Geotech Report · OPW26/0094

Project No. PG-10899                                                                                                 Page | 9
November 2023
Ref: PG-10899, 2023-10-16, GR VER 1
DD Sunnyside CD Sunnyside Pty Ltd – Geotechnical Investigation - Proposed Residence, 30 Mossman Court, Noosa Heads




Foundation Design Parameters

For footings founded 3m below the ground level, an allowable bearing capacity of
200kPa could be adopted, following confirmation that a minimum Density Index of
75% is present for a minimum depth of 750mm below the footings.

If the exposed sands are disturbed or loosened during the excavation operation,
they should be recompacted. Consideration should be given to placing a 50mm
thick (minimum) concrete binding layer across the footings, immediately following
excavation.

Settlement Potential

Settlements from underlying improved sands and natural soils beneath a pad footing
of 2.5m width (i.e. maximum column load of 1500kN) at a nominal founding depth of
3.0m below existing ground level at a design bearing pressure of 200kPa have been
calculated to be in the order of 15mm with anticipated differential settlements of
between 25% and 50% of the calculated total settlement for pad footings founded
at a similar depth.

The settlement will occur as “immediate” settlement as the building loads are
applied.

Hydrostatic Uplift Forces

For a fully tanked basement, hydrostatic uplift forces should be considered in the
design. It is recommended hydrostatic uplift forces in groundwater levels to at least
RL1.5 to 2.0m be taken into consideration on this site and checked for the Q100 flood
condition.

12.0 DEWATERING CONSIDERATIONS

Construction of the basement and foundation system will likely require dewatering.
The groundwater will need to be lowered to a minimum of 0.9m below the final
excavation depths, to allow the compaction of the founding soils to be achieved.
Groundwater levels are expected to be similar to the tidal levels in the neighbouring
canal. The HAT for Noosa Heads (Noosa River) is expected to be in the order of RL
1.07m and approximately 1.6m below ground level at the site. As the proposed
excavation of up to 3m plus an additional 0.9m for ground improvements, where
required, a drawdown of approximately 2 to 2.5m is therefore expected to be
required.

Dewatering is typically achieved through the use of perimeter spears and internal
wells.

Settlements inducted by 2m of drawdown (i.e. 20kPa effective stress increase) are
estimated to result in settlements of less than 5mm. Therefore, it is considered that
the effects of dewatering on the adjoining structures will be minimal.