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I'm hoping to use 215mm concrete breeze blocks to build a 20m long retaining wall at 1m high. Do I need to build the blocks on their flat or can it be a regular single skin wall with piers every so often?

The land behind has no structures and the ground is pretty much all rocky parent material covered with about 6 inches of top soil.

I'll be installing a french drain etc.

  • What does "on their flat" mean here, and what is a "regular single skin wall"? – isherwood Feb 13 at 22:28
  • @isherwood So the blocks are normally stood on their edge but for extra strength they can be laid on their side. perigordvacance.typepad.com/.a/… – Fony Tinlay Feb 13 at 22:58
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It depends on soil conditions. I build on the flat many walls with stable soil behind the wall I build flat to flat no mortar straight up but will back fill with pea gravel or river rock to allow it to drain.

In some cases I will stager each block back a 1/4 or ~ 6mm back this works without mortar with stable ground.

With the above cases for loose soil that may shift I will mortar in place without pillars.

I have seen pillars used and rectangular blocks on edge , it was interesting and much less expensive but I did not think it would hold up as well, I cannot say if it has held up as I haven’t been in that town for years. 1 meter is not a lot but it depends on soil conditions and I like free standing blocks or rail road ties pinned together for small walls so some opinion and some soil conditions make a difference.

  • The soil is really stable and compacted, almost shale like material with sandy grit in between. The fave of it has been lying exposed for about 8 years and there has been no collapses – Fony Tinlay Feb 13 at 23:02
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Retaining walls resist overturning from 1) soil expansion, 2) moisture behind wall pressing on wall, 3) a surcharge on wall, and 4) moisture freezing behind wall.

1) Clay type expansive soils is the big problem here. It sounds like your soil is not clay-ish.

2) You indicate that you’ll be installing a french drain and your soil will allow moisture to flow through to the drain, so no problem with moisture.

3) Do not park cars near the top of the wall and you won’t have trouble with a surcharge issue.

4) Freezing can be an issue, but a french drain will remove most of the water, so I wouldn’t anticipate a problem with freezing.

It sounds like your retaining wall is a “gravity retaining wall” rather than a “reinforced retaining wall”. That is to say, the weight of the wall material will keep it from overturning, which is fine up to about 1m or so. Higher than that, and it should be reinforced.

I’d lay the bricks flat (not on edge) and if you mortar them together, make sure the french drain is at or near the base of the wall.

  • Thanks for the response. I'll have a 5 or 10 tonne excavator on the ground to do some grading prior to the wall going up, so it'll give me an idea of the integrity of the ground, after though the only thing that will be on top of the ground retained by the wall will be my family. I'll lay on the flat as this is my forever house and want it done right, but just wanted to be sure it wasn't overkill. – Fony Tinlay Feb 14 at 10:28
  • I think on the flat looks better. It sounds like the base is stable. I usually dig a trench and use crushed quarry as a base for short walls like this. On taller walls I will pour a concrete footing. I use crushed quarry rock 3-4” deep and then compact, this provides a good base and allows water to drain through the rock so the little back fill you have once the wall is in place settles and doesn’t push the wall out with a heavy rain while it is loose. FYI, remember if you plan on planting close to the wall after backfilling over fill by 6-8” so when it settles it will be close to the top. – Ed Beal Feb 14 at 14:01

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