Building a retaining wall: Will this work?

[Sam]

Sorry, dg, but you are wrong on this one. A 330mm blockwork wall will never be strong enough to retain 3m of soil, even with no surcharge on the retained side, no water etc. Your recommendation of bed joint reinforcement is of no use unless each end of the wall is connected to return walls (i.e. it can span both horizontally and vetically), and even if they are then I am sure that the maximum area of steel (before it becomes over-reinforced) would not be sufficient. Also, if the cutting is made into a sloping hillside the soil pressures will be greater and overall failure of the slope will also need to be checked. There is insufficient information in the OP for ANY details....

I realise you are only passing on your opinion, but in the same posting you say that your are qualified in this area and you know what you are talking about. You normally talk sense, so some readers may take all of your advise without question!

Sam

[Al Reynolds]

There's an awful lot of assertions in this thread.

Outside my back door is an earth bank, sloping backwards at about 10 degrees to the vertical. There is an un-reinforced concrete footing which is 1.5ft high and about the same deep, followed by 3.5ft of dry stone walling, maybe 8" deep. The remaining 6ft of bank is not retained by any structure.

In total, I am guessing that this is considerably less support than a 330mm blockwork wall, and it has all been in place for about 80 years.

My point is that no-one can make absolute assertions about what is or isn't sufficient to retain an earth bank unless they investigate further. The best advice is to consult someone who knows what they are talking about, and is prepared to guarantee their conclusions. It doesn't help slagging off other people on here.

Just my two cents, Al

[The Natural Philosopher]

Clay is probably the most dangerous soil to work with.

Make that 'unlucky'

it is very difficult to force

I'll try and check the piccies out later. Structiral engineer who knows your soil is invaluable - shold only cost a couple of hundred.

Wall will cost far more than that.

[The Natural Philosopher]

For a start thats only 5 ft, not ten foot high as a structure, and secondly, I bet the remaining 6 ft slopes well back from the lip.

True. I have a 2'6" wall that I built very very conservatively, and so far its fine. Block inside, with brick decorative facing. Convex but I tied every block together and about every other brick, and tied the two together.

No signs of movement BUT its a terrace behind it, that is flat. Its not holding back a whole hillside, and it isn't in Boscastle either :-)

To make the point that any structure can go if enough unusual conditions are imposed on it. Boscastle has been there a couple of hundred years, not a mere 80....:-)

The best advice is to

True.

Biggest danger from a wall like this is water, If the hillside is treeless, and there is not much vegetation and not much drainage, you can get a landslip starting that nothing will stop. Deforestation is a Bad Thing on slopes.

Even a structural engineer can only guess at 'worst cae' conditions.

Traditionally such things got rebuilt until they stopped falling down for long enough to be regarded as 'permanent' 80 years is not 'permanent'

[Sam]

No, I always get a contractor to do the work that I design, never DIY.

OK, 100% of the vertical pressure of water act horizontally and for soil only a fraction (about 30-40%). But waterlogged soil is heavier (approx 2x) than water & there is still usually some water pressure behind the wall. Engineers will always look at the case of the drains being blocked, but with a reduced Factor of Safety. Also, soil can have a surcharge and it can slope up behind the wall - this produces worse pressures than flat as it is effectively extra surcharge. Friction at the back of the wall has only a very minor effect on the lateral load.

Only if you have somewhere to take the thrusts.

Steel is good if placed correctly. Blockwork is not as good as you think. You can put that much steel in a masonry wall as the blockwork will crush long before the steel yields.

Unless it is reinforced with steel otherwise the reinforcement is useless....

Not cheap and usually means a larger overdig to get the thing in.....

But this wont stop a shear failure in the horizontal mortar planes.

No, it will illustrate the first failure mode to happen, which, unless the model is accurate, may not be representative.

[Al Reynolds]

Jut for info, the structure may only be five foot high, but the bank is eleven foot high. The whole thing slopes at about ten degrees to the vertical. Above the dry-stone wall it slopes at about 45 degrees for about a foot, then carries on up at ten degrees from the vertical for the next five foot.

All this is incidental, of course. The reason for my earth bank being retained the way it is is because of the local soil type, which is structurally quite stable unless exposed to severe weathering. My point was that unless you investigate the local conditions then any assertions along the lines of "this will work" or "that won't work" are unlikely to be completely accurate.

Very good point. Must have a look at that earth bank some time...

Al

[dg]

Well as I stated previously, I believed that my post contained sufficient information to enable further enquiry, but not enough so that the OP would take my post verbatim and rush out and construct the wall.

Foundation type, mortar mix, reinforcement spacing, end detail, soil type, local conditions etc etc were not mentioned, so would require further details.

With regards to the wall thickness, then I would say that the wall is not retaining the thrust of 3m of soil over its continuous height, rather more pressure will be exerted near the lower 1/3 of the wall. It may well be uneconomical to construct the wall the same thickness for its total height.

But I'll take you comments onboard

dg

[dg]

I am not sure that you have grasped the purpose of newsgroups. My understanding is that a question is posed, and then a number of opinions will be given, from which the OP will form his own opinion.

If we followed your reasoning, then no answers or opinions will ever be given - except perhaps consult a qualified bricklayer/painter/electrician/engineer etc in every case.

If you review past posts, then you will see many posts of a structural nature and replies which are given without full information and without the benefit of a visual inspection. How someone can answer a question about some structural work without a proper site survey appears to be impossible. Yet, contrary to your views, this appears to go on daily on much of Usenet. I find that odd.

And then, in total contradiction to the basis of your argument of commenting without the full facts, you proceed to add your own little [doubtful] comment. I find that odd too.

Your advice to consult a structural engineer is wrong too. We have a very good structural engineer in our office, and yet he knows little about soils except for basic soil loadings. Should he be consulted?

[dg]

Sorry, I posted before finishing my reply.

So if you are going to give advice on who to consult, then please be accurate in the profession you advise to use. The OP should use a 'competant' or 'suitably qualified' person. That person may or may not be a structural engineer . Being pedantic? - well if the ill informed person asked the structural engineer from our office to design his wall, then he could be in just as much trouble as if he built the wall himself.

dg

[Pete C]

Maybe, but a competent professional will know their limits and refer them on to someone with the right knowledge.

Looking at the photos the OP subsequently posted it gets even more scary :^) I would suggest RSJs piled deep underground and a cast concrete wall reinforced with rebar, but the first step would be a chat with an SE.

cheers, Pete.

[The Natural Philosopher]

You missed teh irony. I was a D-I-Y electronics tinkerer with a degree when I joined the Marconi company and found myself instantly a 'professional' designer of e.g. guided missiles. :D

No argument with that.

Agreed. You need to use buttresses and maybe anchors at teh end, ort cut deeper in to leave some soil being at teh'wings'.

Agreed. But its still masively better n compression than tension sans teh steel.

*shrug* The mediaveal guys didn't have the steel. Or the concrete. The arrival of wrought iron - able to take tensile stresses - revolutioneised structural design in the 18th and 19th centuries.

Nevertheless the romans built massive aqueducts without it.

Not if the wight of material is sufficient. Or of course use steel. Or uise interloking blocks not laid as continuous courses.

Then tyou rebuild it to get to teh second failure mode.

No-one really wants to know the second failiure mode anyway if they are buried under the first one.