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How Much Pressure Can A Single Skin Block Wall Take


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#1 Supadupagt4

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Posted 19 August 2013 - 10:52 AM

HI all,

im wanting to put a koi pond in the garden.

It will have a back wall @ 8' long, then a left side @ 5'long and a bowed side to join the ends together.
3' will be above ground.3'below. The inside will be fibreglassed.
I want a window in the bowed side so I can see the fish from the conservatory.

How thick should I make a rebar reinforced concrete base?
Will 3.6n blocks be Ok as a single skin on end, or should I lay them flat?
Thanks for any help.

Tony.

Edited by joiner, 19 August 2013 - 05:08 PM.
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#2 jsharris

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Posted 19 August 2013 - 11:45 AM

Welcome.

Any chance of a sketch, please?

Generally you can go to a retained height of around 600mm or so with a single skin 100mm block wall, but that's holding back soil that exerts far less pressure than water. As you're going over this height and this is a pond then I'd be inclined to use hollow blocks with steel in the hollows, then the hollows backfilled with concrete. 140mm wide hollow blocks should be OK for the 900mm or so. If this wall is really around 1.8m though it will need at least 215mm wide blocks. As I understand it the wall total height may be around 6ft, 3ft below ground, 3ft above ground, is that right? If not then the advice above may well be off the mark, as I've assumed that the wall will be partially supported against water pressure by the soil behind the lower part.

150mm would be OK for the base, laid on a compacted bed of MOT 1 and would give you a good base for the block work I think. if you stick a bit of steel fabric in the base and wire in some bends at the edges to poke up vertically and line up with the holes in the blocks then you'll get a good rigid structure. Wire the vertical steels to these bends (10mm should be OK) and lay some thinner steel (6mm) in the mortar courses to hold the whole lot together.

Edited by jsharris, 19 August 2013 - 11:47 AM.


#3 Supadupagt4

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Posted 19 August 2013 - 05:13 PM

Thanks for the reply. I have added a sketch.
Even with the inside fibreglassed, will I still need to go for filled concrete blocks?
3' above ground and 3' below, plus a 150mm base. Oh and there will be a 110mm drain in the centre bottom.

Tony

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#4 jsharris

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Posted 19 August 2013 - 05:46 PM

Thanks for the sketch, Tony, that makes things a lot clearer.

The fibreglass won't add any appreciable stiffness at all, I'm afraid, so all the load will be taken by the rigid concrete blocks (the fibreglass will be too flexible to do more than just be a waterproof skin, even if it's quite thick, as it has a pretty low Young's Modulus compared to concrete blocks).

I'd be inclined to build this in 215 wide hollow blocks, as the total height is 6ft, with 10 to 12 mm diameter rebar down each hollow and the hollow filled and vibrated with a reasonably strong concrete, around RC35. 150mm concrete for the base/foundation, with a layer of A393 steel fabric in the centre (supported on chairs or bits of broken concrete slab around 70mm thick) should be OK. Use L shaped 10 or 12mm steels at the edges, wired to the A393 and arranged so that the verticals poke up through the hollows in the blocks. Wire extra lengths of rebar to the L verticals to get up to the full height (it'd be OK to stop the vertical rebars one course down from the top).

This is a significant structure in terms of the forces on the walls, as a 6ft depth will mean quite substantial forces on the retaining walls.

As a comparison, I've recently built a 2.5m (8ft 4in) high retaining wall, holding back soil (which exerts less force than water) and I had to use a double thickness of 215 hollow blocks for the first metre (so 440mm thick, inc the mortar bond) and double 12mm steels up every hollow, with a vibrated RC35 fill.

#5 Supadupagt4

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Posted 19 August 2013 - 06:55 PM

I've calculated 144 blocks.. How much cement?
All that for a pond.. Lol :wacko:

Edited by Supadupagt4, 19 August 2013 - 06:56 PM.


#6 Supadupagt4

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Posted 19 August 2013 - 06:57 PM

Can't I just lay 215's flat?

#7 jsharris

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Posted 19 August 2013 - 07:10 PM

You really need the reinforcement here to tie all the blocks together. There's a significant shear load on a water retaining wall of this height, more than mortar alone would be able to take in my view. The vertical steel will form reinforced concrete beams inside the hollows of the blocks, making them resistant to shear and bending loads.

This is a mighty deep pond, though, deeper than many swimming pools, hence the loads that are being exerted on the blockwork.

