RFI - 21370-001-Rev1 66 Seaview Tce

                              PROPOSED RESIDENCE
                              66 SEAVIEW TERRACE, SUNSHINE BEACH

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.
Table 5: Slope/W Analysis Results

   Analysis                                   Description                                       Factor of Safety


                 Vertical cut into the toe of the dune caused by erosion scouring
 Case 1                                                                                                0.7
                 during a high sea event.

                 Lower slope failure following reprofiling of the slope due to
 Case 2                                                                                                0.9
                 failure/erosion from Case 1.

                 Retrogressive upper slope failure following reprofiling of the slope
 Case 3                                                                                                1.2
                 due to the Case 2 failure

                 Slope stability analysis following reprofiling of the slope from
 Case 4          Case 1 to Case 3, with the eastern property boundary protected                        1.5
                 by a micro-piled (or similar) wall.
                 Slope stability analysis following reprofiling of the slope from Case
 Case 5          1 to Case 3, with the eastern edge of the residence protected by a                    1.5
                 micro-piled (or similar) wall.


Results of the analyses indicates the following:

       Case 1 – The assessed FOS of 0.7, being much less than 1.0, indicates a slope failure. A vertical
        erosion cut would lead to retrogressive failures moving up slope, primarily within the very loose to
        medium dense sands. 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 zones), however the modelled slip surface is consistent with previous experience in
        similar conditions.
       Case 2 – The FOS of 0.9 indicates that further slope failure is likely following reprofiling of the slope
        due to erosion/failure from Case 1.
       Case 3 – Following reprofiling of the slope from retrogressive translational failure(s), further instability
        of the slope extending under the eastern property boundary is at high risk of occurring based on the
        calculated FOS of about 1.2.
       Case 4 – If the eastern boundary is protected by an engineer designed and certified micro-pile (or
        similar) structure the potential critical slip surface takes the form of a deep-seated failure into the
        underlying medium dense (or denser) sands. Such a failure results in a FOS of 1.5 or greater, even
        following a vertical erosion cut and reprofiled slope from Cases 1 to 3.
       Case 5 – If the eastern edge of the proposed residence is protected by an engineer designed and
        certified micro-piled (or similar) wall structure the potential critical slip surface also takes the form of a
        deep-seated failure into the underlying medium dense and/or dense (or denser) sands. Such a failure
        also results in a FOS of 1.5 or greater, including allowance for a vertical erosion cut and reprofiled
        slope from Cases 1 to 3.




28 April 2022
Report No. 21370-001-Rev1                                14