Cob-Coated Plastic Water Storage Tank?

Hang with me here...
Working with a local community development group in coastal Kenya with low rainfall to store water.  There are many examples of failed plastic tanks due to punctures/slashes around the village (vandalism or survival in severely drought stricken area the last 15 years).  There are also many examples of failed ferro-cement tanks around the village (delaminated/failed walls).  Have been in discussions with another organization in western Kenya (Friendly Water for the World) that trains and makes very durable/maintainable 25,000 liter water tanks from curved interlocking soil stabilized blocks (compressed earth blocks) with interior BRC mesh and timber roofs.  However, these cost them about $4000 to make which is largely labor and materials to make the curved ISSB and procure other building materials ($2400), plus construction labor ($800) and construction management & inspection oversight ($800).  Though much of the labor funding could go into the local economy, this is not likely an approach that could be readily adopted on a homestead scale in the area and is not likely affordable even to local institutions either (schools, churches, health clinics, etc).

I'm curious about the combination of plastic water tanks (more affordable but not robust from a vandalism standpoint) + cob exterior protective coating (a readily available material and locally familiar construction method).  I have not been able to find any examples of this online to explore it's viability, but have some ideas on potential challenges and solutions based on prior experience with plastic water tanks.  Would like this community's input/feedback...

Plastic water tanks swell when loaded and shrink back a bit when unloaded - especially in the midsection.  This dynamic action exacerbates holes/punctures in tanks and tends to turn them into growing cracks.  Attempts to patch holes/cracks at or below the midsection tend to fail due to this dynamic action.  

Coating a tank in cob would provide an extra layer of rock hard armor around the tank making it much more difficult to puncture the tank.  In addition, it would help protect the tank from UV degradation and help keep the water cooler.  (Probably would need to use a form of stabilized cob for durability in rain that tends to come in extreme amounts infrequently these days.)  However, these cycles of swelling/shrinking would eventually destabilize and delaminate the cob exterior coating from the tank and break down.

Potential solution:  When setting up the empty tank, reinforce the midsection of the tank with banding of some sort to prevent the bulging of the midsection when loaded with water.  I've got some experience with this for aesthetic purposes back in Texas when we put up cedar fence pickets around our water tanks at the community garden and banded them in place at 25%, 50%, and 75% height levels with stainless steel banding and a banding tool.  They looked great and the tanks no longer swelled.   My thought for Kenya would be slightly different to use fewer and more readily available materials: perhaps only 8 boards (staves) at 45 degree sections around the tank; wetted polyrope with turniquet-style tensioning (shrinks even tighter after drying) tied off against 1 of the 8 "staves" as banding; potentially chicken wire attached to the staves/tension ropes as a substrate for the cob if straw is not readily available for the cob.

Thoughts? Further Issues? Suggestions?

I've thought of this issue a lot, although I admit, simply because of my thinking that the plastic tanks aren't very pretty additions to many of the locations I've seen them used in.

Do you have access to chicken wire? I have seen pictures of it tied to earth bag walls to help a cob coating stay attached. If you could put a layer of something compressible like straw, under the wire, would that be enough to prevent the cob from gradually cracking?

Alternatively, does bamboo grow in the area you're working with? Bamboo can be surprisingly strong - it's used for scaffolding in Asia. If you made a bamboo shield for the tank, and grew useful plants up it, that might also make it harder for people to reach through the thickness of the shield and plants to harm the tank?

Good luck at finding a solution.

I like the idea of using bamboo to make a sort of shack to build over the water tank to keep the sun from degrading the tank.

To me that would be less labor intensive than doing cob.

This is an interesting problem.

My first thought is that if you can build a cob structure strong enough to support the plastic tank... could you make it a bit stronger and simply line it with clay? And not need the plastic at all?

Then I wondered if there is any rock in the area? Where I am from (which admittedly has good rainfall), there have been cisterns which were just big holes, lined with rocks and coated with either clay or lime. If it was submerged in the ground, you would not need to worry about the rain eroding the edges... nor would you need to worry about how much force is pushing out on the walls. You would need to devise some sort of cover for it, and place it so that contaminated water would not run in... but it could deal with a lot of the issues you are describing.

For that matter, could you dig a hole the size and shape of the plastic container and just drop it in?

Matt McSpadden wrote:For that matter, could you dig a hole the size and shape of the plastic container and just drop it in?

This is the issue with unreinforced in-ground swimming pools. They're fine if they're always filled with water, but when the water level is low while waiting for the next rainy season, they will collapse in due to the pressure of the ground outside.

You need to buy special tanks rated for underground use and a) they tend to be much more expensive and b) you now need more electricity to pump the water out of the tank rather than using gravity.

Jay Angler wrote:This is the issue with unreinforced in-ground swimming pools. They're fine if they're always filled with water, but when the water level is low while waiting for the next rainy season, they will collapse in due to the pressure of the ground outside.

Good point! I had not thought about the collapsing possibility when it was empty.

