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

Total Concept Homes                                                                              20332-003-Rev0
                                                                                                    16 June 2022

3.0     ASSESSMENT OF LAND STABILITY
3.1   Existing Conditions
The crest area of the spur running south-west to north-east through the site, which incorporates the area of
the existing residence, has been mapped by NC as presenting a Low/Very Low landslide susceptibility
(clear/no shading). However the initial flanks of the crest, where the granny flat is proposed are mapped as
a Moderate hazard area (pale yellow brown shading). Towards the inverts of gullies away from construction
areas, High risk areas have been assigned (pale orange shading). An extract of the NC mapping is given
below in Text Figure 9.

                                                                                  Lot boundary




                                   Approx. granny flat site




Text Figure 9: Extract from NC Landslide Hazard Overlay Mapping

It should be noted that the NC map is a broad-scale indication of the potential landslide susceptibility only,
largely based on ground slope, and is superseded by this site-specific assessment.

The site does not exhibit any indicators of global, natural slope instability. No landslide back scarps or
areas of natural ‘hummocky’ ground were observed, the existing structures on site were in good condition,
with only minor erosion features observed over the existing access track alignment, or other earthworks
features (e.g. horticultural terraces, granny flat earthworks pad). In addition, the subsurface conditions
were favourable with stiff to very stiff soils present from near surface, and weathered phyllite rock logged
at relatively shallow depth in both the boreholes and exposed in earthworks cuttings.

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 on the
following page and the Qualitative Terminology for Use in Assessing Risk to Property is also attached.




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