Appendix D - Geotechnical Report

Table 3: Details of Qualitative Risk Assessment for Property (AGS 2007)

 Hazard                Likelihood   Consequence   Assessed Risk    Comments
 Failure through         Unlikely       Minor          Low         Given the gentle to moderate gradients
 future fill and/or                                                across the building areas on proposed Lots 1,
 natural residual                                                  2, 4 and 5, and very stiff to hard nature of the
 soils above the                                                   in situ residual soils/XW material, it is
 foundation depth                                                  considered Unlikely that a landslide would
                                                                   develop, subject to implementation of the
                                                                   recommendations in Section 7. For a
                                                                   potential Minor impact from such shallow
                                                                   instability the resultant risk would be Low as
                                                                   per AGS 2007.

 Deep seated              Rare          Major          Low         The likelihood of a deep failure through the
 failure within very                                               residual soils/XW materials or very low
 stiff to hard                                                     strength (or stronger) weathered phyllite,
 residual soils/XW                                                 below the foundation depths is assessed as
 materials or very                                                 Rare due to the strength of these materials,
 low strength (or                                                  apparent lack of adverse structure in the rock
 stronger)                                                         mass (e.g. persistently dipping layering) and
 weathered phyllite                                                no indications of persistent groundwater
 below the                                                         seepage. There were also no signs of
 foundation depth                                                  previous deeper seated instability on the site,
                                                                   and the existing residence was dilapidated
                                                                   and required some maintenance, did not
                                                                   appear to show signs of distress that could
                                                                   be attributed easily to slope instability.
                                                                   Although the consequence of such a failure
                                                                   could be Major, the resultant risk is Low as
                                                                   per AGS 2007.


6.2.2         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 20 kPa surcharge,
to represent a two-storey residence (without fill) or 1 m height of fill.
The section was developed as a generic ground surface profile, allowing for the steepest slopes present
within the building areas and immediately downslope. The subsurface profile for this scenario was
modelled on the conditions encountered in the test pits, and assumes topsoil and unsuitable materials (if
encountered, Ref. Section 7) would be stripped from future development and building areas prior to
construction or filling.
Although groundwater was not encountered within the test pits, groundwater conditions were modelled
to assess the FOS during an adverse temporary (short-term) case with a piezometric surface at about
1 m BGL (close to the interface between the residual soils and the underlying ‘rockhead’). Results of the
numerical slope stability analysis are illustrated in Text Figure 13 overleaf. The key points from our
analysis are summarised as follows (overleaf):




Geotechnical Investigation
10 June 2026 | 26073-001-Rev0                                                                                    15