4. 220148 - Slope Stability Report (2) · 132006.2554.01

Seachange Builders                                                                                      22134-001-Rev0
                                                                                                           26 April 2022

3.2       Stability Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 1 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
Table 1: Stability Risk Levels

 Risk Level             Example Implications(1)
                        Unacceptable without treatment. Extensive detailed investigation and research, planning
          Very High
 VH                     and implementation of treatment options essential to reduce risk to Low; may be too
          Risk
                        expensive and not practical. Work likely to cost more than value of property.
                        Unacceptable without treatment. Detailed investigation, planning and implementation of
 H        High Risk     treatment options required to reduce risk to Low. Work would cost a substantial sum in
                        relation to the value of the property.
                        May be tolerated in certain circumstances (subject to regulators’ approval) but requires
          Moderate
 M                      investigation, planning and implementation of treatment options to reduce risk to Low.
          Risk
                        Treatment options to reduce to Low risk should be implemented as soon as practicable.
                        Usually acceptable to regulators. Where treatment has been required to reduce the risk to
 L        Low Risk
                        this level, ongoing maintenance required.
          Very Low
 VL                     Acceptable. Manage by normal slope maintenance procedures.
          Risk
 Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
 depend on the nature of the property at risk; these are only given as a general guide.

Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ for slope stability where
the surface gradient is about 30° (58%), although often lower (26˚ to 28˚) for very loose sands. Our
measurements indicate that the ground surface gradients around the proposed development area range up to
about 12°. Mapping indicates that the foredune within the reserve area to the east of the site has slopes in
the order of 30° over short distances (up to about 10 m), however the reserve is densely vegetated and about
100 m wide.

The limit of equilibrium is the ‘tipping point’ to instability, where the factor of safety (FOS) of the slope is
approximately 1.0. The FOS of a slope is the balance (ratio) between the forces driving instability (e.g. soil
weight, groundwater pressure) and the resisting forces (e.g. soil friction and cohesion).

Sand dune slopes generally remain stable (i.e. FOS greater than 1.0) when they are:

          well vegetated (mix of ground cover, shrubs and small trees);
          not subjected to concentrated stormwater flows;
          not surcharged with additional loads (e.g. fill); and
          not undercut by excavation or erosion.
Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation, and development of generally localised instability.

The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion, such as due to extreme weather events. At this site, the toe of
the dune (along the eastern property boundary) is setback about 100 m from the beach and protected by a
densely vegetated reserve including a well developed foredune. Therefore, water erosion or significant
disturbance of the dune by natural forces, in the vicinity of the site, is considered unlikely.

Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of global slope instability
affecting the proposed development. Regulators (NSC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in Table




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