Attachment 3 - Revised Slope Stability Assessment

Brett Hage                                                                                                 22328-001-Rev1
                                                                                                         17 November 2022

Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007
                                                                Assessed
 Hazard                            Likelihood   Consequence
                                                                  Risk
                                                                           Comments


                                                                           Given the presence of stiff residual soils at the
                                                                           natural surface, and stiff to very stiff XW materials
                                                                           below at shallow depth, the rounded nature of the
                                                                           spur crest where construction is proposed, and
                                                                           minimal catchment for water above the site and
 1: Proposed residence on
                                                                           apparent lack of groundwater, and no evidence of
subdivided Lot 2- Failure
                                                                           existing instability in the vicinity of the proposed
through fill materials natural      Unlikely      Medium             Low
                                                                           residence, it is considered Unlikely that a landslip
soils above the foundation
                                                                           would develop, subject to implementation of the
depth
                                                                           recommendations in Section 4 (particularly with
                                                                           attention to remediation of existing fill). 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 the XW
                                                                           materials or weathered meta-mudstone rock below the
 2: Proposed residence on                                                  foundation depth is assessed as Rare due to the
subdivided Lot 2 - Deep                                                    strength of these materials, no evidence of existing
seated failure in XW material or     Rare          Major             Low   instability, and relatively consistent nature of the
weathered meta-mudstone                                                    sloping ground across the development area.
below the foundation depth                                                 Although the consequence of such a failure could be
                                                                           Major, the resultant risk is 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.

3.3    Quantitative Assessment
In order to calculate the factor of safety (FOS) against slope instability for the proposed building works for
proposed Lot 2, numerical analysis has been conducted using Slope/W. Based upon the results of our
investigation, an adverse ground profile was adopted, considering relatively high groundwater table
conditions (1 m BGL, noting that this was not recorded during our investigation), and larger scale, relatively
deep seated failure modes.

A 20 kPa surcharge load has been applied across the building envelope to represent a residence of up to
two storeys height.

Results of the numerical slope stability analysis are illustrated in Text Figure 9 overleaf.




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