4895-Geotech Assessment

Mr Max Richardson                                                                                                   20362-001-Rev0
C/- Long View Consulting Engineers Pty Ltd                                                                             28 July 2021

3.2    Qualitative 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 which is attached.
Table 1: Stability Risk Levels
 Risk Level               Example Implications(1)
                          Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation
        Very High
 VH                       of treatment options essential to reduce risk to Low; may be too expensive and not practical. Work likely to
        Risk
                          cost more than value of property.

                          Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options
 H      High Risk
                          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, planning
 M      Moderate Risk     and implementation of treatment options to reduce risk to Low. 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 this level, ongoing
 L      Low Risk
                          maintenance required.

 VL     Very Low Risk     Acceptable. Manage by normal slope maintenance procedures.

 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 proposed development works, it is assessed that
there would be a Low (L) Risk of global slope instability affecting the proposed driveway and house site as
described in Section 1.1 above, on new Lot 429 subject to the implementation of recommendations given
below. 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 site. A summary of qualitative terminology for use in assessing risk to property is
attached (taken from AGS 2007).
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007
                                                                     Assessed
 Hazard                             Likelihood     Consequence
                                                                       Risk
                                                                                    Comments

                                                                                    Given the presence of stiff residual soils at surface,
                                                                                    and phyllite materials below at shallow depth, the
                                                                                    apparent lack of groundwater, and no evidence of
 1: Failure through natural soils                                                   existing instability in the proposed development
or proposed fill materials at        Unlikely         Medium             Low        areas, it is considered Unlikely that a shallow
relatively shallow depth (<2 m)                                                     landslip would develop, subject to implementation of
                                                                                    the recommendations in Section 4. With a potential
                                                                                    Medium impact from such shallow instability the
                                                                                    resultant risk would be Low as per AGS 2007.


                                                                                    The likelihood of a deep failure through the phyllite
                                                                                    rock is assessed as Barely Credible due to the
                                                                                    strength of these materials, no evidence of such deep
 2: Deep seated failure in            Barely
                                                        Major         Very Low      instability, and the favourable topography across the
weathered rock                       Credible
                                                                                    development area. Although the consequence of such
                                                                                    a failure could be Major, the resultant risk is Very Low
                                                                                    as per AGS 2007.




The potential impacts on slope stability of the development components have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts.




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