Preliminary Geotechnical Investigation · OPW21/0315

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A guard layer of ‘ag lime’ should also be applied over the base of the basement excavation. This
guard layer of lime should be applied at a rate of 2.5 kg/m2.

It is generally preferable if an ‘ag lime’ with a purity of 95% or better is used (ie. NV >95, where NV is
the neutralising value, a term used to rate the neutralising power of different forms of materials relative
to pure, fine calcium carbonate which is designated NV = 100). The above lime dosing rates should be
adjusted by a factor of 100/NV.

Temporary dewatering for basement excavation will result in the localised lowering of the water table.
How the groundwater will be extracted and treated will depend on the groundwater dewatering
contractor at the time of construction. It is assumed dewatering will probably comprise a ring of
dewatering spears around the outside of the sheetpiles.

It should be anticipated that for a properly designed and constructed retention and dewatering system
would result in minimal drawdown of the groundwater outside of the excavation and mitigate the extent
of aeration of permanently saturated potential acid sulfate soils. The dewatering contractor would need
to develop their own dewatering management plan to ensure negligible environmental impact.

The dewatering management plan would need to include how the extracted groundwater shall be
treated to meet the water quality targets for the Noosa River prior to discharge.

To further mitigate any potential risk associated with short-term dewatering, the following management
strategies should be included in the contractors dewatering management plan:
•    minimise periods of active dewatering as much as possible; and
•    quality of any groundwater inflows into the excavations be monitored as well to ensure insitu
     acidification is not occurring. Monitoring would involve pH testing of inflows during daily
     dewatering periods. If the monitored pH levels fall more than 1.0 unit over the course of
     construction monitoring, or is consistently below pH 4.0, then further assessment would be
     required to determine whether there has been any significant environmental impact.


8.     References

Ahern, C R; Ahern, M R; Powell, B. (1998). Guidelines for Sampling and Analysis of Lowland Acid
Sulfate Soils (ASS) in Queensland 1998. Resource Sciences Centre, Indooroopilly: Department of
Natural Resources.
Ahern, C. R., McElnea, A. E., & Sullivan, L. A. (2004). Acid Sulfate Soils Laboratory Methods
Guidelines. In Queensland Acid Sulfate Soils Manual 2004. Indooroopilly, Queensland, Australia:
Department of Natural Resources, Mines and Energy.
AS 2870. (2011). Residential Slabs and Footings. Standards Australia.
AS 4678. (2002). Earth-retaining structures. Standards Australia.




Proposed Residence                                                                      95120.00.R.001.docx
33 Wyuna Drive, Noosaville                                                                     October 2020