Poor drainage is the number one cause of retaining wall failure in Melbourne. With an average of 648mm of annual rainfall and reactive clay soils across much of the metropolitan area, every retaining wall needs a properly designed drainage system to prevent hydrostatic pressure build-up.
Essential Drainage Components
- Agricultural pipe (ag pipe) — Slotted pipe at the base of the wall, wrapped in geotextile fabric to prevent silting.
- Gravel backfill — 200–300mm of 20mm drainage gravel behind the wall, providing a path for water to reach the ag pipe.
- Weep holes — PVC pipes through the wall at regular intervals (every 1.5–2m) to relieve pressure.
- Geotextile fabric — Separates drainage gravel from the clay backfill to prevent clogging.
Melbourne’s Clay Soil Challenge
The reactive clay soils in eastern suburbs like Glen Waverley, Doncaster, and Ringwood hold water and swell significantly. Without drainage, this expanding soil can push against the wall with enormous force, causing leaning, cracking, and eventual failure.
Learn about retaining wall construction or check if you need council approval.
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