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    • CommentAuthorcrt
    • CommentTimeMay 19th 2009
     
    Anyone ever built an underground ferrocement water tank? We are planning an 11,000 litre tank to take our roofwater and then use it in the garden (and also solve our drainage problem).

    Although cement usage might be thought of as ungreen, as I understand it, you use a lot less cement than equivalent concrete and will have a longer lasting tank than the equivalent in plastic.

    Someone I've been speaking to recently claims some form of membrane is required to prevent water ingress from the surrounding ground. I've not seen this suggested in any of the literature.

    Also, constructing the roof looks challenging: any views on this aspect?

    CRT
    • CommentAuthorjon
    • CommentTimeMay 19th 2009
     
    It's not ungreen if what you're constructing has an extended lifespan that allows the effective annual CO2 emission to be reduced.

    Membranes are required if it's drinking water or if you have contaminated ground (and the water has a use that any contamination would affect)

    I've often seen these used under buildings (for instance as at Hockerton if memory serves me right): In this instance, the ability of the roof to resist loading isn't as severe as if it were subject to occasional car or other loads.

    What exactly do you mean by ferrocement? (eg: http://en.wikipedia.org/wiki/Ferrocement). Pins are sometimes used in newer applications:
    • CommentAuthorcrt
    • CommentTimeMay 19th 2009
     
    Thanks. That's my view of "green", too.

    My idea of ferrocement is a couple of layers of mesh (probably chicken wire, but I am aware of the criticisms highlighted in the wikipedia article) and then a couple of inches of sand and cement mortar plastered on and left to dry in a controlled manner. Strong, flexible and durable.

    What sort of "pins" were you referring to?
    • CommentAuthorjon
    • CommentTimeMay 19th 2009
     
    Hi crt

    There are stainless steel pins (also non-metallic versions) that you can add to the concrete mix to increase the tensile capacity rather than using traditional wire or reinforcement.

    However, pre-cast units (generally 1.2 metres up to about 3 metres) are generally used for this sort of thing as the quality control is higher leading to thinner wall sections. For these, you can buy off-the shelf covers and access together with foot-rungs and so on.

    If you're seriously looking at doing this, it may be worth a visit to Hockerton and chat with Simon Tilley: They charge a bit for the day's visit but (for the hassle it may save you if you're not familiar with this sort of thing) it may be worth it.
    • CommentAuthorcrt
    • CommentTimeMay 19th 2009
     
    I think we may be talking at slightly cross-purposes. We're looking at ferrocement rather than ferroconcrete. For a given area or volume you have to use much less cement mortar than the equivalent concrete. I think this is because of the way the mesh and the mortar combine to form a strong, flexible wall - a couple of inches altogether for ferrocement, compared to probably twice that if using concrete.
    • CommentAuthorjon
    • CommentTimeMay 19th 2009
     
    Hi crt

    They've variants on the same thing except that the aggregate mix is different: It's somewhat unlikely that you would use less cement per unit volume but depends on the exact application. The reason concrete has to be thicker is to get an increased lifespan (so cover requirements go up). However, you don't have to use reinforcement for this type of application if your design uses the passive restraint of the soil (eg circular tanks or other forms of arched mechanism with no flexure). Some applications can also use active restraint.

    Hope this helps!
    • CommentAuthorcontadino
    • CommentTimeMay 20th 2009
     
    I have a 100,000 and a 50,000 litre underground tanks both made from ferrocement. Neither are lined but neither are potable water tanks. The big tank was in place when I bought the house, the little one I've built since. The advice I got when I built it was that ferrocement is advisable only where the tank is being built into rock. If you're building into less solid ground, a precast or blockwork tank is needed.
    • CommentAuthorsquowse
    • CommentTimeMay 20th 2009
     
    yes the pressure that the ground applies will be an important factor in determining the wall thickness. if there is groundwater this can be especially high. in this case uplift on the base slab will be very high, much worse than the load on the lid. you have to design for the worst case scenario - this may be when the tank is empty.
    the strength of ferrocement and lightly reinforced or fibre reinforced (the "pins" mentioned earlier) concrete will be approximately the same for a given thickness. the advantage of ferrocement is that you don't need to set up formwork and you can keep the same thickness even on an undulating surface.
    • CommentAuthorcrt
    • CommentTimeMay 20th 2009
     
    This is really helpful stuff, thanks.

