RFI - slope stability reporting

RECOR Pty Ltd                                                                                                                   22393-001-Rev0
                                                                                                                                26 October 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 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
 M        Moderate Risk 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.

 VL       Very Low Risk 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 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 natural gradients ranged up to about 12°. 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. As the downslope allotment is occupied by an existing
residential structure, the potential for significant erosion of the toe of the dune is considered negligible,
provided that future disturbances (e.g. excavations etc.) are managed with sufficient engineering
consideration by the downslope owner. It should be noted that this assessment excludes geotechnical risk
associated with the unretained cutting adjacent to the north-eastern corner of the site, which should be
retained with an engineered designed and certified structure as soon as possible by the downslope property
owner.

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 (NC) normally require that a Very Low or Low Risk of
landslide affecting the property must be demonstrated to enable development approval. Summarised in




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