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 some examples of drainage systems that could be required in Terraforce retaining walls:
1. Fin drains also known as wick drains installed at an angel of 45° to 60° at predetermined centres against the cut face.
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. May be waived in some conditions.
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.
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.
5. In and outlets over and through the wall.
Top tip: precast elements work well both over and through the wall.