Ask any experienced retaining wall builder what makes walls fail and you will get the same answer: water. A wall can be strong enough to hold back dry soil and still fail when that soil is saturated after a week of rain. Good drainage relieves that pressure, and it is the part of a retaining wall most often done badly or skipped.
Why drainage matters so much
Water in the soil behind a wall adds weight and builds up hydrostatic pressure against the back of it. Clay soils, common across Sydney, also swell when wet. Without a way out, that water pushes on the wall, softens the ground under the footings, and carries fine soil through the wall face.
The parts of a proper drainage system
- Ag pipe: a slotted agricultural drainage pipe at the base of the wall, behind it, to collect water.
- Geotextile sock or fabric: wraps the pipe and gravel so fine soil does not clog them.
- Drainage gravel: clean aggregate backfill behind the wall so water can move freely down to the pipe.
- Weep holes or gaps: openings through the wall face, depending on the wall type, so water can escape.
- A fall and an outlet: the pipe must fall to an outlet, and that outlet needs somewhere legal and sensible to discharge.
- Surface drainage: grading or a dish drain at the top of the wall so surface water does not pour into the backfill.
Where the water goes
Collected water should be directed to the stormwater system or an approved discharge point, not onto a neighbour's property. On larger or engineered walls, the drainage design is part of the engineering, and connections to stormwater may need approval.
Drainage by wall type
- Concrete sleeper and timber walls: ag pipe and gravel behind, with the sleepers retaining the backfill.
- Block and reinforced concrete walls: drainage behind, weep holes and sometimes a waterproof membrane where the wall backs onto living space.
- Gabion walls: naturally free draining, but still need geotextile behind to stop soil washing through.
- Sandstone walls: drainage behind and gaps for water to escape.
Signs your wall's drainage has failed
- Water seeping through the wall face or pooling at its base.
- White staining or efflorescence on the wall.
- The wall starting to lean or bulge.
- Soil washing out through gaps.
- Soggy ground behind the wall long after rain.
Retaining walls with drainage done right
Every wall we build includes ag drainage, aggregate, geotextile and weep holes as standard, and walls are designed to AS 4678 where engineering applies. See our retaining wall services or get a free site assessment.
Frequently asked questions
Does every retaining wall need drainage?
Almost every retaining wall needs drainage behind it. Without it, water pressure builds up in the soil, which is the most common cause of retaining walls leaning and failing.
Where should retaining wall drainage discharge?
To the stormwater system or an approved discharge point on your property. Water should not be directed onto neighbouring land.
Can drainage be added to an existing retaining wall?
Sometimes, by excavating behind the wall to install pipe and gravel, but it can be difficult without disturbing the wall. Where the wall is already failing, rebuilding with proper drainage is often the better option.
