slope stability · MCU22/0172

Integra Partnerships                                                                                                22339-001-Rev0
                                                                                                                 13 September 2022

It should be noted that the NC landslide hazard map is a broad-scale indication of the potential landslide
susceptibility and is superseded by this site specific assessment. No visual signs of global slope
instability were observed in the location of the proposed residences. No landslide back scarps,
groundwater springs, or areas of naturally ‘hummocky’ ground were observed.

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 implementation of
 VH       Very High Risk   treatment options essential to reduce risk to Low; may be too expensive and not practical. 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 and
 M        Moderate Risk    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, ongoing
 L        Low Risk
                           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 gradients ranged up to about 15°. 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. At this site, the dune around and below the development area is
covered by existing structures, retaining walls and a road (Grant Street); and therefore water erosion or
significant disturbance of the dune by natural forces, in the vicinity of the site, is considered extremely 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 (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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