Geotechnical Engineering. Eastern Suburbs.

The brief. A two-level rear extension, a garage with a studio above on the rear lane, and an in-ground pool, all carried on steel screw piles. The piling design needed the depth of dense sand and of rock.

On the ground. Four rig boreholes with penetrometer tests found up to 0.8 metres of fill over loose sand, then clayey sand becoming dense from about 2.4 to 2.7 metres, to auger refusal on inferred weathered sandstone at 3.65 to 4.1 metres. At the lane end, wet sand sat above inferred rock at 3.1 metres.

The outcome. Class P under AS 2870, because the loose sand cannot carry footings at practical depths. The report set screw pile parameters area by area, with the pool, shoring, earth pressures and groundwater control for the excavations.

The brief. A ground-floor rear extension and a second storey over the existing house, with a carport and a low retaining wall at the street. The classification supported the construction certificate.

On the ground. Two hand-auger boreholes and a hand-dug hole found up to a metre of uncontrolled fill with building debris over very loose to loose sand, with no clay to two metres. When the structural engineer asked about a higher end bearing for the bored piers, three more penetrometer tests found medium dense sand from about 2.3 metres.

The outcome. Class P under AS 2870, with deepened footings. Before concrete, eight 450 millimetre pier bases were inspected: each at least 2.5 metres below ground in natural sand, clean and dry.

The brief. An in-ground pool beside a garage kept as a pool house, with the shell carried on steel screw piles. The structural engineer needed the ground and groundwater conditions and the parameters for the piles and for uplift on the shell.

On the ground. Two hand-auger boreholes and two penetrometer tests, one beside the garage and one in the shored pool area. Very loose silty sand of the Botany Sands, with the water table at about the level of the pool base and medium dense sand from some 1.9 metres below it.

The outcome. Class P under AS 2870 for the garage and any new ground-supported work. The report gave the screw pile parameters and the design basis for the shell against uplift.