25289-001-Rev0 - The Esplanade (1)-compressed

5.          Slope Stability Assessment
5.1         Existing Conditions
The Noosa Planning Scheme 2020 includes a landslide hazard overlay map (Ref. extract in Text Figure 8)
which indicates that the eastern edge of the site, and steeper dune face to the east, have been mapped
as Moderate and High Hazard areas, respectively.




                                           Subject site




                    Text Figure 8: Extract from NC landslide hazard overlay mapping

It should be noted that the NC landslide hazard mapping is a broad-scale indication of the potential
landslide susceptibility and is superseded by this site-specific assessment. The allotment and adjacent
dune face did not exhibit any visible surface features which would indicate larger scale global slope
instability during our walk-over. No landslide back scarps, noteworthy groundwater seepage or areas of
naturally ‘hummocky’ ground were observed. Medium dense sands were encountered at relatively
shallow depths below surface down the dune face, with the dune standing at around 30° overall.
Generally, sand dune slopes are close to the ‘limit of equilibrium’ where the surface gradient is about 30°,
although often lower (26˚ to 28˚) for very loose sands (as were indicated to be present down the dune in
DCP1 to DCP3 to depths of up to 1.9 m BGL). 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).


5.2         Stability Assessment
The risk assessment for this project has been carried out following AGS 2007 Practice Note Guidelines for
Landslide Risk Management. Relative levels of risk and their implications are given in Table 2 on the
following page.




Geotechnical investigation
19 November 2025 | 25289-001-Rev0                                                                           10