210533 - 108 Black Mountain Range Road, BLACK MOUNTAIN - Slope Stability Report

Total Concept Homes                                                                                             20332-003-Rev0
                                                                                                                   16 June 2022

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 granny flat, it is assessed that there
would be a Low (L) Risk of global slope instability affecting the proposed development 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 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 settlement
due to consolidation of any uncontrolled fill materials over time which could affect non-piled structures (e.g.
pavements, landscaping structures). This risk assessment also excludes geotechnical risk associated with
the existing structures and areas of the site outside of the proposed development area.
Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007
                                                                     Assessed
 Hazard                               Likelihood   Consequence
                                                                       Risk
                                                                                Comments

                                                                                Given the relatively shallow depth of fill (1.2 m),
                                                                                presence of stiff (or stiffer) residual soils below, the
                                                                                rounded nature of the spur crest where construction
 1: Proposed granny flat -                                                      is proposed, apparent lack of groundwater, and no
Failure through earthworks fill, or                                             evidence of existing instability in the vicinity of the
                                       Unlikely      Medium            Low
natural soils above the                                                         proposed granny flat it is considered Unlikely that a
foundation depth                                                                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 extremely
                                                                                weathered materials or weathered phyllite below the
 2: Proposed granny flat -                                                      foundation depth is assessed as Rare due to the
Deep seated failure in extremely                                                strength of these materials (based on drilling refusal),
weathered materials or                  Rare          Major            Low      no evidence of existing instability, and relatively
weathered phyllite below the                                                    consistent nature of the sloping ground across the
foundation depth                                                                development area. Although the consequence of such
                                                                                a failure could be Major, the resultant risk is 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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