20220109 - Geotech · OPW23/0182

                                     PROPOSED RESIDENCE
                                     21 MAHER TERRACE, SUNSHINE BEACH

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 C.
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.

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 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 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).
Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)
                                                                  Assessed
 Hazard                               Likelihood   Consequence      Risk      Comments

                                                                              Given the low to moderate natural gradients through
                                                                              the proposed building area, it is considered Unlikely
                                                                              that a landslip would develop in these materials,
 1: Failure through residual soils
                                       Unlikely      Medium         Low       subject to implementation of the recommendations in
above the foundation depth
                                                                              Section 4. Given a potential Medium impact from
                                                                              such instability the resultant risk would be Low as per
                                                                              AGS 2007.


                                                                              The likelihood of a deep failure through the residual soils
                                                                              or weathered sandstone below the proposed foundation
 2: Deep failure through the
                                                                              depth is assessed as Rare due to the lack of precedence
residual soils or weathered
                                        Rare          Major         Low       of such movements locally, no signs of any existing slope
sandstone below the foundation
                                                                              instability and a lack of groundwater. Although the
depth
                                                                              consequence of such a failure would be Major, the
                                                                              resultant risk is Low as per AGS 2007.

                                                                              The likelihood of a slip/failure through the temporary
                                                                              retention system (sheet pile wall, or similar) as outlined in
 3: Temporary instability through                                             Section 6.5, is assessed as Rare, subject to detailed
solder/sheet pile temporary                                                   design by a suitable qualified RPEQ engineer and
                                        Rare          Major         Low
retention system, or similar, as                                              construction by a specialist contractor which accounts for
discussed in Section 6.5.                                                     the required factors of safety against slope instability.
                                                                              Although the consequence of such a failure would be
                                                                              Major, the resultant risk is Low as per AGS2007.




16 July 2021
Report No. 21236-001-Rev0                                     8