Reference Document - Geotech Report - MCU24 0094 · MCU24/0094

                             PROPOSED RESIDENCE
                             33 ROSS CRESCENT, SUNSHINE BEACH

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 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 erosion by storm tides, which could
theoretically lead to larger scale failures.

Modelling has therefore been carried out considering a hypothetical storm and wave scenario nominated by
coastal engineering experts (RHDHV), 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 storm erosion profile (piled wall protecting eastern edge of
        building indicative only).
       Case 2 – Lower slope failure due to the regression from Case 1.
       Case 3 – Retrogressive upper slope failure following the Case 2 failure.
       Case 4 – Slope stability analysis following regressive failure of the slope from Case 1 to Case 3, with
        the high dune ultimately assuming an inclination of 28° (eastern property boundary and ground
        above protected by a shear pile wall). Case 4a illustrates shear wall protection along the eastern
        edge of the building area, and Case 4b demonstrates the feasibility of the pile wall approximately
        along the eastern site boundary.

The modelled ground surface profile for the initial Case 1 was based on provided survey information across
the subject site (Ref. AJS Surveys, Plan Ref. 23311-D1-A, dated 6 November 2023) with surface profiles
beyond the eastern property boundary estimated from slope angles and measurements in the field by
Tectonic. The horizontal extent of erosion adopted in Case 1 was based on information provided in the
referenced RHDHV report (Ref. Section 1.0), which included a combination of short-term erosion from a
100 year average recurrence interval (ARI) storm event. It was indicated in the RHDHV assessment, that
virtually the entirety of the high dune face would be affected by a future erosion cut, extending approximately
5 m to 10 m laterally into the dune face; with a localised scarp crest at around RL 13 m AHD. In addition, the
erosion cut is shown to extend into the subject site, within the proposed building envelope. A toe level of
around RL 2.4 m AHD was adopted for all cases; generally consistent with the larger scale erosion cut given
by RHDHV in their modelling.
Subsurface conditions for Cases 1 to 4 were developed using the borehole and DCP information collected
during the investigation.

Based on the subsurface conditions at the site, and the extent of earthworks, it is anticipated that building
loads for the proposed residence and ancillary structures will be transferred by a mixture of high-level and
piled footings into the natural medium dense (or denser) sands underlying the site. 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 areas and 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 lower level and two full floors above.

The soil parameters adopted for the analyses are summarised in Table 4 on the following page 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 slopes and high
permeability soils, a groundwater table is not anticipated in the slope and therefore, was not modelled.




18 June 2024
Report No. 24096-001-Rev0                              11