    We will be digging into ground that is a mix of clay and stone, not solid rock. There is a lot of ground water about.

    If, as suggested above, ferrocement is not suitable (although my reading elsewhere seems to suggest we ought to be ok - or at least others have been ok in developing countries - even though not building into rock) for these conditions, what about if we used conventional reinforced concrete? I've read that concrete tanks are prone to cracking and leaks. If we went with reinforced concrete, would be then need to put any form of waterproof tanking material on the inside?

    Re blockwork, is the idea here to use interlocking blocks, or standard blocks? In either case, would a waterproof tanking material on the inside be recommended?

    How did the Victorians get away with building brick cisterns all over the place, many of which still seem to be holding water 100 years on? Did they line with render or just leave as brick?
    • CommentAuthorstephendv
    • CommentTimeMay 20th 2009
     
    Have you considered Fibreglass reinforced polyester tanks? Might be more economical than building. The last round of quotes I got here in Spain was around 3000 euro for a 20 000 litre tank including 200 km transport.
    • CommentAuthordjh
    • CommentTimeMay 20th 2009
     
    <blockquote><cite>Posted By: jon</cite>I've often seen these used under buildings (for instance as at Hockerton if memory serves me right)</blockquote>
    You must be thinking of somewhere else. The drinking water at Hockerton in stored in an open pond (not the big lake, a pond across the field behind the houses). There's nothing under the houses except insulation. I'd be interested to know where you have seen it :)

    Cheers, Dave
    • CommentAuthorjon
    • CommentTimeMay 20th 2009
     
    It must have been one of the others Dave. I was convinced that Simon told me there were two additional holding tanks near or under the new office area down by the second filtration pond. I'm sure it wasn't Earthship so I'll have to search the memory banks.

    Cheers

    Jon
    • CommentAuthorjon
    • CommentTimeMay 20th 2009
     
    I'm sure it's Hockerton Dave: The pond across the fields to the back of the houses (which has the 'barrel' filter leading off it) I think is their old system. I'm certain they 'upgraded' a few years back to put in a couple of tanks.

    Cheers

    Jon
    • CommentAuthordjh
    • CommentTimeMay 21st 2009
     
    Could be, I suppose. Strange they didn't mention it when I toured last summer.
    • CommentAuthorjon
    • CommentTimeMay 21st 2009
     
    From memory they used two tanks, both pre-cast, about 1.5 to 1.8 diameter
    • CommentAuthorcrt
    • CommentTimeMay 29th 2009
     
    Back this topic...

    Bearing in mind some of the comments above, we are now looking at building the tank from reinforced concrete, using stainless steel bars for longevity. 200mm thick walls, floor, and roof; stretched octagonal shape 4m long, 3m wide and 2m deep.

    Any further views on this type of underground tank structure?
    • CommentAuthorstephendv
    • CommentTimeMay 29th 2009
     
    Sounds extremely expensive! have you compared costs to ready bought models?
    • CommentAuthorjon
    • CommentTimeMay 29th 2009
     
    As stephendv's comments
    • CommentAuthormike7
    • CommentTimeMay 29th 2009
     
    Posted By: crt We are planning an 11,000 litre tank to take our roofwater and then use it in the garden (and also solve our drainage problem).

    CRT


    If you have room, why not just have a pond? If the ground is stable enough for say a 45 degree slope on the sides, a 1 m deep pond would be a bit over 4m square to store 11000 litres. Quite cheap to line it with a membrane if needed, or if water seeps into it, why bother? Maybe it could be covered over if space is short.

