Geotechnical Engineering. Parramatta.

The brief. A consultancy needed soil samples from beneath a car park on a development site. The programme was booked over two days.

On the ground. We brought the 4WD-mounted rig and drilled through the car park pavement. The consultancy's engineer logged each hole and took the samples. On the day, the scope settled at five boreholes.

The outcome. All five were drilled in a single day, so the second day was not needed.

The brief. A new car park over existing uncontrolled fill, part of a larger church expansion. The investigation characterised the subgrade for the pavement design.

On the ground. Three rig boreholes and three penetrometer tests, on an active site after rain. The fill ran from about 0.5 metres to more than 1.5 metres deep, with brick, concrete, glass and steel that stopped the auger at two offset positions. Below it, very stiff to hard residual clay graded into extremely weathered siltstone.

The outcome. The existing stabilised pavement could be kept and extended as a cement-modified pavement over a proof-rolled, geogrid-reinforced subgrade, rather than the unbound pavement first designed.

The brief. A knock-down rebuild with a basement garage cut 2.4 to 4.1 metres below the surface, close to both side boundaries. The structural engineer asked for excavation support parameters and the bearing capacity at basement level.

On the ground. Three rig boreholes with penetrometer tests found residual silty clay with ironstone gravel to 2.3 to 3.0 metres, soft to firm near the surface in places and stiff to hard below, passing into weathered shale at 4.0 to 4.4 metres. No groundwater was encountered.

The outcome. The basement platform sits in hard clay and extremely weathered shale over most of the footprint, and in shale at the front. The site was found suitable for the excavation with every face supported, and the report gave the shoring, bearing and pile inputs.