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 does 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) without external funding.
I’m curious about an incremental improvement that leverages resources and knowledge already readily available in the community: 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 may shrink back when unloaded - especially in the midsection furthest away from the top and bottom that help maintain more consistent diameter at those locations. This dynamic action exacerbates holes/punctures in tanks and tends to turn them into growing cracks. Attempts to patch holes/cracks near and especially 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. The cob protects the tank from punctures and UV degradation as well as protecting the polyrope banding from UV degradation.
Thoughts? Further Issues? Suggestions?