Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                             PROPOSED RESIDENCE
                             33 ROSS CRESCENT, SUNSHINE BEACH

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
    Future Granular Fill                 18*                          0*                           33*
* Depends on type of fill used and compaction achieved

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, with the Slope/W outputs attached in Figures 3 to 7.
Table 5: Slope/W Analysis Results

   Analysis                                    Description                                    Factor of Safety

                 Lateral dune failure caused by storm erosion event
 Case 1b                                                                                             1.7
                 (Site protected by shear pile wall)
 Case 2          Lower slope failure due to regression from Case 1                                   1.0

 Case 3          Retrogressive upper slope failure following the Case 2 failure                      0.8
                 Slope stability analysis following regression of the slope from
 Case 4a         Case 1 to Case 3, with the eastern edge of the building area                        1.6
                 protected by a shear pile wall
                 Slope stability analysis following regression of the slope from
 Case 4b         Case 1 to Case 3, with the eastern property boundary protected                      1.5
                 by a shear pile wall

Results of the analyses indicates the following:

       Case 1 – It was noted that the storm erosion cut would affect the eastern part of the site (including
        impacting the proposed main residential building). A residual FOS of less than 1.0 was shown in that
        scenario, and as such the model for Case 1 includes protection of the eastern edge of the building
        with a piled wall, a FOS of 1.7 is assessed following this type of event.
       Case 2 – The over steep face of the dune would lead to retrogressive failures progressively moving
        up slope. The nature of the failures will be dependent on numerous factors such as surface soil
        stabilisation provided by vegetation and local variations in subsurface conditions (e.g. indurated sand
        or dense sand zones), however the modelled slip surface is consistent with previous experience in
        similar conditions. The FOS of 1.0 indicates that the slope would be in a marginal state of stability
        following Case 1.
       Case 3 – Following retrogressive translational failure(s), further instability of the slope extending up to
        the eastern property boundary would occur based on the minimum calculated FOS of 0.8.
       Case 4 – If the eastern edge of the proposed building (Case 4a) or property boundary (Case 4b) is
        protected by an engineer designed and certified shear pile structure the potential critical slip surface
        takes the form of a deep-seated failure into the underlying dense (or denser) sands. Such a slip
        surface results in a FOS above 1.5, allowing for retrogression of the slope from Cases 1 to 3
        (including a slight landward migration of the dune toe). It is noted that the position of the shear pile
        wall would be dependent on the future maintenance and expectation of land use to the east of the
        property. The pile wall structure would be significant, with deep piles (up to 12.5 m BGL) and a
        concrete capping beam.

18 June 2024
Report No. 24096-001-Rev0                               12