The difference between building on the deep basaltic clays of Doncaster versus the weathered sandstone profiles of the Yarra Valley is like night and day. In Melbourne, residual soils dominate across the eastern and northern suburbs, where the parent rock has decomposed in place over millennia. These soils behave nothing like transported alluvium — they often retain relict jointing and fabric from the original rock, which can dramatically alter bearing capacity and drainage. That's why we treat every site as a distinct geological problem. Our approach to residual soil characterization in Melbourne starts with understanding the specific weathering profile, from the upper horizon of completely decomposed material down to slightly weathered bedrock. Complementing this with a resistivity survey helps us map lateral variability in the weathering front without drilling every meter.
Residual soils retain the structural legacy of the parent rock; a fact that can either stabilize or undermine a foundation depending on how it is characterized.