Retaining Wall Drainage, Explained

If you read one thing before commissioning a retaining wall, this is the one worth the time. It is also the part of the job you will never be able to inspect once it is finished.

Two different loads

A retaining wall is designed to resist the pressure of the soil behind it. That load is predictable and it is what every wall calculation starts from.

Water is a second, separate load. When the ground behind a wall becomes saturated, water fills the voids between the soil particles and develops pressure of its own, acting outward on the wall independently of the soil. Because that pressure increases with depth, it is greatest at the base of the wall, which is where the leverage against overturning is worst.

A wall with fully saturated backfill can be carrying substantially more than the same wall with drained ground behind it. None of that extra load appears in the design unless somebody deliberately designed for a saturated condition, which is rare because it would make most walls uneconomic.

The whole purpose of a drainage layer is to make sure that condition never arises.

Why this matters more here

In a climate with modest, well-spaced rainfall, ground behind a wall gets wet and then drains before the next event. Saturation is a transient condition.

In a place taking two metres a year, much of it concentrated, the ground can stay saturated for weeks. The wall is not being tested by a downpour, it is being held under sustained load for months.

That is why walls in this region fail in a pattern that puzzles their owners: years of nothing, then movement during one wet period. The wall did not weaken. The condition it was never designed for finally lasted long enough.

The four parts, and how each one fails alone

The subsoil drain. A perforated pipe at the base behind the wall, laid to a fall, collecting water where the pressure is highest. Without it, water collects at the base and has nowhere to leave from.

Free-draining backfill. Aggregate behind the wall so water can actually reach that pipe. This is the part most often substituted with the clay that came out of the cut, and when that happens the pipe is sitting behind a material water cannot travel through. The drain exists and does nothing, which is worse than obvious absence because everybody assumes it is working.

The filter layer. Geotextile between the aggregate and the natural soil. Without it, fine particles migrate into the aggregate over a few seasons and fill the voids. The drainage layer silts up and stops draining, and again nothing visible changes.

The outlet. Somewhere for the collected water to go: a stormwater connection, a lower point where it can daylight, a legal discharge. A subsoil drain that ends in the middle of the yard is a buried pipe full of water.

Weep holes

On solid walls, weep holes through the face let water out at the front. They are the secondary route rather than a replacement for the drain behind, and they get blocked by render, by garden beds banked against the wall, and by paving.

If your wall has weep holes and they never run when everything else is soaked, they are blocked, and clearing them is the cheapest maintenance available on any property.

What to ask for

Ask any quote to state what goes behind the wall and where the water discharges to. A contractor who answers that clearly is telling you something about the whole job.

See retaining wall drainage, including retrofitting it behind a wall that never got any, or call 0468 066 055.