Geotechnical Engineering. St George & Sutherland Shire.

The brief. A single-storey granny flat at the rear of the lot, its slab carried on bored piers, with low blockwork retaining walls. The structural design needed the depth to rock and bearing pressures against founding depth.

On the ground. A rig borehole inside the footprint found residual sandy clay over very dense clayey and silty sands with ironstone gravel and fragments of extremely weathered sandstone. Seepage was met at about 1.6 metres, and the auger refused at 1.84 metres on inferred weathered bedrock.

The outcome. Class M under AS 2870, read from the logged profile, with allowable bearing pressures set against founding depth for the piers.

The brief. A single-storey garden studio at a revised position in the rear yard, for a returning client.

On the ground. A hand-auger borehole and two penetrometer tests found about a metre of topsoil and non-engineered fill with building debris over high-plasticity residual clay, with seepage at the base of the fill. The auger and the penetrometers refused at 1.2 to 1.3 metres on inferred weathered sandstone.

The outcome. Class P as it stood, because of the fill. With the fill cleared from beneath the footings and slab, the report gave a Class S design basis and two footing options.

The brief. A civil contractor needed the working platform under a large mobile crane verified at 150 kPa at each outrigger, before precast stormwater pits were lifted in.

On the ground. Penetrometer tests at the four outrigger positions met practical refusal within the first 100 millimetres of the compacted granular platform.

The outcome. The platform was confirmed for 150 kPa at each outrigger, about twice the highest pressure in the lift study, on two conditions: full-size crane pads under every outrigger, and every pad kept 4 metres clear of the sheet-piled pit beside it.