Geotechnical Stability Report · OPW24/0017

GMW Projects Pty Ltd                                                                                    24008-001-Rev0
                                                                                                        31 January 2024

It should be noted that this assessment excludes geotechnical risk associated with non-piled structures (e.g.
driveway, landscaping etc.) due to settlement of the upper loose sands. The design of at grade structures
(including landscaping) should consider differential settlements between any piled areas (hard/rigid surfaces)
and surrounding fill and natural sands. This risk assessment also excludes geotechnical risk associated with
the existing structures (including retaining walls).

Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)

                                                            Assessed
 Hazard                          Likelihood   Consequence
                                                              Risk
                                                                       Comments

                                                                       Given the gentle to moderate gradients across the
                                                                       proposed development areas, it is considered
                                                                       Unlikely that a landslide would develop in these
 1: Failure through very loose
                                                                       materials, subject to implementation of the
to loose natural sands above      Unlikely      Medium         Low
                                                                       recommendations in Section 4. It is assessed that
the foundation depth
                                                                       there could be a potential Medium impact from
                                                                       such instability so the resultant risk would be Low
                                                                       as per AGS 2007.

                                                                       The likelihood of a deep failure through the medium
                                                                       dense (or denser) natural sand below the proposed
                                                                       foundation depth is assessed as Rare due to the lack
 2: Deep failure through the
                                                                       of precedence of such movements locally, no signs of
medium dense (or denser)
                                   Rare          Major         Low     any existing slope instability, apparent lack of
sands below the foundation
                                                                       groundwater and the negligible risk of erosion at the
depth
                                                                       toe of the slope. 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 4 have been designed to mitigate those impacts.



4.0        RECOMMENDATIONS
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached. These Guidelines should be adopted in the design
and construction of works on the site.

4.1       Site Layout and Development Limitations
Should any revisions be made to the proposed extent of the development, as described in Section 1.1,
Tectonic must review these revisions prior to construction to assess if there are any impacts on slope stability.

To achieve a Low Risk of instability, the following is recommended:
          The proposed development should not exert additional load on the existing structures or footing
           system, unless assessed and approved by a structural RPEQ, who must also confirm the existing
           footings and structural members have sufficient capacity to carry additional loads imposed by the
           extensions.
          All footings must found below the zone of influence of the existing structures (including retaining
           walls).
          Cut and fill earthworks should not exceed 1.5 m depth/height unless assessed and approved by a
           geotechnical RPEQ.
          All retaining walls exceeding 1 m high and/or subject to surcharge loads must be engineer designed
           and certified by an RPEQ. The design must include an assessment of global and internal stability.
          Design and construction of footings shall be undertaken in accordance with Section 4.3.
          Formal drainage should be constructed to ensure all stormwater is directed away from the existing
           and proposed structures.




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