A few years back, during the foundation design for a multi-storey residential building near the River Torrens in central Adelaide, the bore logs showed clean sands with a shallow water table—classic conditions that make a geotechnical engineer pause. Adelaide sits on a mix of Quaternary alluvium, estuarine clays, and the Proterozoic basement rocks of the Mount Lofty Ranges, and while the city isn't perched on a plate boundary, the 1954 Adelaide earthquake (magnitude 5.5) reminded everyone that intraplate seismicity is real. When loose saturated sands are present, a standard bearing capacity check simply isn't enough. We routinely trigger a soil liquefaction analysis when the SPT N-values drop below 15 in sands within 15 metres of the surface, especially in suburbs like Hindmarsh or along the Torrens floodplain where the groundwater can be less than 2 metres deep. In these settings, a detailed evaluation of cyclic stress ratio versus cyclic resistance ratio becomes the backbone of the foundation strategy—not an afterthought.
A factor of safety of 1.0 against liquefaction isn't a pass—it's a gamble on post-earthquake settlement that can exceed 150 mm in loose Torrens alluvium.
