Retaining Wall Drainage

This is a service page rather than a line item on other pages because in this climate the drainage is the wall. Everything above ground is what you look at. What is behind it is what decides whether you are looking at it in fifteen years.

What the pressure actually is

Soil pushes on a wall. That load is what a retaining wall is designed for and it is predictable.

Water is a different problem. When the ground behind a wall saturates, the spaces between soil particles fill and the water develops its own pressure, pushing outward independently of the soil. Because water is dense and the pressure builds with depth, a wall with a fully saturated backfill can carry substantially more load than the same wall with dry ground behind it, and none of that additional load was in anybody’s calculation.

In a place that takes the rain Cairns does, over months rather than in single events, saturated backfill is not a rare condition. It is the default for a good part of the year, which is why walls that stood through five dry seasons go over in one wet one.

What a proper drainage layer is

Four parts, and each one fails differently if it is left out.

A perforated subsoil drain sits at the base of the wall behind it, laid to a fall, collecting water at the lowest point where the pressure is greatest.

Free-draining aggregate goes behind the wall so the water can actually reach that drain. Backfilling with the clay that came out of the excavation is the single most common shortcut in the trade, and it means the drain is sitting behind a material water cannot move through, doing nothing.

Geotextile filter fabric separates the aggregate from the surrounding soil. Without it, fines migrate into the aggregate over a few seasons, clog the voids, and the drainage layer quietly stops working while everything still looks fine.

An outlet, which is the part most often botched. The drain has to discharge somewhere real: to a stormwater connection, to daylight at a lower point, to a legal discharge. A subsoil drain that stops in the middle of the yard is a pipe full of water, not a drain.

Weep holes

On solid walls, block and rendered walls in particular, weep holes through the face at the base let water out at the front. They are not a substitute for the drain behind, they are the secondary route, and they get blocked by render, by garden beds and by paving pushed up against the wall. Clearing them is one of the cheapest maintenance jobs on any property and almost nobody does it.

Retrofitting behind an existing wall

Where a wall is structurally sound but was built without drainage, the drainage can usually be installed without dismantling the wall. It means excavating behind it, which means access and disposal, but it is a far smaller job than a rebuild and it addresses the actual cause.

That is the most common repair we carry out and it is very often the honest answer for a wall that has begun to move but has not moved far.

Call 0468 066 055 or read why walls fail here first.