Jay Angler wrote:You need to buy special tanks rated for underground use and a) they tend to be much more expensive and b) you now need more electricity to pump the water out of the tank rather than using gravity.

On flat ground, you would lose gravity feed, so there would need to be a way to get the water out again. I have seen some bicycle powered pumps, and the old fashioned bucket and rope. These options are not as easy as an electric pump, but would be less costly. A stone/clay lined cistern style tank would not be as convenient... but neither are all the failed options that don't seem to hold any water right now.

Matt McSpadden wrote: I have seen some bicycle powered pumps, and the old fashioned bucket and rope. These options are not as easy as an electric pump, but would be less costly.  

Yes!  I remember seeing a nifty bicycle powered pump - very simple, easy to maintain and the design was free to use.

The bucket and rope is a learned skill - my friend is proficient in it, but alas, I seemed to lack the essential "flick of the wrist at the right moment".  Something I haven't had long term need for, so it never made it high on my list.

There is also some simple tech called a "jiggler" - basically a ball in a shaped cup at the end of a semi-rigid tube. You pump the tube up and down that the water goes into the tube, but the ball then blocks it from leaving at the same rate. It wouldn't be anywhere as efficient as the bicycle pump, but I've used it to get water out of rain barrels.

Great thoughts/questions/suggestions! Thanks and keep them coming please!

What an interesting problem, vandalism and poor quality tanks.
I have done a bit of looking around and I see that some of the plastic tanks are rubbish compared with what I see in Australia.
I understand why, no regulations.
I can see why your idea of banding is worthwhile. I have seen it on high pressure timber pipes made before steel pipes were available.
The tightness / spacing of the bands changed for the water pressure involved.

I think your idea of banding, perhaps with fencing wire and 'gripples' that allow fencing wire to be joined and tensioned would be a good idea, low tech  high outcome..
Special tensioning tools are available also.
If that technology is not available, old fence tensioning tools may be available.
And using the special fencing knots it will work.

Then on the outside again, lay adobe bricks made from the soil and often used in Africa, which are air dried and then laid around the tank as a protective wall,
set on stone / rubble foundation ring.
Then the exterior is earth plastered using traditional techniques also well know in parts of Africa.
I would like to see more examples of the efforts.

Living where I do, I immediately thought of ratchet straps for the banding.
Your ropes seem more appropriate.
A bamboo or cane enclosure daubed in cob seems like it could be cheaper than cob on chicken wire, plus the canes themself would offer some protection.

I'm very surprised the ferrocement is delaminating.
I would expect it to be quite durable in any place without freeze thaw cycles.
My first inclination would be to fix those tanks, patching them with cob, and lining them with sheets of plastic.

As for the in ground option, a submersible hollow frame can keep a flexible liner in place when the water gets low.
Think of a pallet sized cube of milk crates , wrapped in plastic and buried to the top edge.
The water could come or go, but the crates will hold the liner against the surrounding soil.
With the right materials, this could be duplicated with basketry/wattle in the place of milk crates.
A  ferrocement inner coating would work the same way, but we have seen there are issues with using ferrocement in your context.


I think there is an underlying issue at hand.
Whatever is built will need maintenance.
Ongoing maintenance of our own homes and infrastructure is seen as an anomaly, but it is closer to the historic normal.
Expecting anything to last and retain function without ongoing care is unreasonable.
Nothing lasts forever, and I know when I do things on the cheap, I pay with extra ongoing labor, but again everything will eventually need maintenance, even when it is made of the most expensive materials.


Is there a local entity that can organize the ongoing labor that keeping the water tanks in good repair will need?
There seems to be active destruction of the tanks, so it doesn't seem likely the community will actively care for them.
Will the finished tanks belong to individuals?
If so, can they afford the maintenance costs in the form of labor/materials?

The most durable/cheapest design will be self sustaining , and that will take human intervention.
I can't get my family to put away clean laundry , so I have no suggestions on how to motivate people a world away, but it seems like necessity for long term success in this project.

Thanks for all the input and questions! Very very helpful!

It is a tough problem, people have been trying to solve it for a long time. Once someone gets close, the prices and availability of materials changes.

Mike Reynold (Earthship) used to build cisterns out of packed tires and pond liner.

One guy on here a long time ago built one out of a bunch of sheet metal bolted together into a ring with a liner.

There are YouTubers that built a cistern out of gabions-wire cages filled with rocks.

Your ideas for reinforcing the tanks sound good. I do think you need to leave space between the cob and and tank for expansion and drying.

It’s hard to come up with a solution when we don’t know what resources you have available. It’s easy to say just buy X or do XYZ when we’re a half world away.

I think Jay said exactly what I was thinking while reading the original post.

I might consider keeping some emergency water jugs in the shade but obvious enough for people who are just desperate, but would rather not vandalize. I think this would be the majority, if for the simple reason of not killing your goose for future use. As a Pacific Crest Trail thru-hiker that is forever thankful to trail angels who leave water and food throughout the mojave sections, providing drinking water in the desert is a great way to make friends.