Retaining Walls in Gordonvale

Gordonvale sits on the Mulgrave floodplain south of the city, in cane country, and the retaining problem here inverts the one in the hill suburbs. There is far less slope to deal with. What there is instead is ground that needs thinking about before anything is built on it.

Alluvial ground and what it carries

Floodplain soils are deposited by water, which means they are layered, variable over short distances, and often soft. A block here can have quite different material a few metres apart, and the profile can change with depth in ways that are not visible from the surface.

For retaining walls the consequence is bearing capacity. A wall’s footings transfer load into the ground, and soft alluvial material may not carry what a standard footing detail assumes. Under-designed footings on soft ground do not fail suddenly; they settle, and a wall that has settled differentially goes out of line and cracks. On a long wall, differential settlement along its length is the characteristic failure.

The honest approach is to establish what is actually there before committing to a footing design. On anything of size that means a proper geotechnical look rather than an assumption, and it is one of the few places where we would suggest spending money before the wall is even designed.

A high water table and long saturation

Ground water sits close to the surface across much of this area, and after sustained rain the profile can be saturated for extended periods.

Two things follow. Excavation is harder: a trench that fills with water needs dewatering, and working in saturated soft ground is slower and messier than the same job on the hill. That is a real cost and it belongs in the quote rather than as a surprise.

And drainage design is constrained. A subsoil drain has to discharge somewhere lower than itself, and on flat ground with a high water table the fall available is limited and the receiving ground may already be wet. Working out where the water can actually go is often harder here than working out the wall.

Flood behaviour

Parts of the area are subject to flooding, and a structure in a flow path is a different design problem to a structure in still water. Anything built where water moves has to account for the forces that flow applies and for the possibility of scour undermining a footing.

Rock walls suit some situations here well for exactly that reason: they pass water rather than resisting it, and mass rather than slenderness is what keeps them in place. Whether that is appropriate depends on the specific location and on what the planning scheme requires for the site.

What people usually want

Levelling house pads and yards, walls along creek and drain edges, walls around sheds and machinery areas, and boundary walls on the newer estates. Cane land soils and long access driveways also mean plant movement is easier here than on a steep block, which offsets some of the extra cost.

Call 0468 066 055 and tell us what the ground does after a wet week.