Appendix4-SlopeStabilityAssessment · MCU18/0135

Structures Engineering Design                                                                           18197-001-Rev0
                                                                                                          6 August 2018

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 below and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached.
 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
 L        Low Risk
                         to 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.

Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°. 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). Site mapping results
indicate that the maximum natural slope is about 12°, therefore landslide activity would be unlikely to occur
subject to adoption of conventional hillside construction practices.

Conventional construction practices on sand dune slopes include:
          Maintaining and/or reinstating vegetation coverage outside of building areas (e.g. mix of ground
           cover, shrubs and small trees);
          Avoiding concentrated stormwater flows;
          Providing adequate temporary and permanent support to excavations and fill embankments;
          Limiting additional loads (e.g. fill); and
          Preventing disturbance to the ‘toe’ of the dune (if possible, i.e. within control of the subject site).
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 localized instability. Cut
and fill earthworks have the potential to overload and/or undermine the stability of sand dune, therefore must
be carefully planned and implemented.
Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is our opinion that there would be a Low (L) Risk of global slope instability
affecting the proposed development. Regulators (Noosa Shire Council) 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 below 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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