Eighteen stratigraphic and material testing rotary mud/core drillholes were drilled along King William Street, as part of a feasibility study for an underground railway. All of the holes penetrated the Hindmarsh Clay and the calcareous Hallett Cove Sandstone, which overlie, with a slight unconformity, sandy and silty marine Tertiary formations. A planned tunnel would enter underground through the Hallett Cove Sandstone at the Torrens Parade Ground in the north, but would soon pass into the Hindmarsh Clay and remain within it until the terminus at Victoria Square. Thus, apart from a small section cut into sandstone of variable strength, some of which may require blasting, most of the tunnel could be excavated by tunnelling machine. Piezometers were set in all of the sandy aquifers encountered, and their readings have indicated that the standing groundwater levels in all aquifers would be below the tunnel, except for a minor perched sandy aquifer in the Hindmarsh Clay.