RFI - 21370-001-Rev1 66 Seaview Tce

                                     PROPOSED RESIDENCE
                                     66 SEAVIEW TERRACE, SUNSHINE BEACH

Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)

                                                                   Assessed
 Hazard                               Likelihood   Consequence
                                                                     Risk
                                                                              Comments


                                                                              Given the low to moderate natural gradients through
                                                                              the proposed building area (8° across majority of
                                                                              building area, 18° to 22° towards eastern boundary),
 1: Failure through very loose to
                                                                              retention system proposed and protection of eastern
loose natural sands above the
                                       Unlikely      Medium          Low      side of site using a shear pile wall it is considered
foundation depth across the
                                                                              Unlikely that a landslip in these materials would affect
building area
                                                                              the proposed development. Given a potential Medium
                                                                              impact from such instability the resultant risk would be
                                                                              Low as per AGS 2007.


                                                                              The likelihood of a deep failure through the medium
                                                                              dense (or denser) natural sand below the proposed
                                                                              foundation depth is assessed as Rare due to the lack of
 2: Deep failure through the                                                  precedence of such movements locally, no signs of any
medium dense (or denser) sands          Rare          Major          Low      existing slope instability, apparent lack of groundwater,
below the foundation depth                                                    and relatively shallow depth to dense (or denser) sands
                                                                              below this intermediate unit. Although the consequence of
                                                                              such a failure would be Major, the resultant risk is Low as
                                                                              per AGS 2007.

                                                                              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
 3: Instability of temporary sheet
                                                                              suitably qualified RPEQ engineer, which accounts for the
pile retention system, or similar,      Rare          Major          Low
                                                                              road embankment fill, required factors of safety against
as discussed in Section 6.4.
                                                                              slope instability, and construction by a specialist
                                                                              contractor. Although the consequence of such a failure
                                                                              could be Major, the resultant risk is Low as per AGS2007.

                                                                              The likelihood of a slip/failure through the future micro-pile
                                                                              wall system along the eastern boundary as outlined in
                                                                              Section 6.0, and advised by the coastal erosion specialists
 4: Retrogressive failure of the
                                                                              (RHDHV); is thus assessed as Barely Credible, subject to
high dune face considering a
                                       Barely                                 detailed design by a suitably qualified RPEQ engineer,
hypothetical storm and wave                           Major        Very Low
                                      Credible                                construction by a specialist contractor, and considering the
scenario, impacting the subject
                                                                              results of our quantitative slope stability analysis outlined
site.
                                                                              in Section 5.2.1. Although the consequence of such a
                                                                              failure would be Major, the resultant risk is Low as per
                                                                              AGS2007.


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 preliminary design drawings, site survey information and slope angles measured local to
the site by Tectonic. The analysed subsurface profile was compiled based on the results of our subsurface
investigation. Some inferences regarding subsurface conditions were made between test locations. A
minimum FOS value of 1.5 is the commonly adopted industry standard for long term conditions.

Considering the moderate gradients recorded in the eastern part of the site, progressing closeby to the main
steep dune face, and the presence of loose sands, the stability of the lower part 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 from
erosion by high seas, could lead to large scale instability.

Modelling has therefore been carried out considering a hypothetical storm and wave scenario nominated by
coastal engineering experts (RHDHV) in their report (Ref. Section 1.0). Each of the cases analysed are
summarised on the following page:


28 April 2022
Report No. 21370-001-Rev1                                     12