Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                               PROPOSED RESIDENCE
                               33 ROSS CRESCENT, SUNSHINE BEACH

5.0      SLOPE STABILITY ASSESSMENT
5.1      Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. Text Figure 10) which
indicates that the site, and most of the main dune below, have Moderate and High Hazard Areas.




                                                    Subject site




Text Figure 10: Extract from NC landslide hazard overlay mapping

It should be noted that the NC landslide hazard mapping is a broad-scale indication of the potential
landslide susceptibility and is superseded by this site-specific assessment. The allotment did not exhibit
any visible surface features which would indicate larger scale significant global slope instability during our
walkover within proposed building areas. No landslide back scarps, noteworthy groundwater seepage or
areas of naturally ‘hummocky’ ground were observed. Medium dense sands were encountered at relatively
shallow depths below surface, with the main dune standing at between 17° and 22°.

Given the presence of very loose to loose sands to depths of in the order of 1.2 m to 2.7 m BGL in slopes
within the site and beyond in the high dune, the sand slopes would be close to the ‘limit of equilibrium’. 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).

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.
Table 2: Stability Risk Levels
 Risk Level            Example Implications(1)
                       Unacceptable without treatment. Extensive detailed investigation and research, planning and
        Very High
 VH                    implementation of treatment options essential to reduce risk to Low; may be too expensive and not
        Risk
                       practical. Work likely to cost more than value of property.
                       Unacceptable without treatment. Detailed investigation, planning and implementation of treatment
 H      High Risk      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 investigation,
        Moderate
 M                     planning and implementation of treatment options to reduce risk to Low. Treatment options to reduce
        Risk
                       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
 L      Low Risk
                       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.



18 June 2024
Report No. 24096-001-Rev0                                    9