RFI - 21370-001-Rev1 66 Seaview Tce

                             PROPOSED RESIDENCE
                             66 SEAVIEW TERRACE, SUNSHINE BEACH

       Case 1 – Vertical cut into the toe of the sand dune slope caused by erosion scouring, and allowing
        for a 500 year average recurrence interval (ARI) storm event.
       Case 2 – Slope failure following reprofiling of the slope due to the failure/erosion from Case 1.
       Case 3 – Retrogressive upper slope failure following reprofiling of the slope due to the Case 2
        failure.
       Case 4 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern property boundary protected by a micro-pile (or similar) structure (piles min 7 m in length
        and with 120 kN shear force resistance).
       Case 5 – Slope stability analysis following reprofiling of the slope from Case 1 to Case 3, with the
        eastern edge of the residence protected by a micro-pile (or similar) structure (piles min 6 m in length
        and with 120 kN shear force resistance).
The modelled ground surface profile for the initial Case 1 was based on provided survey information across
the subject site (Ref. Minstaff Survey, Dwg. No. M3973 3003 2, Rev B; dated 19 October 2021) 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 500
year average recurrence interval (ARI) storm event. It was indicated in the RHDHV assessment, that while
the foredune and incipient dunes may be affected, significant erosion at the toe of the high dune face would
not occur. During our walk-over assessment at the site, it was noted that a steepening at the toe of the dune
was present. This was taken in to account as part of the development of the modelled ground profile. It is
noted that this resulted in a slightly more conservative, higher dune slope (following the retrogressive
failures); than would be proposed where considering coastal erosion only. An eventual toe level in the order
of RL 2.6 m AHD resulted, in line with the most onerous erosion case outlined by in the RHDHV modelling
(i.e. 500 year ARI event).

Subsurface conditions for Cases 1 to 5 were developed using the borehole and DCP information collected
during the investigation.

A surcharge load of 5 kPa was applied across crest of the dune to the eastern boundary to replicate future
landscaping fill. Building loads for the proposed residence and ancillary structures were included in the
modelling, given the height of the structure proposed (three full storeys above ground to the seaward side),
but will largely be transferred by a piled footing system into the natural medium dense (or denser) sands at
depth and would be anticipated to have a partial surcharge effect on the ground surface.

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
range of values recommended for similar materials in Appendix D of Australian Standard AS4678-2002
Earth Retaining Structures. Due to the relatively steep slopes and high permeability soils, a groundwater
table is not anticipated in the slope and therefore, was not modelled.
Table 4: Soil Parameters Adopted for Slope/W Analyses
                                                                                        Friction Angle, ɸ
  Material Description        Unit Weight (kN/m3)           Cohesion, c (kPa)
                                                                                            (degrees)


    Very Loose/Loose
                                        17                           0                          27
          Sands


  Medium Dense Sands                    18                           0                          34


 Dense (or denser) Sand                 19                           0                          36


  Hard Clays (Residual)                 19                           7                          30




28 April 2022
Report No. 21370-001-Rev1                             13