Strong blocks on a solid footing are not enough on their own to keep a retaining wall up. When rain soaks the soil behind a wall and has nowhere to go, the pressure on the wall rises sharply, and that is the usual cause of failure. Good drainage and backfill are what keep most walls standing over time.
Why water is the main problem
Dry soil pushes on a retaining wall. Wet soil pushes harder, because it is heavier and because the water itself adds pressure. When water collects behind a wall, it acts like a reservoir pressing against it.
The National Construction Code (NCC) recognises this. Its structural rules require buildings and structures to resist earth pressure, liquid pressure and groundwater pressure, among other loads. A retaining wall designed without drainage is often assumed to have dry soil behind it. In a storm, that assumption no longer holds.
What a drainage system includes
Most retaining wall designs use the same basic parts:
- Subsoil drain. A slotted pipe, often called an ag pipe, runs along the base of the wall on the high side. It collects water that seeps down through the soil.
- Filter sock or fabric. A geotextile wrap stops fine soil from clogging the pipe slots.
- Free-draining backfill. Clean gravel or crushed rock fills a zone directly behind the wall, so water can drain down to the pipe rather than build up.
- Outlet. The pipe needs a fall and an outlet to a legal point of discharge, usually the stormwater system. A pipe that simply ends in the garden returns water to the soil.
- Weep holes. Some walls have openings in the face to let water out at the base.
The engineer's design or the manufacturer's system details will specify the pipe size, gravel type and depth of the drainage zone. Follow them, because substitutions can change how the wall performs.
Backfill and compaction
The fill placed behind the wall is just as important as the pipe. Clay, builders' rubble and topsoil hold water and can expand when wet. Clean, free-draining material placed in layers and compacted as specified gives the wall the support it was designed for.
Loads close to the wall's upper edge also count. A driveway, shed, pool or pile of stored material close to the top adds weight, called a surcharge. If it wasn't allowed for in the design, it can push a wall past its limits.
Water and your neighbours
Retaining walls alter the way rain flows over and through a site. For exempt walls, the NSW Planning Portal states the work must not redirect surface or ground water, or wash sediment, onto neighbouring land. Approved walls are held to similar expectations through their conditions and stormwater plans. Our article on Retaining walls on the boundary: neighbours, footings and access covers the wider neighbour issues.
Warning signs to watch for
- The wall leaning outward or bulging in the middle.
- New cracks, or blocks and sleepers moving apart.
- Water seeping through the face or staining at the base.
- Soil washing out through joints or under the wall.
- Pooling or soggy ground along the upper side after rain.
If you see these, get an engineer to look at the wall. Early repairs to drainage are much cheaper than rebuilding a collapsed wall.
Maintenance
Keep weep holes and pipe outlets clear of soil, mulch and leaves. Where the design includes inspection points, flush the subsoil pipe from time to time. Avoid adding garden beds against the high side that trap water.
The NCC's site preparation provisions also call for care with earthworks so soil doesn't collapse or subside onto buildings and neighbouring property. Drainage is part of that.
For when a wall needs approval and an engineer, see Retaining wall heights: when you need approval and an engineer. CertifiBuild certifies retaining walls as part of Class 1 and Class 10 projects across NSW. Book an inspection at least 48 hours ahead, or have your plans checked before you build.
Local help across NSW
CertifiBuild certifies Class 1 and Class 10 work in 39 council areas. Local rules on these council pages are a good place to start: Penrith City Council, Inner West Council, Hawkesbury City Council, Bayside Council, Lithgow City Council, Randwick City Council.
Suburb pages with local planning notes: Kingswood, Lilyfield, Richmond, Sans Souci, South Bowenfels, South Coogee, Vaucluse, Wallerawang, Warriewood, Wongawilli. You can also browse every service area.
Sources
- Part H1 structure, NCC 2022 Volume Two (ABCB)
- Exempt earthworks and retaining walls, NSW Planning Portal
- Housing Provisions Part 3.1, site preparation (ABCB)