Steep Sites And Landslip Risk Land In Sydney

A steep site in Sydney can be built on safely, but the engineering starts with the ground and the water in it, not the building. Where a council has mapped the land for landslip risk, a geotechnical engineer has to show the slope stays stable before, during and after construction, and that assessment shapes the footings, the retaining walls and the drainage.

The map is a trigger for that assessment rather than a ban. Plenty of mapped land carries good buildings. What changes is how much the design has to prove.

What a landslip map means

Councils map land where the slope, the geology or a history of movement points to a risk of instability. On mapped land, a development application needs a geotechnical report that assesses the landslide risk for the proposal.

Councils commonly ask for those reports to follow the Australian Geomechanics Society's landslide risk management guidelines, which weigh how likely a slope is to move against what happens if it does. Some councils go further. On the Northern Beaches, land in the former Pittwater area that is mapped for geotechnical hazard comes under a risk management policy that asks for the geotechnical engineer's sign-off at the development application, again before the construction certificate, and again at the occupation certificate.

What makes a Sydney slope move

Water. Rain soaking into a slope raises the water pressure in the soil and lowers the friction holding it in place. That is why slips tend to follow long wet spells, when the ground has had days to fill with water.

Weak layers. On parts of the Northern Beaches the layer that moves is weathered claystone under the sandstone, or old slide debris called colluvium sitting on rock. In the sandstone country around Hornsby, Mt Colah, Sutherland and Engadine, the risks are loose boulders, undercut rock ledges and joints in the rock that dip out of the slope.

Earthworks. Cutting into the bottom of a slope removes the support for everything above it. Placing fill or stockpiles at the top adds weight where it does the most harm.

What the investigation looks at

The investigation reads the slope as a whole, not only the footprint of the building.

Boreholes run down the slope to find the depth of soil, colluvium and rock. Rock outcrops are mapped for their joints and how those joints are angled. Groundwater is observed rather than assumed. The engineer also looks for the signs a slope leaves behind when it has moved before: leaning trees, cracked walls and kerbs, and steps in the ground. How to read the resulting report is covered in [understanding geotechnical investigation reports](/blog/understanding-geotechnical-investigation-reports).

We then model the slope with the groundwater at its worst credible level. A slope that is stable in a dry month proves nothing about the wet one.

Footings and retaining walls

Footings on a slope go through the soil and the colluvium to rock or to stiff ground below them. On Sydney sites that usually means piers or bored piles socketed into sandstone, designed as described in [founding in Sydney sandstone and rock sockets](/blog/founding-in-sydney-sandstone-and-rock-sockets).

Retaining walls carry the full slope above them, not just the height of the cut. Sloping backfill and anything sitting above the wall add to the load, and taller cuts are held with anchors or soil nails, the systems compared in [soil nailing and anchored retaining walls](/blog/soil-nailing-and-anchored-retaining-walls).

Drainage on a slope

Every retaining wall on a slope needs drainage behind it, or it ends up holding back water as well as soil.

Stormwater from roofs and paving is collected and piped away, never sent into an absorption trench on the slope, because that puts water exactly where it lowers stability. Where the site falls away from the street, the drainage options are set out in [stormwater for sites that fall away from the street](/blog/stormwater-for-sites-that-fall-away-from-the-street).

The order of the cuts

On a slope, the construction sequence is part of the design.

Excavation goes in short sections, with each exposed face supported before the next is opened. Rock faces are inspected as they are uncovered, because the joints seen on site are the ones that matter. Stockpiles stay off the crest, and cuts are not left open over a forecast of heavy rain.

How we approach it

Our geotechnical, structural and civil engineers work the slope as one problem. The slope model sets the footing depths and the wall loads, the civil design keeps water out of the ground, and the excavation sequence is written onto the drawings the builder actually uses.

During excavation we inspect the cuts and the pier bases to confirm the ground matches the model, and we provide the stage sign-offs a council policy asks for. On a slope, the ground has already decided where it wants to go, and good engineering finds that out before the excavator does.

ACSES provides geotechnical engineering, including slope stability assessment, across Sydney. Talk to us about a project.

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