Slope Stability Assessment

Jane & Cameron Blake                                                                                           22178-001-Rev0
                                                                                                                  26 May 2022

 Table 1: Stability Risk Levels

 Risk Level              Example Implications(1)
                         Unacceptable without treatment. Extensive detailed investigation and research,
        Very High
 VH                      planning and implementation of treatment options essential to reduce risk to Low; may
        Risk
                         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 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
 L      Low Risk
                         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 subject to the implementation of
recommendations given in Section 4, it is assessed that there would a Low (L) Risk of global slope instability.
This is provided a suitable soil test report, building plans and earthworks drawings are reviewed by Tectonic
prior to construction. Regulators (NC) normally require that a Very Low or Low Risk of landslide affecting
property must be demonstrated to enable development approval. Summarised in Table 2 below 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).
It should be noted that this risk assessment excludes geotechnical risk that may be associated with the
existing structures, dams and areas of the site outside of the proposed development areas.
 Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)


                                                                     Assessed
 Hazard                               Likelihood   Consequence
                                                                       Risk
                                                                                Comments



                                                                                The likelihood of a failure through future fill or natural
                                                                                residual soil materials impacting the proposed
                                                                                development is assessed as Unlikely due to the low
                                                                                to moderate ground slopes across the proposed
 1: Shallow failure through future
                                                                                development area, favourable topography and
fill or natural soils above the        Unlikely       Minor            Low
                                                                                apparent lack of groundwater. The consequence of
foundation depth
                                                                                such a failure would be Minor considering the
                                                                                anticipated confined extent of instability, and the depth
                                                                                of foundations recommended, with the resultant risk
                                                                                being Low as per AGS 2007.


                                                                                The likelihood of a deep failure through very stiff residual
                                                                                soils or weathered phyllite below the foundation depth is
 2: Deep failure through very stiff
                                                                                assessed as Rare due to the strength of these materials
(or stiffer) residual soil or
                                        Rare          Major            Low      and no signs of previous deep-seated instability across
weathered phyllite below the
                                                                                the proposed development areas. Although the
foundation depth.
                                                                                consequence of such a failure could be Major, the
                                                                                resultant risk is Low as per AGS 2007.


The potential impacts of slope instability on the proposed development have been assessed, and the
measures recommended below in Section 4 have been designed to mitigate those impacts so that a Low Risk
of instability would be achieved for the proposed secondary dwelling.




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