Reference Document - Geotechnical Investigation Report - DBW24/0090 & RAP24/0025

                Rebecca Stead                                                                                                20248-001-Rev0
                                                                                                                           3 September 2020

                Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface gradient is about 30°. The
                overall slope across the proposed development area is generally in the order of 10˚ to 22˚, being steepest in
                the north-eastern corner. 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).

                Steep sand dune slopes generally remain marginally stable (i.e. FOS just greater than 1.0) when they are:

                         well vegetated (mix of ground cover, shrubs and small trees);
                         not subjected to concentrated stormwater flows;
                         not surcharged with additional loads (e.g. fill); and
                         when the ‘toe’ of the dune is not disturbed.

                Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
                dunes to stand up relatively steeply. Areas outside of structures and concrete ground slabs are generally
                covered by vegetation being either grass, shrubs or trees. Discharge of stormwater onto dune slopes may
                cause channel erosion, undermining and disturbance of vegetation, resulting in development of generally
                localised instability. Stormwater runoff must be collected (or re-directed) and discharged from the site as
                discussed in Section 4.5 below.

                The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
                be caused by excavations or natural erosion. Sand dune instability due to undercutting of the toe generally
                occurs as a translational-type slide where a ‘slab’ of sand on the dune face slides down slope. The depth of
                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 the ocean or high flow
                natural drainage line 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 extensions. Regulators (Noosa Shire Council) 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 position of the site near a dune crest, and
                                                                                             moderate natural gradients (up to 20˚ to 22˚) on the
                                                                                             downslope side of the residence, relatively limited
                                                                                             depth of very loose/loose material (about 1.3 m to
                  1: Shallow landslide through
                                                                                             1.6 m BGL), and depth of footings recommended, it
                 very loose to loose natural soils    Unlikely      Medium             Low
                                                                                             is considered Unlikely that a landslip would develop
                 above the foundation depth
                                                                                             in these materials, subject to 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 landslide through the medium
                                                                                             dense (or denser) natural sand below the foundation
                                                                                             depth recommended is assessed as Rare due to the
                  2: Deep landslide through the
                                                                                             moderate overall ground slopes, strength of the
                 natural medium dense (or
                                                       Rare          Major             Low   materials, lack of groundwater, and no evidence of
                 denser) sand below the
                                                                                             similar large scale previous instability slope instability
                 foundation depth
                                                                                             on the site. 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.




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Document Set ID: 23080488
Version: 1, Version Date: 27/06/2024