Breeze block for garden wall

savings37

I intend to rebuild my garden retaining wall of approx 80cm high. Thinking of using a standard breeze block which will be rendered. I'd like to use lightweight blocks but worried about them soaking up all the moisture.

Section62

savings37 said:

I intend to rebuild my garden retaining wall of approx 80cm high. Thinking of using a standard breeze block which will be rendered. I'd like to use lightweight blocks but worried about them soaking up all the moisture.

You'll need to use dense concrete blocks, ideally hollow ones which allow reinforcing rod to be placed vertically through the holes before they are packed with concrete.

The blocks will need to resist moisture (but you will need to allow for drainage in your design) but more importantly they need to be frost resistant.  Damp lightweight blocks will just crumble when frosted.

Most masonry retaining walls also work on the basis that the mass of the wall develops friction with the ground it is built on, and that provides the resistance to the retained soil which is trying to make the wall slide or overturn. The heavier the wall, the more able it is to resist the loads imposed by the retained soil.

stuart45

As you are rebuilding the wall, is it because the old one is leaning out of plumb?
Render on a retaining wall will often fail in a short space of time.

savings37

Good information, thanks

I've thought about using a land drain pipe behind the wall 👍

casper_gutman

savings37 said:

Good information, thanks

I've thought about using a land drain pipe behind the wall 👍

A properly constructed drain behind the wall may well help stop the wall from failing, as there will be less pressure on the back of the wall. The wall would still need to have enough strength and weight to hold the material back though. A drain also won't stop the wall from being damp, so low density blocks would still give problems with frost damage.

Don't underestimate how chunky a retaining wall needs to be. Even a relatively modest wall like yours will need to be built quite thick to stand up to the pressure of soil pushing against it. Larger retaining walls often have wide reinforced concrete bases you can't see, like an "L" or an upside down "T", so that the weight of earth overlying the base helps add to the effective weight of the wall.

If in any doubt, consult a professional civil engineer, or at least look for examples of properly designed walls for similar conditions. Certainly don't make your new wall any less strong/heavy than the one you're replacing - if it needs to be rebuilt, it's presumably failed in some way, suggesting the replacement needs to be an upgrade if anything!

stuart45

The recommended height at which you need an SE to design a retaining wall is 1000mm with LABC, but only 600mm with the NHBC. 
A lot depends on ground conditions. A lot of builders think a 9 inch wall is adequate for a 3ft high wall, but it can still fail as one I've looked at rebuilding shows.

savings37

I've removed a stone wall for cosmetic purposes and want to replace it with a modern style wall, hence the block and render idea. 

savings37

I'm thinking of using dense concrete blocks lay sideways rather than upright to increase the width and strength. The wall has a couple of 90 degree turns (hopefully this will add to the strength) I could build some pillars behind also.

It's getting a little too warm for DIY building at the moment 😬

Section62

savings37 said:

I'm thinking of using dense concrete blocks lay sideways rather than upright to increase the width and strength. The wall has a couple of 90 degree turns (hopefully this will add to the strength) I could build some pillars behind also.

It's getting a little too warm for DIY building at the moment 😬

If you need a wider wall then use wider blocks, or two blocks side by side with a staggered bond.  Blocks laid on their side have a single mortar join between them which creates a failure plane.  Hollow blocks filled with concrete and the addition of steel reinforcement rods creates continuity along the vertical axis which helps resist the shear and overturning forces.

Shorter lengths with return walls will help with overall stability, but the wall needs to be thick enough to start with.

weeg

My rule of thumb starting point for a masonry retaining wall is 2/3 width to height, so start at about 400mm wide and step in as you go up. 

That said, almost all wall failures I'm called out to fail because of lack of drainage.

You want to paint the back of the wall with a couple of coats of bitumen, and backfill immediately behind the wall with a looseish stone. (No fines or single size) with weepholes every couple of feet along the base.