Concrete retaining blocks let you build a stepped, open-faced wall that holds back a bank without a reinforced concrete wall. In South Africa the best-known systems are Terraforce and Infraset’s Löffelstein. Both work the same basic way: the blocks are stacked with each course set back against the bank, the wall leans into the soil, and the weight of the blocks and the soil in them holds the bank. This guide sets out what their published data says about blocks, heights and when you need a design.
Terraforce L11
Terraforce’s L11 datasheet gives:
- 11 blocks per m² of wall face;
- a block 225 mm high and 350 mm deep, weighing about 29 kg;
- a table of maximum unreinforced wall heights depending on the soil (firm clay or compact silt, or silty sand and sand), the slope of the ground behind the wall, and how steeply the wall leans back.
The lean makes a large difference. In firm clay with level ground behind, the table allows about 3.7 m at a 60° face, 1.4 m at 80° and only 0.8 m when the wall is built vertical. Sandier soils and sloping ground behind the wall reduce the heights further. The top of the foundation should be at least 150 mm below the ground in front of the wall.
Löffelstein
Infraset’s Löffelstein blocks come in three lengths:
| Block | Length | Width | Height | Mass |
|---|---|---|---|---|
| L300 | 300 mm | 450 mm | 170 mm | about 34 kg |
| L500 | 500 mm | 450 mm | 170 mm | about 52 kg |
| L750 | 750 mm | 450 mm | 170 mm | about 82 kg |
Infraset gives about 9.5 blocks per m² of wall in a straight line, rising to 10 to 12 on curves, and notes that the count is not affected by the wall’s angle. The blocks are laid at 180 mm spacings and about 1.6 units per linear metre. Infraset is clear about height: structures over 1.2 m need engineered designs and plans submitted to the local authority.
Counting blocks
Measure the face area of the wall: its length along the face × the height along the slope of the wall, not the vertical height. A wall leaning back at 70° is longer on its face than it is tall. Then:
Formula: blocks = face area × blocks per m² × (1 + waste)
Worked example: a Terraforce L11 wall 10 m long with a face of 1.2 m is 12 m², or 132 blocks at 11 per m²; allow a few extra for cutting and breakage. The retaining wall calculator works this out for each system and shows the L11 height limits for your soil and slope.
Drainage and backfill
Water behind a retaining wall is its biggest enemy: wet soil is heavier and pushes harder against the wall. Follow the maker’s installation guide for drainage behind the wall, the backfill and filling the block cavities, build the backfill up in compacted layers as the wall rises, and keep stormwater from pouring over the top of the wall.
Choosing between the systems
The published data points to a few practical differences. An L11 block weighs about 29 kg and is 225 mm high, so two people can handle it and the wall rises quickly. Löffelstein blocks are 170 mm high and weigh from about 34 kg for an L300 to about 82 kg for an L750, which may call for more hands or a machine on a big wall. Löffelstein’s block count per m² stays the same whatever the angle of the wall, which makes ordering simple. For both, the height you can build depends on the ground and the lean of the wall, so decide the angle before you count blocks. On a tight site where the wall must be nearly vertical, the allowed height drops sharply and an engineered wall may be the better answer.
When you need an engineer
- Any Löffelstein wall over 1.2 m, under Infraset’s own guidance.
- Any Terraforce wall beyond the heights in the datasheet table for your soil, slope and angle.
- Walls with a driveway, building or other load near the top.
- Walls on clay that swells, on fill, or where water collects.
A free-standing wall that ends up holding back soil is also a retaining wall; see building a boundary wall for the difference.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.