    (I dug a 300,000 litre pond like this a while ago with earth sides, 2m deep. Had to line it, though. Hope it will be warm enough to swim in soon. I recon we are now drought-proof!)
    • CommentAuthortrule
    • CommentTimeMay 29th 2009 edited
     
    <blockquote><cite>Posted By: crt</cite>Back this topic...

    Bearing in mind some of the comments above, we are now looking at building the tank from reinforced concrete, using stainless steel bars for longevity. 200mm thick walls, floor, and roof; stretched octagonal shape 4m long, 3m wide and 2m deep.

    Any further views on this type of underground tank structure?</blockquote>

    You can buy a precast underground tank with 11000 litre capacity for around 3000 £. Sizes are available up to 36000 litre.
    • CommentAuthorcrt
    • CommentTimeJan 20th 2010
     
    An update on this...

    We had a design for a reinforced concrete tank that would have cost anything between £7,000 and £10,000 (before the cost of the hole). Very tricky and therefore expensive getting a single pour of concrete into an underground structure of the size we were contemplating. However even with standard rebar it would have had a life of over 60 years, probably over 100.

    The last suggestion above by trule is the way we ended up going. Pre-cast 15,000 litre tank with our own specified outlets and inlets and lid delivered and lifted into our hole for about £3,000. Costs more than plastic but with a lifetime many times of plastic. Warranted for at least 50 years, but in practice opinion seems to be that it will last indefinitely. A 15,000 litre precast tank is both enormously heavy (c.10 tonnes in all) and enormous (a 3mx3m cylinder, so needing a hole 4x4mx4m, which produces a vast amount of earth). Carlow Precast in Ireland if anyone is interested.

    Despite lots of discussion and the odd game offer to have a go, no one really knows anything about the true ferrocement tank construction that you see throughout the third world.
    • CommentAuthorbene
    • CommentTimeJan 20th 2010
     
    i gotta say the pond idea sounds best to me.
    line it with clay then you don't need any cement whatsoever... or is space an issue?
    just think of all those happy duckies you could eat! :tongue:

    anything under ground will have to be so over engineered as it will be nearly impossible to maintain and repair. will need to be able to resist weight of surrounding earth when empty.
    i would go down the plastics road if it was to be buried.

    ben
    • CommentAuthorcrt
    • CommentTimeJan 20th 2010
     
    The problem with plastic is that it needs to be anchored so needs a hefty amount of concrete anyway. And has nowhere near the design life of concrete or ferrocement.

    The real problem with underground tanks is not so much the weight on them from the outside when empty (this is largely overcome by a circular reinforced design, but the risk of high ground water levels popping them out (hence plastic tanks needing to be anchored).

    The engineering problems are overcome in practice by using precast concrete and, if as we do, you have a low ground water level, the tank is anchored by earth on a bed of pea gravel and scalpings.
    • CommentAuthorjamesingram
    • CommentTimeJan 20th 2010 edited
     
    Thought this might be of interest
    Twelve cubic metre ferrocement water tank, Thailand
    http://www.cd3wd.com/cd3wd_40/UWWKDTU/12MFCTNK/EN/12MFCTNK.htm
    look under water & sanitation may be more, just started looking , tons of stuff http://www.cd3wd.com/cd3wd_40/cd3wd/index.htm

    got link from here http://www.lowtechmagazine.com/2009/10/how-to-make-everything-yourself-online-lowtech-resources.html

    How to make everything yourself - online low-tech resources
    great other than you need a computer to start with etc. to look at it! anyway years & years of reading for the survialist types.

    More here RECOMMENDATIONS FOR DESIGNING RAINWATER HARVESTING SYSTEM TANKS PDF
    http://www.cd3wd.com/cd3wd_40/UWWKDTU/a6/PDF/a6.pdf
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