RFI - 21327-001-Rev0 Plover St

                                    PROPOSED RESIDENCE
                                    34 PLOVER STREET, PEREGIAN BEACH

Table 3: 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 retention system
 1: Failure through very loose to                                          proposed it is considered Unlikely that a landslip
loose natural sands above the        Unlikely      Medium         Low      would develop in these materials, subject to
foundation depth                                                           implementation of the 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 of
medium dense (or denser) sands        Rare          Major         Low      precedence of such movements locally, no signs of any
below the foundation depth                                                 existing slope instability and a lack of groundwater.
                                                                           Although the consequence of such a failure would be
                                                                           Major, the resultant risk is Low as per AGS 2007.


                                                                           The likelihood of a slip/failure through the temporary
                                                                           retention system (sheet pile wall, or similar) as outlined in
 3: Instability through temporary                                          Section 6.4, is assessed as Rare, subject to detailed
sheet pile retention system, or                                            design by a suitable qualified RPEQ engineer and
                                      Rare          Major         Low
similar, as discussed in Section                                           construction by a specialist contractor which accounts for
6.4.                                                                       the required factors of safety against slope instability.
                                                                           Although the consequence of such a failure would be
                                                                           Major, the resultant risk is Low as per AGS2007.


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.

5.2.1        Quantitative Assessment
In order to estimate the FOS against slope instability for the proposed development, numerical analysis has
been conducted using the software Slope/W. The ground surface profile has been modelled using a
combination of preliminary design drawings and slope angles measured in the field by Tectonic. The
analysed subsurface profile was compiled based on the results of our subsurface investigation. Some
inferences regarding subsurface conditions were made between test locations. A minimum FOS value of 1.5
is the commonly adopted industry standard for long term conditions.

No groundwater was encountered at the time of investigation. Given the gradients across the site and highly
permeable nature of the sandy soil, development of shallow groundwater is considered unlikely. Should
groundwater seepage be encountered during construction Tectonic must be notified to assess the impacts
on temporary instability.

To replicate future landscaping, a surcharge load of 5 kPa has been applied upslope of the proposed
building. Most of the building will be supported by piled foundations taken into the underlying medium dense
(or denser) natural sands and therefore, building surcharge loads have not been included in the model.

Soil shear strength was based upon effective parameters which were estimated based on our interpretation
of the subsurface investigation results, while also considering the values recommended for similar materials
in Appendix D of Australian Standard AS4678-2002 Earth Retaining Structures. Adopted soil parameters
are shown on the Slope/W output presented in Text Figure 8 on the following page. Analysis was
undertaken for a specified range of entry and exit points that could impact the proposed development.

The analysed profile represents the critical section (i.e. ‘worst case’ scenario) for the proposed development,
which covers the deepest cut. Our analysis indicates that the proposed development would achieve at least
the required minimum FOS for long-term stability (i.e. 1.5), subject to permanent retention of cuts using
engineer designed structures.




10 September 2021
Report No. 21327-001-Rev0                                   7