Geotehnical Investigation Report · OPW22/0140

                                    PROPOSED UNIT DEVELOPMENT
                                    37 BOTTLEBRUSH AVENUE, NOOSA HEADS

Given the geology, topography and particularly the relatively moderately sloping terrain across the proposed
development area, it is considered rare that a landslide could affect the proposed development. Sand dune
slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°, although
possibly lower in the order of 26˚ to 28˚ for very loose sands. The limit of equilibrium is the ‘tipping point’ to
instability, where the factor of safety (FOS) of the slope is approximately 1.0. The FOS of a slope is the
balance (ratio) between the forces driving instability (e.g. soil weight, groundwater pressure) and the resisting
forces (e.g. soil friction and cohesion). Inferred natural slopes around this site are between 9° and 11°.
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 development. 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
3 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 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, it is considered Unlikely
 1: Failure through very loose to                                          that a landslip would develop in these materials,
loose natural sands above the        Unlikely      Medium         Low      subject to implementation of the recommendations in
foundation depth                                                           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 any temporary
                                                                           retention system (sheet pile wall, or similar) as outlined in
 3: Temporary instability through                                          Section 6.4, is assessed as Rare, subject to detailed
sheet pile temporary retention                                             design by a suitably qualified structural RPEQ and
                                      Rare          Major         Low
system, or similar, as discussed                                           construction by a specialist contractor which accounts for
in Section 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 factor of safety (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 survey information, 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 instability. Nonetheless a relatively high groundwater level was modelled as a conservative approach.




13 May 2022
Report No. 22151-001-Rev0                                   8