Retaining Walls in Freshwater

Freshwater sits at the point where the valley floor meets the base of the range, and that transition is the whole character of the suburb. Blocks run from flat ground up onto rising slope, the creek system runs through it, and water moving off the range crosses the area on defined paths rather than as diffuse runoff.

Blocks with a drainage line across them

The particular thing about working here is how many properties have a real flow path across them. Not a soggy corner: a defined line that carries water during sustained rain, sometimes an overland flow path, sometimes a gully that only runs when the range is shedding.

A retaining wall built across one of those without recognising it is a dam. It will hold, or partly hold, and then it will be overtopped or undermined or pushed out at the point where the flow concentrates. The failure will be sudden and it will be at the low point.

The right approach is to work with the flow rather than to interrupt it: identify the path, keep the wall out of it where possible, and where the wall genuinely has to cross it, design it as a structure that passes water rather than one that holds it back. That is a design question to settle before anything else, and it is the main reason we would rather see a block here in person and ideally know how it behaves during rain.

Where rock walls earn their place

This is the part of Cairns where rock and boulder walls genuinely make sense more often than manufactured walls.

They drain through themselves, so a flow path crossing a rock wall is far less of a problem than the same flow reaching a solid one. They suit the informal, vegetated, creek-edge character of a lot of these blocks in a way a straight sleeper wall does not. And on the larger properties toward the range there is usually room for the batter they need.

The caveat is the filter layer. A rock wall passing water freely will also pass soil fines unless there is geotextile behind it, and on a block where water moves in volume that erosion is not theoretical.

Saturation, not just flow

Even away from the defined lines, ground at the base of a range spends a long time wet. Water that fell on the slope above emerges lower down, and the ground can be carrying water long after the rain has stopped.

For a wall that means the saturated condition is the design case, not the exception, and the drainage outlet has to work when the ground it discharges into is already wet. An outlet that daylights into ground that is itself saturated is not draining anything.

Call 0468 066 055 and tell us whether water crosses the block when it rains properly.