Why is drainage necessary?
When groundwater comes into contact with a retaining wall it creates hydrostatic pressure. That pressure begins pushing water in through the capillary voids of the concrete. A membrane on its own will fail at detail points if it is left under constant pressure. A drainage system collects the water and carries it away before it can bear against the wall, removing the load on the membrane.
Layer order
External foundation drainage is built from the bottom up in the following sequence:
- Excavation and surface preparation. The retaining wall surface is cleaned, formwork tie holes are filled with repair mortar and sharp protrusions are ground back.
- Primer. A bitumen or polyurethane based primer is applied so the membrane bonds to the concrete.
- Waterproofing membrane. Applied in one or two layers depending on groundwater conditions. Overlaps must be at least 10 cm and fully bonded.
- Drainage board. An HDPE dimpled board is fixed over the membrane with the dimples facing the wall. This cavity lets water run downward.
- Filter geotextile. Laid on the soil side of the drainage board. Its job is to stop fine grained soil from clogging the cavities. If this layer is skipped, the system loses its function within a few years.
- Drainage pipe. A perforated corrugated pipe is laid at foundation level with a minimum fall of 0.5 percent. The pipe is bedded in gravel.
- Backfill. Carried out with washed gravel or free draining material. Clay backfill renders the system useless from the start.
Where the water goes
The collected water has to go somewhere. If the site slopes, natural discharge is possible; if not, an inspection chamber and a lifting pump are required. In pumped systems a backup power supply should be considered, because a downpour coinciding with a power cut puts the system out of service.
The four most common mistakes
- Skipping the filter geotextile or laying it on the wrong side
- Failing to give the pipe a fall, leaving water standing inside it
- Reusing excavated clay soil as backfill
- Installing the drainage board the wrong way round
How long does it last?
A correctly installed external drainage system works for the service life of the materials, which is more than 25 years. What shortens a system is not the material but application error and clogging. Checking the inspection chambers once a year is enough.