Geotechnical Report (1) · MCU21/0212

                             PROPOSED RESIDENCE
                             66 SEAVIEW TERRACE, SUNSHINE BEACH

       Case 1 – Vertical cut into the toe of the sand dune slope caused by erosion scouring, and allowing
        for a 500 year average recurrence interval (ARI) storm event.
       Case 2 – Slope failure following reprofiling of the slope due to the failure/erosion from Case 1.
       Case 3 – Retrogressive upper slope failure following reprofiling of the slope due to the Case 2
        failure.
       Case 4 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern property boundary protected by a micro-pile (or similar) structure (piles min 7 m in length
        and with 120 kN shear force resistance).
       Case 5 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern edge of the residence protected by a micro-pile (or similar) structure (piles min 6 m in length
        and with 120 kN shear force resistance).
The modelled ground surface profile for the initial Case 1 was based on provided survey information across
the subject site, with surface profiles beyond the eastern property boundary augmented with slope angles
and measurements in the field by Tectonic. The horizontal extent of erosion adopted in Case 1 was based
on information provided in the referenced RHDHV report, which included a combination of short-term erosion
from a 500 year ARI storm event and shoreline recession due to sea level rise for the 2100 climate extreme
event scenario.

Subsurface conditions for Cases 1 to 5 were developed using the borehole and DCP information collected
during the investigation.

A surcharge load of 5 kPa was applied across crest of the dune to the eastern boundary to replicate future
landscaping fill. Building loads for the proposed residence and ancillary structures were included in the
modelling, given the height of the structure proposed (three full storeys above ground to the seaward side),
but will largely be transferred by a piled footing system into the natural medium dense (or denser) sands at
depth and would be anticipated to have a partial surcharge effect on the ground surface.
The soil parameters adopted for the analyses are summarised in Table 4 and are based on interpretation of
the site investigation, in situ testing and laboratory test results. The parameters are consistent with the
range of values recommended for similar materials in Appendix D of Australian Standard AS4678-2002
Earth Retaining Structures. Due to the relatively steep slopes and high permeability soils, a groundwater
table is not anticipated in the slope and therefore, was not modelled.
Table 4: Soil Parameters Adopted for Slope/W Analyses
                                                                                        Friction Angle, ɸ
  Material Description        Unit Weight (kN/m3)           Cohesion, c (kPa)
                                                                                            (degrees)


    Very Loose/Loose
                                        17                           0                          27
          Sands


  Medium Dense Sands                    18                           0                          34


 Dense (or denser) Sand                 19                           0                          36


  Hard Clays (Residual)                 19                           7                          30




Commonly adopted industry standards generally require minimum FOS values against slope instability of 1.3
and 1.5 for short-term and long-term conditions, respectively. Administering authorities such as Noosa
Council normally require a minimum FOS of 1.5 or greater for long-term conditions. Results of the stability
analyses are presented in Table 5 overleaf, with the Slope/W outputs attached in Figures 3 to 7 at the end of
the report.


23 November 2021
Report No. 21370-001-Rev0                             13