Reference Document - OPW25 0085 (AS Published)

Table 4: Details of Qualitative Risk Assessment for Property (AGS 2007)

    Hazard                    Likelihood   Consequence     Assessed Risk   Comments
    Landslide through           Unlikely        Minor           Low        Given the gentle to moderate gradients
    future fill and natural                                                across the development areas and
    soils above the                                                        relatively shallow depth to stiff/medium
    foundation depth                                                       dense (or stronger) residual soils it is
                                                                           considered Unlikely that a landslide
                                                                           would develop, subject to
                                                                           implementation of the
                                                                           recommendations in Section 6. Given a
                                                                           potential Minor impact from such
                                                                           shallow instability the resultant risk
                                                                           would be Low as per AGS 2007.

    Deep seated failure           Rare          Major           Low        The likelihood of a deep failure through
    within weathered                                                       the weathered sedimentary rock below
    sedimentary rock                                                       the footing depths is assessed as Rare
    below the foundation                                                   due to the strength of these materials,
    depth                                                                  moderately sloping ground and no
                                                                           signs of previous deep-seated
                                                                           instability. 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 6 have been designed to mitigate those impacts, so that a Low
Risk of instability is achieved and can be maintained for the proposed development.

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 30 kPa surcharge,
to represent filling up to 1.5 m high. As all footings for the future structures on slopes greater than 10°
will be founded into weathered rock, minimal building loads are expected to surcharge the soil profile.
The subsurface profile for this scenario was modelled on the conditions encountered in the test pits
across the likely building areas, comprising stiff (or stiffer) residual clayey silt/silty clay overlying
weathered siltstone. The model assumes any existing topsoil and any firm/loose (or weaker) materials
will be stripped from the development areas prior to filling.
Groundwater was encountered within five of the twelve test pits and the investigation was completed
just after a period of prolonged heavy rainfall at the site. As such, groundwater conditions have been
modelled with a piezometric surface at about 0.5 m BGL to assess the FOS during an adverse temporary
(short-term) case.
Results of the numerical slope stability analysis 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.



Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                                 14