By Matthew Baring-Gould, Kingscliff-based drainage plumber. Servicing Tweed Heads for 4 years. 5.0 stars across 173 Google reviews. Licensed NSW 383340C and QLD 15036450. ABN 74 605 392 348.
Customer in Tweed Heads built a new retaining wall to open up more usable yard. We prepared the drainage trench behind the wall and installed two parallel 100mm socked aggie lines in geofabric, filled to the top with 20mm blue metal drainage gravel.
What the customer called us for
Owner had just built a new retaining wall in the yard to gain more usable area, and needed the drainage done properly behind it before backfilling.
What we found when we got there
New retaining wall in place, open trench behind it ready for drainage. Our job was the drainage design and install.
Why we did it this way
A retaining wall without proper drainage is a wall slowly getting pushed over by hydrostatic pressure. Two parallel 100mm socked aggie lines carry water away twice as fast as a single line and gives you redundancy if one ever chokes. Blue metal rather than recycled gravel keeps the drainage sock free of concrete dust, which is the number one cause of long-term drainage failure.
What we did on site
- Brought in the excavator and prepared the drainage area behind the wall.
- Laid geofabric along the trench floor and walls.
- Installed two parallel 100mm socked aggie lines along the trench.
- Filled the trench to the top with 20mm blue metal drainage gravel.
- Wrapped the geofabric over the top to seal the drain.
- Connected the discharge to the existing stormwater system.
- Backfilled and handed back to the customer.
The result
Retaining wall now drained with parallel aggie lines and a blue metal French drain. Water from behind the wall has a planned fast path out, not a slow build-up against the structure.
If you have the same issue
If you are building a retaining wall over about 600mm high, the drainage behind it is not optional. Done right, the wall lasts decades. Done wrong, you get bulges, cracks, and eventually failure. Spend a little more on 20mm blue metal and socked aggie now; you only build the wall once. Call Matty on 0447 188 394 or request a quote.
Customer names and addresses have been withheld. Individual results vary. Drain Masters Plumbing is licensed in NSW (383340C) and QLD (15036450).
Frequently asked
Why two aggie lines instead of one?
Double the capacity, plus redundancy. If one line ever silts up or gets damaged, the other is still carrying water. On a wall you want to last decades, that matters.
Is 100mm aggie enough?
100mm socked aggie is the standard for residential retaining wall drainage. On commercial or very large residential walls we would step up to 150mm. For a standard retaining wall opening up more usable yard, 100mm parallel lines is the right spec.
Why spend more on blue metal instead of recycled gravel?
Recycled gravel has concrete dust and fines mixed through it. Over time those fines wash down into the aggie sock and reduce flow. Blue metal has clean voids, holds flow for decades, and is specifically specced for drainage. The extra cost pays back the first time you do not have to re-dig.
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