ᐅ Is a strip foundation necessary for a small wall?

Hello everyone,

we want to build a wall using these "Lego concrete blocks." 40cm (16 inches) thick, 2 blocks each 80cm (31.5 inches) high, and about 25m (82 feet) long in total.
I think you know which blocks I mean. Linking is not allowed in the forum, and I am respecting that.

Currently, the base is a gravel layer that was compacted three years ago when construction took place there. Opinions differ somewhat regarding the base:

- Re-compact the soil, level/grade with gravel, and place the concrete blocks directly on top.
- Excavate (how deep?) and pour a lean concrete footing, then place the concrete blocks on top.
- Re-compact the soil, pour a lean concrete footing (is 13cm (5 inches) enough?), and place the concrete blocks on top.

What would you do or recommend? Is reinforcement necessary? I was thinking of making the footing slightly wider than 40cm (16 inches), or does it not matter if it is exactly as wide as the wall thickness?

Best regards,
kanuddel

One possibly relevant detail: the wall is intended to serve as a retaining structure for the slope. We have a terrain difference of 3–4 meters (10–13 feet) and want to save at least 1.6 meters (5 feet) here.

The background is that we have built a hall on the site, and around the hall— including the area planned for our future garden—there is only 3 meters (10 feet) of level ground before a 45-degree incline leading up to the natural terrain. This slope causes a significant loss of garden space. Therefore, we want to build this wall about 1 meter (3 feet) away from the hall (allowing room for potential repairs if necessary). This wall can also be used to contain any excavation material from the house construction. Beyond that, the ground rises steeply to the natural terrain, and a safety barrier will be installed at the top to prevent accidents.

Hello kanuddel,

You are probably referring to concrete form blocks. I’m not familiar with 40cm (16 inches) ones, but 30cm (12 inches) blocks, yes. And 80cm (31 inches) high per block? I know of 20cm (8 inches) and 25cm (10 inches) heights.

But keep in mind, these blocks have to withstand the earth pressure of 160cm (63 inches) of soil. They can only do this if they have a proper footing to support them. So: dig 80cm (31 inches) deep, compact 15cm (6 inches) of crushed stone well. Then pour a footing (concrete with a 1:4 cement-to-aggregate ratio, not lean concrete) about 100cm (39 inches) wide with reinforcement. You can build the concrete form blocks on this footing to resist the pressure. Make sure to put vertical rebar (at least 12mm (1/2 inch)) from the footing up as far as possible to the top of the wall. Add two horizontal rows of rebar as well. Fill the inside fully with concrete mixed 1:4 up to the top. Otherwise, it won’t hold.

Alternatively, use L-shaped blocks designed specifically for this purpose.

Otherwise, the wall will lean toward the slope.

Steven

@Steven : I think he means concrete blocks, not formwork blocks.

Exactly, concrete blocks, not formwork blocks.
80x80x40cm (31.5x31.5x15.7 inches) and 900kg (1984 lbs) heavy per piece

1m³ of soil weighs between 800-1400kg (1760-3086 lbs), so the wall still experiences significant earth pressure. Without anchoring the stones (steel reinforcement + concrete foundation) or building the wall in a stable interlocking manner like Lego so that it becomes 80cm (31 inches) deep, the wall will "fall over" (it won’t simply topple, but depending on local conditions it may shift, tilt, or deform).

At a height of 1.6m (5 feet 3 inches), several factors need to be considered:
• Is this allowed?
• If yes, is a structural engineering certificate required? (I believe it is for 1.6m)
• A photo of the situation might help to find alternatives to the probably very expensive small stones.