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

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?

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.

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.

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?

<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

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

From memory they used two tanks, both pre-cast, about 1.5 to 1.8 diameter

Sounds extremely expensive! have you compared costs to ready bought models?

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!)

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.

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.