220328 - Edwards Street - Slope Stability Assessment

Bev Jenner Const.                                                                                       22073-001-Rev0
C/- Westera Partners                                                                                     25 March 2022

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 to
 L       Low Risk
                       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 was generally in the order of 8˚ to 9˚. 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 supported surface 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 (or intermediate
steepenings over the face) 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 such sliding may range up to a few metres during one event,
but on-going erosion can cause retrogressive failure of dune slopes leading to multiple sliding events, if left
unchecked. The risk such an occurrence at this site is minimal, particularly considering the moderate ground
surface gradients and the protection afforded by the road infrastructure downslope of the site.
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 deples. 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 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).

This slope stability assessment excludes geotechnical risk that may be associated with settlement due to
consolidation of loose sands over time which could affect non-piled structures (e.g. pavements, driveways,
landscaping structures).




                                                            8/14