20013-001-Rev0 Flooded Gum Pl - geotech report

Mr Andrew Jensen                                                                                                    20013-001-Rev0
                                                                                                                    4 February 2020

Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given in Section 4, it is our opinion that there is a Low (L) Risk of global slope instability
affecting the proposed residence. Regulators (NC) normally require that a Very Low or Low Risk of landslide
affecting the property must be demonstrated to enable development approval. Summarised in Table 2 below
is our qualitative assessment of landslide risk for the site. A summary of qualitative terminology for use in
assessing risk to property is attached (taken from AGS 2007).
 Table 2: Details of Qualitative Risk Assessment for Property (AGS 2007)

                                                     Assessed
 Hazard                     Likelihood Consequence
                                                       Risk
                                                                Comments


                                                                The likelihood of a failure through residual soils in the vicinity of
 1: Shallow failure                                             the proposed residence is assessed as Unlikely due to the
through residual soils                                          moderate natural gradients, shallow depth to rockhead, apparent
above foundation             Unlikely    Minor         Low      lack of groundwater, and topographic position near the crest of
depths                                                          the ridge. The consequence of such a failure would be Minor
                                                                considering the anticipated limited effects of such shallow
                                                                instability, with the resultant risk being Low as per AGS 2007.


                                                                The likelihood of a deep failure through the weathered rock is
 2: Deep failure                                                assessed as Barely Credible due to the strength, and shallow
through weathered                                               occurrence of this material (and shallow transition to very low
rock below the               Barely
                                         Major       Very Low   strength rock expected), moderate ground slopes, and no evidence
foundation depth            Credible
                                                                of such instability in the area. Although the consequence of such a
                                                                failure could be Major, the resultant risk is Very Low as per AGS
                                                                2007.

                                                                The likelihood of a failure through embankment fill in the vicinity of
 3: Embankment                                                  the proposed residence is assessed as Unlikely due to the relatively
failure through fill near                                       flat batter slopes (1V:2H), the quality of the fill materials used
the residence                Unlikely    Minor         Low      (gravelly clay), apparent lack of groundwater, and topographic
                                                                position at the crest of the ridge. The consequence of such a failure
                                                                would be Medium considering the anticipated effects of instability,
                                                                with the resultant risk being 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.

4.1    Geotechnical Constraints
The results of our assessment indicate that construction should be avoided along central drainage line/
surface depression; and otherwise where slopes exceed 15° (26 %).

Construction should therefore be focused on the ridge on the southern boundary. Observations made on site
indicate that shallow rock is present in this area, at approximately 1 m depth (from the top of the ridge).

Both low-set and high-set type construction should be possible, noting that footings must penetrate through
any fill or natural soils to socket at least 300 mm into the underlying weathered rock.
The embankment located on the western side at the top of the driveway needs to be re-profiled to 1V:2H. It
is noted that the fill here appears to cut off surface water flows from the slope above so a culvert or some
form of adequate diversion drainage system should be developed to enable surface water flows to be
directed around or under the embankment.




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