Key Themes

  • Why do retaining walls fail?
  • Key components in successful design
  • Importance of drainage coil
  • Where to put it, where it goes.

In my experience, whenever I’ve had to look at a retaining wall that’s a little bit wobbly or seems to be constantly leaking water, the one common feature of the failure is the absence or the incorrect installation of the drainage coil. Incorrect drainage means the material behind the retaining wall becomes saturated and the added weight is greater than the wall can withstand. Best practice includes a combination of free draining material as backfill and correctly placed drainage coil that allows stormwater and groundwater to be directed away from the wall and into the storm water system via a catchpit.

On this recent project we have built 3 significant terraces of around 30 metres long, with the intention that the grassy area above the retaining wall will be able to be used all year round. Getting the drainage right was critical. We installed slotted 110mm Bailey’s Black Snake drainage coil at the base of the retaining walls before backfilling with scoria before adding a layer of geotextile cloth before levelling the terrace with topsoil before sowing the grass.

As you can see in the pic 1, we have added a filter sock around the slotted Black Snake drainage coil which will help prevent fine sediment from entering the drainage coil. The drainage coil is laid level at the bottom of the wall, and this will allow moisture to drain along the Black Snake coil to a piped connection at the end of the retaining wall before being directed to a catch pit.

The role of a catch pit is not to be underestimated when ensuring the overall performance of the retaining wall. The catch pit is there to catch sediment, so it does not enter the storm water system. Sediment drops to the bottom of the catch pit and can be cleaned out as required. The other benefit of the catch pit is that you can then see and observe the drainage coil working after it rains as the water will flow from the drainage coil into the catch pit.

Bailey Black Snake drainage coli is available in 110mm and 160mm diameter, either slotted or unslotted, and in a variety of coil lengths. To find out more about Black Snake click here.