21054-001-Rev0 Tristania Dr

Peter Williams                                                                                          21054-001-Rev0
                                                                                                       25 February 2021

It should be noted that the NSC natural hazard overlay mapping is a broad-scale indication of the potential
landslide susceptibility only, and is superseded by this site specific assessment. It is inferred that the
landslide hazard rating is a result of natural ground slopes exceeding 15%.

At the time of our visit, no signs of global slope instability were observed across the site or surrounding
properties. No landslide back scarps, groundwater springs, or areas of naturally ‘hummocky’ ground were
observed. Tilting trees were noted on the subject site which can be an indicator of shallow soil creep.
However, the allotment and areas immediately around did not exhibit any indicators of larger scale global
slope instability.

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.
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
overall slope across the proposed development area is generally in the order of 10˚ to 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).
Steep sand dune slopes generally remain marginally stable (i.e. FOS just 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
          when the ‘toe’ of the dune is not disturbed.

Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Areas outside of structures and concrete ground slabs are generally
covered by vegetation being either grass, shrubs or trees. Discharge of stormwater onto dune slopes may
cause channel erosion, undermining and disturbance of vegetation, resulting in development of generally
localised instability. Stormwater runoff must be collected (or re-directed) and discharged from the site as
discussed in Section 4.5 below.

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. Sand dune instability due to undercutting of the toe generally
occurs as a translational-type slide where a ‘slab’ of sand on the dune face slides down slope. The depth of




                                                            6/11