Appendix B - Geotechnical Investigation Report

elevated characteristic surface shrink swell movements in the range of a Class M to H1 site could be expected
and would again be expected to be differential.

The upper residual sands are of variable relative density and very loose sands and soft to firm residual clays
are not considered to be suitable foundation materials due to the low allowable bearing capacity and
potential for high elastic settlements. The very loose sands are also not considered suitable for supporting
building loads as they are susceptible to additional settlements due to vibrations and infiltration of surface
water run-off.

For uniformity of support and for stability considerations, the footings supporting the proposed house should
be uniformly founded in bedrock which we expect to be encountered at depths between about 2.2m and
2.8m. Footings may be designed for an allowable bearing capacity of 600kPa, subject to inspection of a
representative number of footing excavations by a geotechnical engineer prior to placing the reinforcement
cages.

Locally over the deeper section of the excavation pad or strip footings will be suitable. However, over the
majority of the proposed building, we expect that bored piles will be suitable but will require rock sockets to
be formed and sacrificial liners will be required to support the upper sandy soils, where encountered. We
expect that powerful pile drilling plant with rock augers fitted with tungsten carbide teeth would be required
although the size of piling rig may be restricted by the limited site access and sloping nature of the site.
Further advice from the piling contractor should be sought in regard to the most suitable piling rig to form
the rock sockets particularly as medium (and possibly higher) strength bedrock may be encountered.

Due to the variability relative density and strength of the upper residual sand and clay profile, the potential
for differential movements and for uniformity of support, we recommend that the later southern extension
also be founded in bedrock. If it is preferred to found the southern extension in the residual sands or clays
then further geotechnical advice would need to be sought. However, we forewarn that the footings and floor
slabs would need to be designed with regard for the recommendations for a Class M or H1 site (due to the
removal of trees) provided in AS2870 and differential shrink swell movements would be expected across the
extension would be expected. A full height movement control joint would also need to be provided between
the extension and the proposed house, which will be founded in bedrock. An allowable bearing pressure of
100kPa may be adopted for design of footings founded in the residual clays of at least stiff strength or residual
sands of at least loose relative density but this would locally require piers to be founded in the order of 2.2m
depth (based on BH1). Differential reactive movements would also be expected and we reiterate that further
geotechnical advice must be sought if the later extension is to be founded in the residual soils.

The weathered bedrock will soften in the presence of water so the piles must be drilled, inspected and poured
without delay and preferably on the same day. All pile holes should be free from all loose or softened
materials prior to pouring. If water ponds in the base of the pile holes, the water should be removed and
then the footing excavation deepened to remove all loose and any water softened materials.




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