Geotechnical Engineering. Canterbury-Bankstown.

The brief. Two dwellings with shared basement garages, a lower subfloor level, a pool and retaining walls, on a large lot running down towards the Georges River. The scope included acid sulfate soil screening.

On the ground. Four rig boreholes met refusal on weathered sandstone between about 0.5 and 3.0 metres, with sandstone outcropping across the site. Up to a metre of fill lay over clayey sand at the road end and stiff to very stiff residual clay towards the river. Ten samples screened for acid sulfate soils showed no reaction.

The outcome. Class M. Both basement levels sit at or below the top of rock over most of the footprint, so most of the reactive clay is dug away. On the evidence, no acid sulfate soil management plan was needed.

The brief. A two-storey attached duplex on raft slabs, replacing the existing house.

On the ground. Three hand-auger boreholes and three penetrometer tests found 0.3 to 0.4 metres of topsoil and minor fill over residual silty clay of medium to high plasticity. Refusal came on extremely weathered shale at about 0.9 metres in one hole and 1.8 metres in another. No groundwater was met.

The outcome. Class M, moderately reactive, with the topsoil and minor fill removed from beneath all footings and slabs.

The brief. A single-storey addition to the rear of a house that stays. The report was to check the site class assumed on the structural drawings.

On the ground. Two hand-auger boreholes and two penetrometer tests inside the footprint. Topsoil over residual silty clay of medium to high plasticity, becoming grey-red high-plasticity clay with ironstone gravel below 0.8 metres. No fill was found.

The outcome. Class H1, highly reactive, under AS 2870, with foundation parameters and allowable bearing pressures for the new footings.