24-01-2020 - Appeal No. 245 of 2019 - Final Order of Judge Rackemann · 51987.2951.01

McInnes Wilson Lawyers


EXECUTIVE SUMMARY
This report presents the results of a geotechnical investigation and slope stability assessment for a proposed
swimming pool at 42 Seaview Terrace, Sunshine Beach, Queensland. This report includes information that
specifically addresses the joint information requests made by the nominated coastal engineering experts on
11 August 2019 and 2 September 2019, being:
             1.       The stability implication of a horizontal erosion cut of 60 metres measured inland from
                      the plan position of Highest Astronomical Tide (i.e. 1.16m AHD) associated with a
                      storm event (i.e. combination of a 100 year ARI storm tide level and 100 year ARI
                      wave condition) occurring with a sea level rise of 0.3 metres by the year 2050.
             2.       The likelihood of sea erosion extending 60 m inland taking into
                      account the underlying soil conditions. Specifically addressing the
                      effect of the underlying soil and rock profiles on the extent of
                      erosion.
             3.       Design of the piling and footing details for the swimming pool are
                      to be shown on a drawing. The piling details must demonstrate
                      how the pool will withstand the erosion cut detailed in item 1
                      above.

In summary, the pool may be constructed with an adequate Factor of Safety (FOS) against slope instability
provided the recommendations given in this report are followed.
The geotechnical investigation fieldwork comprised seismic testing, borehole drilling and in-situ testing
(standard penetration and dynamic cone penetration tests).
The ground investigation results indicate that the upper portion of the dune, in the vicinity of the existing
residence/proposed pool, comprises about 1 m to 2 m depth of very loose or loose sand from the ground
surface, over medium dense to very dense sand. The sand shows some variability in density but appears to
generally increase in density with depth. At about 8 m to 10 m below ground level (BGL), the sand typically
becomes very dense and weakly cemented in parts. Groundwater seepage was encountered in borehole
BH1 at about 8.5 m BGL during drilling.
The lower portion and toe of the dune generally encountered about 1 m to 2 m depth of very loose or loose
sand from the ground surface, over medium dense sand. From about 3 m to 4 m BGL, the sand becomes
dense with zones of very dense sand (including inferred cemented sand ‘coffee rock’) and increases in
density with depth. The seismic testing results indicate that the coffee rock is consistently present at the toe
of the dune below the site, but may vary in thickness and density. Coffee rock is generally significantly more
resistant to erosion than typical beach or dune sands. This is evidenced by its long term (years) exposure at
around mean sea level at many locations along the sunshine coast.
Shear-wave velocity values and patterns typically associated with weathered sedimentary rock were
encountered from depths ranging between about 12 m to 20 m BGL. The testing indicates that the rock dips
towards the ocean and that its strength generally increases with depth.
Details of the investigation test locations and the ground conditions are given in Figures 1 and 2 and in
Appendix A.
Quantitative slope stability modelling was undertaken to analyse factor of safety (FOS) values for the sand
dune and the proposed development, as follows:

           ▪ Case 1: Vertical cut into the slope caused by erosion and subsequent failure of
                  the slope above the cut. The horizontal extent of erosion was based on
                  information provided by Omtrek Pty Ltd.
           ▪ Case 2: Lower slope failure following reprofiling of the slope due to the
                  lower failure/erosion from Case 1.
           ▪ Case 3: Retrogressive upper slope failure following reprofiling of the
                  slope due to the lower failure from Case 2.
           ▪ Case 4: Slope analysis following reprofiling of the slope from Case 1 and
                  Case 2, with the proposed pool supported on micro-piles (10 m in length and
                  with 100 kN shear force resistance).

J000867-003-R-Rev0                                     053
                                                       Page ii
                                                                                                                   62