For any project falling under AS 1170.4 structural design actions, the starting point in Adelaide is rarely straightforward. The city sprawls across four distinct geological domains—the Proterozoic basement of the Mount Lofty Ranges, Tertiary alluvial fans of the plains, Quaternary marine sediments near the coast, and the notorious Keswick Clay formations that have caused decades of foundation movement. A simple site walkover won't tell you whether you're on Class C shallow rock or Class E soft clay requiring a full dynamic analysis. That's where a properly executed MASW survey, processed with the active-source approach and inverted to a reliable VS30 profile, becomes the foundation of the seismic brief. Our team runs these tests weekly from Outer Harbor to Mount Barker, and the variation in shear wave velocity across less than 20 km can be astonishing—something the generic code maps simply don't capture. When the borelog suggests stiff clay but the seismic microzonation data from the Adelaide CBD indicates potential amplification, we always recommend verifying with direct measurement rather than relying on proxy correlations alone.
A VS30 measured at 180 m/s in the Keswick Clay versus 360 m/s just 500 metres east changes the site class from D to C—and the seismic design actions by up to 40%.
