We decided some time ago that it would be a good idea to build water tanks out of earthbags. Plastic or steel are expensive, concrete is just too much work. Plus, it gives us a chance to practice. The plan is to build water tanks over weekends, get water delivered to the site, then we can use that water during the main build (it won't rain enough between then and now).
For houses not connected to mains water the minimum on-site storage capacity that the council will approve in connection with a dwelling is 110,000L (90,000L for domestic use, 20,000L dedicated fire-fighting supply). As in, they will not approve a dwelling that includes a smaller water storage capacity. We asked, but apparently the lower demand for water resulting from use of waterless toilets doesn't count towards the mandatory capacity, so we still need to put in massive tanks. Realistically we know this is only a good thing, and we'd want to do it anyway, it's just a bummer to have to do it now.
First we had to work out how big they should be. It makes logical sense to want smaller tanks instead of one big one. Materials and so on come out a bit more expensive, but it means you can do maintenance on one while still having water storage in the other. I triple-checked with the Council and apparently on land zoned RU1 (which we are) above-ground tanks of any size count as exempt development, meaning you do not need to apply for council approval for them (in most other zones you need approval if it's bigger than 10kL). This stumped me a bit initially, as it didn't make sense that we needed extensive engineering consultations for the house, but the water tank (bigger, heavier, severe consequences if something goes wrong and multiple kL of water go rolling down the hill) is good to go regardless.
So we went through a process of working out whether to worry about that. We are aware of quite a number of 15-20kL tanks made from earthbags in various parts of the world. This proves the concept works, but what about a bigger tank? Will an increase in the diameter of the tank increase the outward pressure of the water on the walls? At first, logically, it seems like it should. More water = more pressure, surely? In a water tank we're primarily concerned with hydrostatic pressure (because the water isn't moving, so the only force we need to worry about is gravity). Pressure is related to the depth of the water, not simply the volume. The parts of the tank wall closer to the bottom will receive more pressure than the parts of the wall closer to the top, but the wall at the same depth doesn't receive different pressure if the volume of water is greater. So if we have a 2m tall tank that is 3m diameter, and a 2m tall tank that is 6m diameter, the pressure on the tank walls is the same even though the volume of water is greater.
The next part to think about is the hoop stress on the walls. Hoop stress is the pressure perpendicular to the axis of a cylinder (so the walls of our tanks), and is related to both the radius and the thickness of the walls. Physics textbooks usually use the example of the pressure on the hoops of a wine barrel (I'm pretty sure that's where hoop stress gets its name). This is where it got hard for us, no one we found had done studies on the maximum hoop stress earthbags walls can withstand. After some very extensive internetting (including some excellent spreadsheets for materials calculation covering all potential water tank materials except earthbags) we realised that this was simply unknowable for us. Someone, someday, might conduct the stress tolerance testing to figure this out, but it won't be us. Given that, it follows logically that the safest approach is to ensure we use the widest possible bags (as hoop stress is related to both radius and thickness of the walls - to be safe within the unknown, maximise wall thickness).
Finally we wanted to thought experiment with what would happen if we were wrong and it all fell apart. There is potential for cracks to form in the tank wall over time, which would be a problem if we were sealing it with concrete and painted sealant. So we're going down the path of a tank liner instead. We think it is a reasonable assumption, though, that if the tank walls did start to fail this would be a very slow process over time and we would be able to monitor it as the tanks age. It seems likely that points where pipes fit into the walls of the tank are the weak points, rather than the structure of the wall itself, and any failure is likely to come from the fittings first. If we do notice the wall beginning to bulge outwards with time we can look into ratchets and strapping and see how we go, but again this is likely to be a process taking years rather than giving way overnight. As a bit of additional structural support, during construction we will use a trick I've seen in a few accounts of earthbag building, particularly in earthquake zones, where metre-long lengths of rebar are hammered down through the wall at regular intervals.
So, we have a bit of leeway with tank size from a structural perspective. Therefore, it comes down to convenience and cost. Given we need 90kL drinking water all up, we end up with the option of 2x 45kL tank or 3x30kL tanks. We also need at least a 20kL tank for fire-fighting water.
Kyle, who by virtue of having a more happy-go-lucky approach to life than ever-practical me, is in charge of ensuring we don't go too fast with this whole project and forget to have fun. In an effort to make his contribution to the water tanks he's suggested we should put crenelations on top and paint the sides to look like stonework. He reasons that since we have to have a bunch of big, round, tower-like structures anyway we might as well make them look cool. We met at the local historical re-enactment society, so this isn't an entirely left-of-field suggestion.
With this in mind, plus practical considerations such as the time it takes to build each tank, we think we'll go with a total of 4x 30kL tanks. My rough maths (aided greatly by Google's cylinder calculator - just google 'volume of a cylinder') tells me that a tank that is 2m tall will need an internal diameter of 4.4m to hold 30kL. Note that the height here is not the total height of the tank but the height to the overflow, as the roof (and any crenelations, if Kyle convinces me this is a good idea) will be a little higher by necessity.
Overall, dealing with the water tanks first is a good move. It will allow us to try out the whole earthbag concept in terms of materials calculations and get the hang of how it will work on site with groups of people doing stuff before we get to the complicated part (cylinders are pretty straight forward). That way, when we get to the house, we're familiar with the process. At least, that's the theory - we'll see over the next few months how that turns out in practice.
My budget hesitantly puts material costs for the build of each tank at around $1500 (including earthbags, tank liner, and fittings), but I'll go into further detail on that once it's a more precise estimate. I suspect our strategy will be to build one or two, then build the house, then do the rest at the ever indefinite 'later'.
