Sydney soils and footings: what sandstone, shale, clay and fill mean for your site class

Much of the Shire, the North Shore, the Northern Beaches, the eastern suburbs and the harbour foreshores sit on Hawkesbury Sandstone, often at shallow depth with a thin sandy or gravelly soil over it. Rock in the footing zone and sandy residual soils are good news for footings: these sites commonly classify A or S. The catches are the slopes, which can push a site to P and sometimes need a geotechnical assessment, and fill in the old gullies between the ridges.

The Wianamatta Group shales, Ashfield Shale on the ridges and Bringelly Shale across the Cumberland Plain, weather into clays that are moderately to highly reactive. From the inner west out through Bankstown, Liverpool, Fairfield, Blacktown, Penrith and Campbelltown, and on the shale caps of the Hills and the Shire, sites commonly classify M to H2, and occasionally E on the deep, dark clays. These are the sites where the classification earns its keep, because the difference between an M footing and an H2 footing is real money and real cracking if it is wrong.

Along the Georges, Cooks, Parramatta and Hawkesbury-Nepean rivers and their creeks, the ground is alluvium: layered sands, silts and clays laid down by water, often soft, sometimes wet, and variable within a single block. Alluvial sites can classify anywhere from S to P depending on what the boreholes find, and P is common where the soil is soft or the water is shallow.

The Botany Basin, Kurnell and the beach suburbs sit on dune and estuarine sand. Clean sand barely moves with moisture, so these sites are usually Class A, but loose sand and a high water table can bring their own settlement questions, and old industrial land can carry fill. The boreholes tell the difference.

Fill is soil that people put there: to level a block, to fill a gully, to bury a demolished house or an old pool. It is the most common reason a Sydney site comes out Class P, because uncontrolled fill can settle unevenly under a footing regardless of what the natural ground beneath it would have classified. A knock-down rebuild, a block that was once a low corner of a paddock, or a suburb built over a creek line are all places to expect it, and a classification finds it before the footing does.

None of the above replaces drilling the block. Geology maps are broad, weathering depth varies from one lot to the next, and fill does not appear on any map. The value of a classification is that it is done on your site, for your footprint, by someone who logged the ground as it came out of the hole. Our area pages describe what we typically find council by council.