Geotechnical Engineering. North Shore.

The brief. A lightweight modular granny flat on piers, on the lower level of a sloping block. Access was by stairs, so the work was done with hand-portable equipment.

On the ground. Five hand-auger boreholes refused in fill at 0.3 to 0.5 metres. Four penetrometer tests refused at 0.6 to 2.5 metres, read as the rock surface. Weathered sandstone was exposed across the garden, and the Sydney 1:100,000 Geological Series Sheet 9130 maps the site on Hawkesbury Sandstone.

The outcome. The rock surface was stepped and irregular, so levels could not be carried between test positions. The founding stratum is to be confirmed at each support.

The brief. A new three-storey home designed on bored piles socketed into rock. The old house had been demolished and the site cut to a level platform, so the work read the ground then exposed against the pile design.

On the ground. Two excavator test pits went to practical refusal through orange-brown to grey silty clay, into material with the look and feel of weathered shale and laminite. Published 1:100,000 mapping shows Ashfield Shale.

The outcome. The inferred rock surface fell from west to east across the site, so it was reported as indicative only, with geotechnical inspection of the first piles to confirm the founding.