If you work on having a 10 to 12mm wide mortar course you can work out the volume from the number of blocks and the mortar area per block. You'll need to add the volume of RC35 concrete for the filling, too, and at that height you'll need to fill the hollows in two fills, as about 3ft is the most you can reasonably fill reliably. This means building the wall to 3ft, then filling the hollows around the steels, then adding the final 3ft and filling the hollows in those. Trying to fill 6ft in one go will result in voids. You'll also need a vibrating poker to ensure that the concrete flows down to the bottom of each hollow, but you can hire one of these easily enough.

If this sounds like a lot of work, it's because you're building a 6ft deep tank, which is a significant project in terms of the forces involved from the water it's retaining.

#8 Supadupagt4

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Posted 19 August 2013 - 07:21 PM

After doing. Some recalculating... The water depth would be around 1700mm.. I normally work in feet, (wall) 3' below ground, 3' above, water will be 3' below ground, and 2'7" on top of that. 1700mm water depth. Does that help..

Oh and what about the window.. 4' wide 2' high in the bowed part.. Extra reinforcement around the area?

#9 jsharris

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Posted 19 August 2013 - 07:56 PM

You're still in the realms of a significant structure, I'm afraid. Reducing the height by 100mm doesn't appreciably reduce the load, at least not by enough to allow reducing the size of the blockwork.

The window thickness depends on the depth so if it's set in the upper part it will be a lot thinner than if set in the lower part. What's the depth (below the water surface) of the lowest edge of the window? This will determine the stiffness and strength required, and in turn will determine the thickness. For safety this will need to be toughened glass as a minimum, ideally laminated toughened glass, as this is more able to tolerate impact without creating a flood.

#10 Supadupagt4

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Posted 19 August 2013 - 08:25 PM

Lowest edge of glass will be 2'5".

#11 notasquickasiwasNeil

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Posted 19 August 2013 - 09:19 PM

View PostSupadupagt4, on 19 August 2013 - 08:25 PM, said:

Lowest edge of glass will be 2'5".
If I was you I would listen to jsh , unless you want to be putting it right in a few years . You might well get away with laying the blocks flat, if you did decide to do this and not go with the hollow blocks then you should use concrete blocks and put 200mm wide EML in every bed joint in continuous lengths along each course , This would probably be just as expensive as reinforced block work, and it is very awkward . If you are ok with timber then a shuttered wall construction can be good.

Edited by joiner, 20 August 2013 - 08:02 AM.
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#12 joiner

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Posted 20 August 2013 - 08:04 AM

The curved front will actually weaken the structure so reinforced isn't an option but a necessary structural element. :)

#13 temp

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Posted 20 August 2013 - 09:00 AM

What soil type? If you have a shrinkable clay you might want to clad the outside with 50mm insulation board before back filling? I believe curved glass is expensive so I hate to think what curved laminated glass would cost. Best get a quote before finishing the design? It might pay to make the wall in a series of say 3 straight sections rather than a curve so flat glass can be used?

#14 joiner

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Posted 20 August 2013 - 09:17 AM

Increasing the stress on the panel joints, though?

#15 wmacleod

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Posted 20 August 2013 - 10:13 AM

View PostSupadupagt4, on 19 August 2013 - 06:55 PM, said:

I've calculated 144 blocks.. How much cement?
All that for a pond.. Lol :wacko:

A cheaper pond can be made by digging a hole in the garden and having the soil support the water! I would go with the steel as suggested but shutter it and pour concrete, be a little cheaper if you can get the timber for shuttering for nothing, maybe do two pours, making sure it is well compacted

#16 Supadupagt4

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Posted 20 August 2013 - 11:16 AM

What's the highest I could go above ground without all the extra reinforment and no curved wall?

#17 jsharris

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Posted 20 August 2013 - 11:27 AM

If you just want to retain a shallow pond then you could go to around four or five 100mm blocks high on their side at a pinch. Even then I'd tie the bond together with EML as Tony suggests.

Structurally, the problem is that a plain mortared block wall is a gravity wall, that is it is the weight of the wall that resists the side pressure from the water. The mortar bond at the base adds very little to the overturning resistance of the wall, as it's in tension on the pond side at the base and mortar is very weak in tension (even though it's relatively strong in compression and shear).

Gravity retaining walls work by being thick, so they are both heavy and have a wide base that resists the overturning force being exerted by the side pressure.

Laying 215 hollow blocks with steel reinforcement would, in my view, be quicker and easier (and probably no more expensive) than building with standard blocks on their sides.

Edited by jsharris, 20 August 2013 - 11:27 AM.


#18 Supadupagt4

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Posted 20 August 2013 - 11:44 AM

I've managed to get 10mm rebar at a very nice price..(free) so I think that's the way we'll go.
Thanks for the help thus far. :-D.