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

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 Slope/W. The ground surface profile has been modelled
using a combination of architectural layout 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.
Although the existing sand dune slope would be considered relatively stable in the short term, it is
acknowledged that there were positive gradients dipping down and away, from beyond the eastern edge
of the likely building area, and very loose/loose sands were present in the near surface profile. The longer
term stability of the site could in addition be affected by other factors such as coastal erosion by storm
tides, which could theoretically lead to larger scale failures.
Modelling has therefore been carried out considering erosion from a hypothetical storm and wave
scenario nominated by coastal engineering experts (M&N), and taking into account subsequent
retrogression of the dune over time. Each of the cases analysed are summarised as follows:
   Case 1 – Reprofiling of slope due to the modelled storm erosion profile.
   Case 2 – Slope stability analysis following regressive failure of the slope from Case 1, with the high
    dune ultimately assuming an inclination of 27° (eastern property boundary protected by a shear pile
    wall).
The modelled ground surface profile for the initial Case 1 was based on provided survey information
across the subject site (Ref. Next Step Surveying, Project No. NSS032, Dwg No. 3000 001, dated 30
January 2025) with surface profiles beyond the eastern property boundary estimated from slope angles
and measurements in the field by Tectonic. Erosion cut adopted in Case 1 was based on information
provided in the referenced M&N report, which included a combination of short-term erosion from a
100-year average recurrence interval (ARI) storm event. It was indicated in the M&N assessment that a
significant portion of the high dune face would be affected by a future erosion cut, extending
approximately 17 m laterally into the dune face; with a localised scarp crest at around RL 23 m AHD. The
erosion cut was shown to extend to within 13 m of the subject site’s eastern boundary. A toe level of
around RL 1.9 m AHD was adopted; generally consistent with the larger scale erosion cut given by M&N
in their modelling. Subsurface conditions for Cases 1 & 2 were developed using the borehole and DCP
information collected during the investigations.
Based on the subsurface conditions at the site, and the extent of proposed earthworks, it is anticipated
that the majority of the building loads for the proposed residence and ancillary structures will be
transferred by high-level footings into the natural medium dense (or denser) sands underlying the site.
However, in some locations where medium dense (or denser) sands will be at depth, piled foundations
will be required. A surcharge load of 5 kPa was modelled in areas where the building loads are to be
transferred via piled foundations to the underlying soils and in landscaped/driveway areas, whilst 30 kPa
was applied across the proposed building area were medium dense (or denser) sands are expected at
bulk earthworks levels to represent the distributed load from the proposed three level structure.
The soil parameters adopted for the analyses are summarised in Table 4 and are based on interpretation
of the site investigation, in situ testing and laboratory test results. The parameters are consistent with
the low to mid-range values recommended for similar materials in Appendix D of Australian Standard
AS4678-2002 Earth Retaining Structures. Due to the dune slope and high permeability soils, a
groundwater table is not anticipated within the slope and therefore, was not modelled.


Geotechnical investigation
19 November 2025 | 25289-001-Rev0                                                                        13