ECM_22343520_v1_Geotechnical Investigation · 132000.200173.1

                                                  PROPOSED RENOVATIONS
                                                  2 ARAKOON CRESCENT, SUNSHINE BEACH


                                                                                    The likelihood of a slip/failure of a temporary retention
                                                                                    system (sheet pile wall, or similar) as outlined in Section
                                                                                    6.3, is assessed as Rare, subject to detailed design by a
                                                                                    suitably qualified RPEQ, which accounts for the required
                                                                                    factors of safety against slope instability, and construction
              3: Instability of temporary sheet                                     by a specialist contractor. Although the consequence of
             pile retention system, or similar,     Rare    Major         Low       such a failure could be Major, the resultant risk is Low as
             as discussed in Section 6.3.                                           per AGS2007.

                                                                                    Note that this assessment relates to the construction of a
                                                                                    new temporary retaining wall between the upper and lower
                                                                                    levels of the building. No assessment has been made of
                                                                                    any elements in loco within the existing structure.



            The potential impacts on slope stability of the development components have been assessed, and the
            measures recommended below in Section 4 have been designed to mitigate those impacts.

            5.2.1         Quantitative Assessment
            In order to estimate the factor of safety (FOS) against slope instability for the proposed development,
            numerical analysis has been conducted using Slope/W. The ground surface profile has been modelled using
            a combination of architectural layout drawings, survey information, NC interacting mapping and slope angles
            measured in the field by Tectonic. The analysed subsurface profile was compiled based on the results of the
            ASI boreholes and our subsurface investigation. Some inferences regarding subsurface conditions were
            made between test locations, particularly across the western part of the site which is occupied by the existing
            building. A minimum FOS value of 1.5 is the commonly adopted industry standard for long term conditions.

            Considering the steep gradients to the eastern edge of the site, and the presence of loose sands; the
            stability of the existing high dune slope is considered marginal. The slope is probably held in place by the
            stabilising effects of negative soil pore pressures and shallow root systems. Any disturbance to the lower
            part of the dune slope, such as erosion by high seas, could lead to larger scale instability.

            Modelling considered a hypothetical storm and wave scenario nominated by coastal engineering experts
            RHDHV, which included a combination of short-term erosion from a 500-year average recurrence interval
            storm event and shoreline recession due to sea level rise for the 2100 climate extreme event scenario.
            Section 7 of the coastal erosion assessment indicates that the existing house site and proposed
            development is not likely to be impacted by beach erosion under this scenario. In accordance with
            previously adopted practices along the Sunshine Beach coastal strip, a 28° line was modelled from the
            current dune toe which does not intersect the eastern boundary or proposed development area. Text Figure
            11 overleaf illustrates our geological model of the site including the extreme storm and wave scenario
            nominated by RHDV and the 28° line from the high dune toe.




             24 February 2023
             Report No. 22484-001-Rev0                              11
Document Set ID: 22343520
Version: 1, Version Date: 16/03/2023