081128 - Lots 5 - 8 Soil Tests · OPW18/0155

                                 LOTS 5 TO 8, SETTLERS COVE, SERENITY CLOSE, NOOSA



Some fill comprising sandy gravel was encountered in borehole BH1 to 0.4 m below ground level (BGL). Silty
sand was encountered in boreholes BH2, BH3 and BH14 to depths of between 0.4 m and 0.5 m BGL.
Sandy gravel was encountered in borehole BH3 from the ground surface to 0.2 m BGL.
Groundwater was not encountered in any of the boreholes at the time of the investigation.
These results are similar to those from previous investigations on Lots 2 and 3, nearby (ref. Golder Reports
06682026(AA) and 97680045(B)).



5.0      COMMENTS AND RECOMMENDATIONS
5.1      Slope Stability
The steep dune face in the northern part of the site is described as a ‘Sand Dune Problem Area’ in Noosa
Council’s Planning Scheme.
Details on the stability of the site have been provided in earlier geotechnical reports by Golder Associates for
the ‘Settlers Cove’ development (Ref. 91680133). These previous reports indicate that the steep dune face
is marginally stable. The existing steep slope of the dune face (up to about 35º) appears to be maintained at
more than the normal angle of repose for loose sand (about 27º) by the mechanical binding action of the
roots associated with the dense cover of vegetation. The edge of the development area, near the crest of
the steep slope, corresponds to the Zone 1/Zone 2 boundary as defined in previous reports. Zone 1 includes
all the steep dune face, plus a margin on the uphill side of the crest to allow for the unlikely event that three
subsequent and effectively cumulative translational slide failures develop down the dune face. The
remaining near level areas are Zone 2.
In accordance with previous practice in the area, it is recommended that all structures are founded below a
plane rising at 27º from the toe of the steep dune face (Ref. Figure No. 2). For this proposed development,
the position and levels are such that relatively shallow foundations will be suitable. Provided the dune face is
managed as detailed in earlier reports (eg. retain existing vegetation), it is our opinion that there is a Very
Low to Low Risk of slope instability affecting the proposed development and that the development should
not detrimentally affect the stability of the dune face. Subsurface disposal of stormwater via RIPs with
outlets at least 2 m below the ground surface must not occur within 5 m of the crest of the dune face (ref.
Figure No. 1). The risk assessment for this project has been carried out following the Australian
Geomechanics Society (AGS) Guidelines for Landslide Risk Management (March 2007). Relative levels of
risk and their implications are given in Table 1 below.




December 2008
Report No. 087682058 002 Rev.1                         2