Geotechnical Investigation

                              PROPOSED CALILE HOTEL NOOSA DEVELOPMENT
                              3 – 7 SERENITY CLOSE, NOOSA HEADS

Given the geology and topography of this site, it is unlikely that a landslide could affect the proposed
development area. Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface
gradient is about 30°, although often lower (26˚ to 28˚) for very loose sands. Our measurements indicate that
the overall natural gradients typically vary between about 8° and 12°. Steeper gradients were recorded in the
southern part of the site, ranging up to about 20°.
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).

Sand dune slopes generally remain stable (i.e. FOS 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
       not undercut by excavation or erosion.

Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Relatively dense vegetation across the southern part of the site will be
retained. Direct discharge of stormwater onto dune slopes may cause channel erosion, undermining and
disturbance of vegetation, and development of generally localised instability. Uncontrolled stormwater will not
be directed into the southern part of the site.

Cut and fill earthworks have the potential to overload and/or undermine the stability of sand dune, therefore
must be carefully planned and implemented.

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. At this site, the dune around and below the site contains dense
vegetation along a natural watercourse. Previous experience in the area indicates that the ground surface to
the south is underpinned by indurated sand ‘coffee rock’ and, as the toe of the dune is heavily vegetated,
significant disturbance by natural forces (e.g. erosion during a flood event) in the vicinity of the proposed
development area is considered unlikely.

In accordance with previous practice along Serenity Close, it is recommended that all structures are founded
below a plane rising at 27° from the toe of the dune to the south of the site.

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 overleaf 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).




7 October 2022
Report No. 22239-001-Rev0                                9