? Pretty grateful that that is not a current concern for me. I don't do well in the cold

Where I am ponds do freeze, but generally not enough to walk on. The freezing can actually help keep the water from evaporating due to wind. People who use tanks often bury them, otherwise they have to put a heater inside. I've been considering using the pond ice to fill an icehouse. Rainwater catchment is mostly illegal on a large scale here too, which is insane. We get about 19.5" of precipitation annually on average +/-5", so pretty dry. I'm a little worried about your sandy soil being able to hold water without a liner. Anyone nearby do what you're trying to do, or can you build a test model? If you're really against plastic, you might have to tile the inside. Are you able to use excavators or are you having to dig by hand?

Illegal to collect rainwater?? Wow. That seems like a massive infringement on a basic and essential human right. And one that is becoming more and more scarce! Here it is becoming law that you have to collect rainwater. The government encourages it to a large degree. That being said, we did almost run out of municipal water a few years ago. The incompetence of the government was not solely to blame, but it sure didn’t help. Looks like our governments are both incompetent, just in different ways. But that’s just my opinion and a digression.

I’m not sure if others in the area are doing the same thing. I will ask around. I will get an excavator to dig it out when I bring one in for the foundation because the ground is rock hard due to years of misuse by the previous steward. I had not considered the effect that the surrounding soil would have on the tank so thanks for that consideration.

Would a cement plaster on the inside not be adequate to seal the tank?

cement is not waterproof.  So, water will leach out thru cement/concrete/plaster.   Cement can be coated with a paint on water proofing, or a pondliner can be used.

Thanks for the info Sue. I definitely thought cement was water tight so thanks for that one. I will continue to explore my options.