Geotech Report - techtonic · RAL22/0021

                               PROPOSED RESIDENTIAL DEVELOPMENT
                               39 PICTURE POINT CRESCENT, NOOSA HEADS

5.2        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 2 and the
Qualitative Terminology for Use in Assessing Risk to Property is also attached in Appendix B.
Table 2: 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.

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’ where the surface
gradient is about 30°, although often lower (26˚ to 28˚) for very loose sands. Our measurements indicate that
the overall natural gradient is around 30°, although ranging to about 34° downslope around the northern site
boundary. 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 layer more resistance to erosion, 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 toe of the dune is over 30 m from the northern
boundary, and is protected to a large degree by the presence of the Noosa Drive road corridor. The sealed
surface of the road, and formalised drainage conditions would make the likelihood of water erosion or
significant disturbance of the dune by natural forces, in the vicinity of the site 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 residential 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 Table 3 overleaf 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).



10 February 2023
Report No. 22511-001-Rev0                                   9