Retaining walls on Cape Town slopes

From Fresnaye to Clifton, Higgovale to St James, a lot of Cape Town is built on the side of a mountain. Building on a slope means holding back soil, and that means retaining walls. They are one of the least glamorous parts of a project and one of the most important. A failed retaining wall can damage your house, your neighbour’s house and anyone standing below it.

What a retaining wall does

A retaining wall holds back soil at a level higher than the ground in front of it. The soil pushes against the wall, and that pressure increases sharply with height. When the soil is wet, it pushes even harder. Winter in Cape Town, with weeks of rain soaking the ground, is when retaining walls are tested.

Common types

Gravity walls: heavy walls of stone, concrete or concrete blocks that resist the soil by their weight. Suited to lower heights.
Reinforced concrete walls: cast-in-place walls with steel reinforcing, designed by an engineer, with a footing that extends under the retained soil. Common for basements and higher walls.
Reinforced masonry walls: hollow concrete blocks filled with concrete and reinforcing steel.
Segmental block systems: interlocking concrete blocks, often planted, built at a slight angle back into the slope. Popular for gardens and embankments.

The right type depends on the height, the soil, the space available and what sits above the wall, such as a driveway, a house or a swimming pool.

Drainage is everything

Most retaining walls that fail do so because of water, not because they were too thin. When water builds up behind a wall, the pressure on it increases dramatically. Every retaining wall needs:

Free-draining material directly behind the wall, such as crushed stone
A drainage pipe at the base of the wall, leading water away
Weep holes through the wall so water can escape
A geotextile fabric to stop fine soil clogging the drainage
Surface water managed at the top, so stormwater does not pour into the backfill

Blocked weep holes and missing drainage are the main things we look for when checking an existing wall.

When you need an engineer and plans

Any retaining wall of significant height, or one that supports a structure, a driveway or a neighbour’s property, should be designed by a structural engineer. In Cape Town, retaining walls generally need building plan approval and an engineer’s appointment. Low garden walls may fall under minor building work, but check with your professional before assuming.

Warning signs on an existing wall

The wall leaning or bulging outward
New cracks, especially horizontal ones or cracks that are getting wider
Water seeping through the face of the wall or at the base
Soil or paving sinking behind the wall
No visible weep holes

If you see any of these, have an engineer inspect the wall before the next winter.

Building on a slope

On a sloping site, retaining walls are often built first, before the house itself, to create level platforms. They can be a large part of the project budget. Budget for a geotechnical report and an engineer early, so there are no surprises once excavation starts.

If you are planning to build or extend on a slope, get in touch.