For houses not connected to mains water the minimum on-site storage capacity that the council will approve in connection with a dwelling is 110,000L (90,000L for domestic use, 20,000L dedicated fire-fighting supply). As in, they will not approve a dwelling that includes a smaller water storage capacity. We asked, but apparently the lower demand for water resulting from use of waterless toilets doesn't count towards the mandatory capacity, so we still need to put in massive tanks. Realistically we know this is only a good thing, and we'd want to do it anyway, it's just a bummer to have to do it now.
First we had to work out how big they should be. It makes logical sense to want smaller tanks instead of one big one. Materials and so on come out a bit more expensive, but it means you can do maintenance on one while still having water storage in the other. I triple-checked with the Council and apparently on land zoned RU1 (which we are) above-ground tanks of any size count as exempt development, meaning you do not need to apply for council approval for them (in most other zones you need approval if it's bigger than 10kL). This stumped me a bit initially, as it didn't make sense that we needed extensive engineering consultations for the house, but the water tank (bigger, heavier, severe consequences if something goes wrong and multiple kL of water go rolling down the hill) is good to go regardless.
So we went through a process of working out whether to worry about that. We are aware of quite a number of 15-20kL tanks made from earthbags in various parts of the world. This proves the concept works, but what about a bigger tank? Will an increase in the diameter of the tank increase the outward pressure of the water on the walls? At first, logically, it seems like it should. More water = more pressure, surely? In a water tank we're primarily concerned with hydrostatic pressure (because the water isn't moving, so the only force we need to worry about is gravity). Pressure is related to the depth of the water, not simply the volume. The parts of the tank wall closer to the bottom will receive more pressure than the parts of the wall closer to the top, but the wall at the same depth doesn't receive different pressure if the volume of water is greater. So if we have a 2m tall tank that is 3m diameter, and a 2m tall tank that is 6m diameter, the pressure on the tank walls is the same even though the volume of water is greater.
Greater water depth = greater pressure, regardless of volume
(this is why people put water tanks on stilts, to get higher water pressure)
The next part to think about is the hoop stress on the walls. Hoop stress is the pressure perpendicular to the axis of a cylinder (so the walls of our tanks), and is related to both the radius and the thickness of the walls. Physics textbooks usually use the example of the pressure on the hoops of a wine barrel (I'm pretty sure that's where hoop stress gets its name). This is where it got hard for us, no one we found had done studies on the maximum hoop stress earthbags walls can withstand. After some very extensive internetting (including some excellent spreadsheets for materials calculation covering all potential water tank materials except earthbags) we realised that this was simply unknowable for us. Someone, someday, might conduct the stress tolerance testing to figure this out, but it won't be us. Given that, it follows logically that the safest approach is to ensure we use the widest possible bags (as hoop stress is related to both radius and thickness of the walls - to be safe within the unknown, maximise wall thickness).
Finally we wanted to thought experiment with what would happen if we were wrong and it all fell apart. There is potential for cracks to form in the tank wall over time, which would be a problem if we were sealing it with concrete and painted sealant. So we're going down the path of a tank liner instead. We think it is a reasonable assumption, though, that if the tank walls did start to fail this would be a very slow process over time and we would be able to monitor it as the tanks age. It seems likely that points where pipes fit into the walls of the tank are the weak points, rather than the structure of the wall itself, and any failure is likely to come from the fittings first. If we do notice the wall beginning to bulge outwards with time we can look into ratchets and strapping and see how we go, but again this is likely to be a process taking years rather than giving way overnight. As a bit of additional structural support, during construction we will use a trick I've seen in a few accounts of earthbag building, particularly in earthquake zones, where metre-long lengths of rebar are hammered down through the wall at regular intervals.
So, we have a bit of leeway with tank size from a structural perspective. Therefore, it comes down to convenience and cost. Given we need 90kL drinking water all up, we end up with the option of 2x 45kL tank or 3x30kL tanks. We also need at least a 20kL tank for fire-fighting water.
Kyle, who by virtue of having a more happy-go-lucky approach to life than ever-practical me, is in charge of ensuring we don't go too fast with this whole project and forget to have fun. In an effort to make his contribution to the water tanks he's suggested we should put crenelations on top and paint the sides to look like stonework. He reasons that since we have to have a bunch of big, round, tower-like structures anyway we might as well make them look cool. We met at the local historical re-enactment society, so this isn't an entirely left-of-field suggestion.
With this in mind, plus practical considerations such as the time it takes to build each tank, we think we'll go with a total of 4x 30kL tanks. My rough maths (aided greatly by Google's cylinder calculator - just google 'volume of a cylinder') tells me that a tank that is 2m tall will need an internal diameter of 4.4m to hold 30kL. Note that the height here is not the total height of the tank but the height to the overflow, as the roof (and any crenelations, if Kyle convinces me this is a good idea) will be a little higher by necessity.
Overall, dealing with the water tanks first is a good move. It will allow us to try out the whole earthbag concept in terms of materials calculations and get the hang of how it will work on site with groups of people doing stuff before we get to the complicated part (cylinders are pretty straight forward). That way, when we get to the house, we're familiar with the process. At least, that's the theory - we'll see over the next few months how that turns out in practice.
My budget hesitantly puts material costs for the build of each tank at around $1500 (including earthbags, tank liner, and fittings), but I'll go into further detail on that once it's a more precise estimate. I suspect our strategy will be to build one or two, then build the house, then do the rest at the ever indefinite 'later'.
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