Attachment 5 - Geotechnical Report · MCU18/0015

J000635-002-R-Rev1                                                                                                 November 2018


7.1       Qualitative Slope Stability Assessment
A qualitative slope stability risk assessment for this project has been carried out following the Australian
Geomechanics Society (AGS) Guidelines for Landslide Risk Management (March 2007). The risk
assessment relates to property. Relative levels of risk and their implications are given in Table 5 below and
the qualitative terminology (i.e. likelihood and consequence definitions) for use in assessing risk to property
are attached in Appendix D. Our risk assessment is summarised in Table 6.
Table 5: Stability Risk Levels.
Risk Level                          Example Implications(1)

                                    Unacceptable without treatment. Extensive detailed investigation and research,
                 VERY HIGH          planning and implementation of treatment options essential to reduce risk to Low;
VH
                 RISK               may be too expensive and not practical. Work likely to cost more than value of
                                    property.
                                    Unacceptable without treatment. Detailed investigation, planning and implementation
H                HIGH RISK          of 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
                 MODERATE           requires investigation, planning and implementation of treatment options to reduce
M
                 RISK               risk to Low. Treatment options to reduce to Low risk should be implemented as soon
                                    as practicable.
L                LOW RISK           Usually acceptable to regulators. Where treatment has been required to reduce the
                                    risk to 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.


Table 6: Qualitative Risk Assessment for Property (Based on AGS 2007).
                                                                Assessed
          Hazard             Likelihood     Consequence                                           Comments
                                                                  Risk
                                                                               Shallow translational failure in the loose sands
     Shallow failure of                                                        is considered Possible, but if the
      soils above the                                                          recommendations in this report are
                              Possible       Insignificant      Very Low
    footing zone (< 3m                                                         implemented, the consequence to property is
           depth)                                                              assessed as Insignificant. The resulting
                                                                               assessed risk is Very Low.
                                                                               The likelihood of a deep failure through the
                                                                               dense (or denser) sand or stiff (or stiffer)
                                                                               colluvium/residual soil is assessed as Rare
       Deeper failure
                                                                               based on the results of the 2003 slope stability
    through soils below         Rare             Major             Low
                                                                               modelling (03632248-001) and the slope
       footing depth
                                                                               profile. Although the consequence of such a
                                                                               failure would likely be Major, the resultant risk
                                                                               is Low.
    Refer Appendix C, Qualitative Risk Analysis Matrix (Australian Geomechanics Vol 42 No 1 March 2007).

Based on the available information and the results of the investigation, it is our opinion that there is a Low to
Very Low Risk of slope instability for the proposed development assessed in accordance with the AGS
guidelines (March 2007).

7.2       Slope Stability Modelling
Slope stability modelling of the proposed access driveway fill and retaining structure was carried out using
Geostudio’s Slope/W software.
The model section was based on an assumed ‘worse case’ profile (i.e. deepest fill and steepest slope), from
the Preliminary Engineering Report (C5756/17/ENG Rev A) by Walsh Professional Engineers.
Groundwater was only encountered in test pit TP5 (2003) at the interface between the soil and rock (outside
the currently proposed development layout area). No groundwater was encountered in the other test pits or



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