Geotechnical Engineering. Inner West.

The brief. A basement of about 325 square metres under a new rear pavilion, with a pool beside it, behind a heritage-listed house that stays. The bulk excavation runs 2.9 to 3.6 metres below the existing ground, near a sewer main.

On the ground. Four hand-auger holes inside the footprint and a rig borehole beside the house, each with a penetrometer test. The soil cover was thin: fill or topsoil over stiff residual clay, with extremely weathered material or rock from 0.1 to 0.7 metres within the footprint and weathered bedrock from 1.6 metres beside the house.

The outcome. The bulk excavation sits within weathered bedrock. The report gave the design team parameters for the foundations, the excavation and its support, the pool, and the protection of the sewer and the retained house.

First visit. An internal pad footing was checked against the 150 kPa on the structural drawings, with a penetrometer test at its base, and passed.

Second visit. A footing beam and the rear footing, founded in high-plasticity residual clay with ironstone gravel. The top 200 millimetres at two test points was not adequately compacted; the contractor recompacted it while we watched, and it was reassessed before the footings were passed.

The brief. A two-storey rear addition built against the common wall, with a large tree to stay about five metres behind it. The report also covered the tree's influence on the footings.

On the ground. A hand-auger borehole and two penetrometer tests. About 0.45 metres of fill over residual silty clay, high then medium plasticity, stiff to very stiff, to hand-auger refusal at 1.6 metres. Towards the tree, one penetrometer met very low resistance to a metre, and that ground is treated as unsuitable for footings.

The outcome. Class P, because the retained tree stands within influence distance of the rear of the addition. The footings are designed on a Class H1 basis, with the tree provided for under AS 2870 Appendix H.