Melbourne's geology varies sharply from the volcanic basalt plains of the west to the deep alluvial silts and clays of the Yarra River corridor. Within a single site we often encounter stiff residual clays over weathered basalt, then a sudden change to soft estuarine deposits near the bay suburbs. This variability makes direct visual inspection essential. An exploratory test pit provides the most reliable method to log soil stratigraphy, identify groundwater seepage, and collect undisturbed block samples for lab testing. Before we commit to any foundation design, we dig test pits to confirm what the boreholes suggest. Complementing the pit with a densidad-cono-arena test directly on the excavated floor gives us immediate field density data for compaction control in the same excavation.
A test pit exposes the actual soil fabric — fissures, root holes, and gravel lenses — that SPT logs simply cannot resolve.