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Ferrocement Educational Forum  |  Ferrocement Discussions  |  General Discussion  |  Topic: Underground water tank « previous next »
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Author Topic: Underground water tank  (Read 396 times)
Posts: 3

« on: November 29, 2017, 12:46:23 AM »

Hello everyone.

I am building a house that i want to fit with some rainwater tanks.
Since my postage stamp property cannot allow anything more above ground for water storage and winter frosts won't help either, my only chance is to go underground.

The best spot is under the porch / terrace as it's wide enough.
I could fit 5 tanks 1.7m (5.6ft) wide and 1.8m (5.9ft) high in this space.

Fist thing i haven't found yet on the interwebs is how well (if at all) FC (laminated or not) works underground.
My soil is 37% clay with the rest silt and very little sand.
If i lay the tank on a gravel/sand bed and also leave a small gap around it and fill with sand i figure it will take care of the soil movements.

Let's say FC is appropriate for use underground in this scenario.
My problem now is how to build the thing.

I could:
1. build it underground in it's final position
2. build it outside, haul and lower it into position
3. build it outside in sections and haul and fix those into position

1. is uncertain to me on how to proceed.
I could wrap the hole in PE film and try to work from there.
But the base is concrete and may be hard to do and also the walls.
Caving in is not a concern but dirt falling from outside is.

2. The best from a manufacturing POV since i can easily check everything but the weight will be in the ton range and i don't have access to a crane.

3. This is also best from a manufacturing POV as above but i don't know how to exactly design the parts and how to fix the cold joints.
Circular rings might be a good idea structurally but may be too heavy or difficult to maneuver/fixate.
Vertical curved panels will be easier to manufacture but may be less structurally sound ?

Please help me with any input on this as in my area FC is an unheard of "thingy".
Concrete is the norm and everyone knows it and nothing else.
This knowledge is pretty poor (in the deep understanding sense) but the response to bad concrete is to put MOAR concrete.
I can't and won't seek any advice from such an establishment.
Full Member
Posts: 99

« Reply #1 on: December 02, 2017, 11:48:29 AM »

Hi Ionel,

Good questions. LFC of old school FC will work fine for an in ground tank. If you can build in place as the weight will get away from you very fast. I have LFC water tanks on my boat and the water is sweet and fresh. Are you trying to keep potable water or just for irrigation ? If irrigation it's easy to just run the roof water to the tank but if you want to drink it purification , filtering etc is a complication.

You say it is under a deck so I guess you will remove any structure so you have clear access, if so I would dig the hole to size and line it with 1/2 inch Styrofoam and then apply the structure directally to the foam. A lid could be done after The bottom and walls are cured and you could cast in place some 4 inch pvc cleanouts for filling points.

You can also make the sidewalls flat on the ground and join then to the slab but that is not as easy as it sounds and if it's leaking it Will be hard to track down and fix. Doing the sidewalls in place is more difficult than flat on the ground but better in the long run leak wise. Do a practise wall to get the kinks out.

The main factor in a waterproof tank is your mix, water content must be under 50% by weight to the cement in the mix. This prevents waste water escaping during the set and in doing so makes capillaries to let water go the other way after the set. The low water content makes for a stiff mix but that will help you on the walls to prevent slip.

Not rocket science but it does require attention to details. Any questions just ask , lots of help on this site. Brad
Posts: 3

« Reply #2 on: December 03, 2017, 08:46:26 AM »

Thanks Brad.

I'm glad to hear LFC or plain FC works underground.
I also am not very pleased to maneuver heavy weights around.

Regarding water, i plan to catch rainwater for washing.
Some filtration will be in place but minimal since it's not for drinking.

The deck is not an actual deck, it's just a side of the house where the roof is extended a lot to cover it.
Nothing is build at this point, except the house foundation.
I'm in the planning stages.

The 1/2 styrofoam for lining the hole is a good idea since the mortar will adhere pretty well to it and will keep the moisture in for proper curing.
Problem is, this sthickness is not available around here but much thicker, rigid boards are available.
Bending those is not really doable and cutting them is a pain unless i develop a hotwire device to cut them in cylinder sections to keep thickness constant.
I have used a diy hotwire tool but for straight, narrow, cuts. I guess i could be making a radial arm fixture wide enough to accommodate a 3' or larger board.

Thanks for reminding me about the water/cement ratio, that's really essential.

What i'm still not sure is how to do the floor and transition into the walls.
I've seen everyone uses reinforced concrete for the base and not LFC or FC itself.
Out of curiosity, why is this ?

Also, if i pour the floor in place, there's no room for me to finish it unless i do the pouring from the outside and finish in the center by standing on a suspend (hanged from above) harness or platform.
Full Member
Posts: 99

« Reply #3 on: December 03, 2017, 11:10:19 AM »

Hi Ionel,

Unless you have a good reason you could do rectangular tank. Round is better if very high and stress is an issue but a short tank is fine with straight sides. Make it big enough so you can work the bottom slab comfortably. Makes the foam easy as well. On the bottom to side detail turn the lath up at the edge of the tank , overlap ie stagger the wire and just proccede up the wall. Do the same at the top for the lid. Do the bottom and sides at the same time so no seams.

Thick foam is fine or whatever you can get. This is just one way to do it but others may have different suggestions.  Whatever makes the most sense to you is the issue.


Posts: 3

« Reply #4 on: December 03, 2017, 11:24:29 AM »

Hi Brad

Reason for choosing round over square is due to much better ability to resist stress (from inside and out) of the circle compared to rectangular.
So, even if my tank is only 1.7m (5.6ft) wide, since it's in the soil, there could be some forces acting on it.
This is just my speculation as i don't know from experience the inground forces.
Its's true that if using EPS, some of those forces could be lowered a lot and using rectangular shape can be a real option.

A rectangular shape will simplify everything tremendously plus give me some extra storage capacity.
To strengthen the walls (if necessary), i could add some small LFC ribs every 1 or 2 ft, should not be very hard.

Good advice overall.
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