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Storm water management on retaining walls

Since most retaining walls are semi-impervious, which means water cannot easily pass through the wall itself, efficient drainage is crucial. When drainage goes unaddressed, hydrostatic pressure can build up behind the wall and cause damage such as bulging or cracking, or even collapse.

How important is the drainage layer?

To put it simply – very. The wall itself and the foundation are important, but the drainage layer is at least as important. It should consist of a layer of coarse, well-draining material of not less than 300mm (1ft) behind the blocks as shown in our typical cross-sections, and be well-compacted. In certain situations, an engineer may also specify a permeable drainage pipe above foundation level. Is also crucial to channel surface water away from the top of the wall in an open swale.

Below are some examples of drainage systems that could be required in Terraforce retaining walls:

1. Fin drains also known as wick drains installed at an angle of 45° to 60° at predetermined centres against the cut face.

Wick drain and Terraforce wall SwazilandWick drain and Terraforce wallWick drain and Terraforce wallwick drain and Terraforce wall

2. Subsurface drains also known as subsoil drains are specified behind the bottom row of blocks.

Top tip: Slotted or drilled drainage pipes should be installed within a gravel bed, wrapped in filter fabric to prevent clogging of the gravel with fine particles from the backfill. Maybe waived in some conditions.

Subsoil drain and Terraforce wallSubsoil drain and Terraforce wallsubsoil drain with Terraforce wall

3. Drainage outlets through the wall and at the end of the wall.

Top tip: Wrapping the open end of the outlet pipe, in this instance for a horizontal drainage conduit, with plastic mesh to prevent spillage of gravel.

Drainage outlet and Terraforce wallDrainage outlet and Terraforce wall

4. Open drainage channels above the wall.

Top tip: you can also use a permeable paver such as Terracrete, installed on an impermeable plastic sheet.

Open drainage channels and Terraforce wallsOpen drainage channels and Terraforce wallsDrainage swale with Terracrete

5. In and outlets over and through the wall.

Top tip: precast elements work well both over and through the wall.

5. In and outlets over and through the wall5. In and outlets over and through the wallIn and outlets over and through the wall

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Reversable hollow-core retaining blocks

4 Huguenot Avenue Oranjezicht
Cape Town
Western Cape
South Africa

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