Slope Stability Assessment Existing Earthworks - OPW21-0279

Anthony Kelly                                                                                               21354-001-Rev0
                                                                                                         17 September 2021

Table 1: 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
 L      Low 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.

Considering the existing site conditions (Ref. Section 2), and proposed development works, it is assessed that
there would be a Very Low (VL) Risk of global slope instability affecting the existing earthworks 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

                                                                             The likelihood of a failure through the existing cut
                                                                             batter or fill embankment faces created as part of
                                                                             the recent earthworks is considered Rare, given the
                                                                             minimal depth/height of the works, shallow finished
 1: Failure through cut or fill                                              inclinations, revegetation of permanent slopes, and
                                     Rare      Insignificant      Very Low
embankments                                                                  drainage installed to date. The consequence of such
                                                                             a failure would be Insignificant given the offset to
                                                                             existing structures upslope and downslope and very
                                                                             limited scope of remediation, with the resultant risk
                                                                             being 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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