Geotechnical Stability Report · OPW24/0017

GMW Projects Pty Ltd                                                                                                            24008-001-Rev0
                                                                                                                                31 January 2024

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 (taken from AGS 2007) is attached at the end
of this report.
Table 1: Stability Risk Levels

 Risk Level                 Example Implications(1)

                            Unacceptable without treatment. Extensive detailed investigation and research, planning and
          Very High
 VH                         implementation of treatment options essential to reduce risk to Low; may be too expensive and not practical.
          Risk
                            Work likely to cost more than value of property.
                            Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
 H        High Risk         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 investigation, planning
          Moderate Risk
 M                          and implementation of treatment options to reduce risk to Low. 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 this level,
 L        Low Risk
                            ongoing maintenance required.
          Very Low Risk
 VL                         Acceptable. Manage by normal slope maintenance procedures.

 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 areas. 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 natural gradients around the site typically range up to about 10°. 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 which can lead to the 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. At this site, the area down slope (to the east) is protected by the
relatively wide Peregian Beach foredune and Reserve Area and therefore, water erosion or significant
disturbance of the dune by natural forces (e.g. from coastal erosion), in the vicinity of the site, is considered
unlikely over the design life of the house.

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 extensions and pool. Regulators (NC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in Table
2 overleaf is our qualitative assessment of landslide risk for the development site. A summary of qualitative
terminology for use in assessing risk to property is attached (taken from AGS 2007).




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