21054-001-Rev0 Tristania Dr

Peter Williams                                                                                               21054-001-Rev0
                                                                                                            25 February 2021

such sliding may range up to a few metres during one event, but on-going erosion can cause retrogressive
failure of dune slopes leading to multiple sliding events, if left unchecked. The risk such an occurrence at this
site is minimal, considering the location of the site being away from features such as high flow natural
drainage lines that could cause toe erosion.

Considering the existing site conditions (Ref. Section 2), and subject to the implementation of
recommendations given below, it is assessed that there would be a Low (L) Risk of global slope instability
affecting the proposed residential development. Regulators (NSC) 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 is our qualitative assessment of landslide risk for the development 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


                                                                            Given the low to moderate natural gradients through
                                                                            the proposed building area and setback from the
                                                                            beach (affording protection to the toe of the dune), it
 1: Failure through very loose to
                                                                            is considered Unlikely that a landslip would develop
loose natural sands above the        Unlikely      Medium             Low
                                                                            in these materials, subject to implementation of the
foundation depth
                                                                            recommendations in Section 4. Given a potential
                                                                            Medium impact from such instability 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
 2: Deep failure through the
                                                                            foundation depth is assessed as Rare due to the lack
medium dense (or denser)
                                      Rare          Major             Low   of precedence of such movements locally, no signs of
sands below the foundation
                                                                            any existing slope instability and a lack of groundwater.
depth
                                                                            Although the consequence of such a failure would 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.

4.1    Building Location & Limitations
We understand that the proposed building envelope is to be as shown in Text Figure 2. This position is
acceptable from a geotechnical viewpoint; however, Tectonic must re-assess slope stability if the proposed
development area is altered.

4.2    Earthworks
It is anticipated that cut and fill earthworks for the proposed development will range up to about
2 m deep and 1 m high, respectively. Should proposed earthworks exceed these quantities, Tectonic must be
notified and a re-assessment of slope stability may be required.
Preliminary plans indicate that the majority of earthworks will be supported by engineer designed retaining
walls. Any fill, and cuts exceeding 1 m, must be supported by engineer designed and certified retaining
walls.




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