Geotechnical Engineering. South West Sydney.

The brief. A granny flat and a studio side by side in a rear yard, with no drawings yet. The classification and bearing pressures had to cover both buildings.

On the ground. Two hand-auger boreholes and four penetrometer tests. Fill was 0.4 metres deep under the eastern building and 1.2 metres under the western one, over medium plasticity residual clay that continued below the 1.5 metre boreholes. Three penetrometer tests stayed in stiff to very stiff ground to 2.6 to 2.8 metres; the fourth refused at 2.4 metres.

The outcome. Class P as it stands, for the deeper fill and a disused sewer trench crossing the rear of both buildings, on a Class H1 reactivity basis with the upper bound of 60 millimetres adopted for design.

The brief. A secondary dwelling in the rear yard of a corner lot, classified before the design was drawn. Most of the footprint was paved, with a service line across it.

On the ground. The rig drilled one borehole to 3.0 metres on the only strip of open ground inside the footprint, beside one penetrometer test. About 0.5 metres of topsoil and uncontrolled fill over stiff to very stiff residual silty clay of medium to high plasticity, with ironstone gravel, to the base of the hole. No groundwater.

The outcome. Class P as it stood, because of the fill, and reclassified Class H2, highly reactive, for design, with founding and bearing parameters for the structural engineer.

The brief. A consultancy needed boreholes on two residential lots for its own reports. We drilled, tested and logged; the consultancy wrote the reports.

On the ground. Four boreholes to the two-metre target depth with the 4WD-mounted rig, with a penetrometer test beside each. Silty clay fill was 0.3 to 0.6 metres deep on one lot and 1.2 to 1.5 metres deep on the other. Every hole was dry on completion.

The outcome. Both lots were drilled and logged in a single visit, with a log set issued for each lot.