Reference Documents

                                                 PROPOSED SUBDIVISION
                                                 41 TALLOWWOOD PLACE, BLACK MOUNTAIN

            Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)
                                                                             Assessed
              Hazard                             Likelihood   Consequence
                                                                               Risk
                                                                                        Comments


                                                                                        Given the presence of very stiff (or stiffer) residual
                                                                                        soils at shallow depth, together with no evidence of
              1: Recommended building                                                   existing instability and apparent lack of groundwater
             area and driveway alignment                                                across the recommended building area, it is
             (Ref. Section 6.1) - Failure         Unlikely       Minor           Low    considered Unlikely that a landslide would develop,
             through fill and natural residual                                          subject to implementation of the recommendations
             soil above the foundation depth                                            in Section 4. Given a potential Minor impact from
                                                                                        such shallow instability the resultant risk would be
                                                                                        Low as per AGS 2007.



                                                                                        The likelihood of a deep failure through the weathered
              2: Recommended building                                                   phyllite is assessed as Rare due to the strength of
             area and driveway alignment                                                these materials, moderately sloping ground and no
             (Ref. Section 6.1) - Deep seated      Rare          Major           Low    signs of previous deep-seated instability across the
             failure within weathered phyllite                                          recommended building area. Although the
             below the foundation depth                                                 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 6 have been designed to mitigate those impacts so that a Low Risk
            of landslide can be achieved for the proposed new building area.

            5.2.2         Numerical Modelling
            In order to estimate the factor of safety (FOS) against slope instability for the proposed development,
            numerical analysis has been conducted using Slope/W, including consideration of adverse short-term
            groundwater conditions. The modelled conditions for future construction comprised a 40 kPa surcharge, to
            represent the future dwelling, landscaping and up to 1.5 m of filling. A 10 kPa surcharge was also applied up
            slope of the building envelope to represent a future driveway. As all footings for the future structures will be
            founded on weathered phyllite, minimal building loads would surcharge the soil profile.
            The subsurface profile for this scenario was modelled on the conditions encountered in the test pits,
            comprising very stiff residual soil up to about 1 m deep overlying XW materials and weathered phyllite. The
            model assumes any existing topsoil, colluvium, uncontrolled fill or firm/loose (or weaker) materials will be
            stripped from the development areas prior to filling. Although groundwater was not encountered within the
            test pits excavated around the proposed building area, groundwater conditions have been modelled to
            assess the FOS during an adverse temporary case with a piezometric surface at about 1 m BGL.
            Results of the numerical slope stability analysis for the proposed new house site are illustrated in
            Text Figure 12 overleaf. The key points from our analysis are summarised as follows:
                     Unit weight and shear strength parameters adopted for the soil and weathered rock strata were
                      estimated based on our interpretation of the geotechnical investigation data and were typically within
                      the mid range of values recommended for similar materials in Appendix D of Australian Standard
                      AS4678-2002 Earth Retaining Structures.
                     Analysis was undertaken for specified slip circle centroids (over a grid) and radii to assess global
                      slope instability. The adopted ground surface profile was based on the steepest slope measured
                      during the field investigation across the proposed building area, that being about 17°. The analysis
                      returned an acceptable minimum FOS value of 1.5, with adverse groundwater conditions included.
                     This result indicates that the industry standard minimum FOS against slope instability for hillside
                      development (≥1.5) would be achievable for the proposed development.




             22 May 2023
             Report No. 23099-001-Rev0                                      11

Document Set ID: 22826802
Version: 1, Version Date: 23/02/2024