24-01-2020 - Appeal No. 245 of 2019 - Final Order of Judge Rackemann · 51987.2951.01

McInnes Wilson Lawyers

6.0       SLOPE STABILITY ANALYSIS
    6.1      Method
Quantitative slope stability modelling was undertaken to analyse factor of safety (FOS) values for a
hypothetical storm and wave scenario nominated by the coastal engineering experts for the proposed
development, as follows:

    ▪ Case 1: Vertical cut into the slope caused by erosion and subsequent upslope failure.
    ▪ Case 2: Lower slope failure following reprofiling of the slope due to the lower failure/erosion from
          Case 1.
    ▪ Case 3: Retrogressive upper slope failure following reprofiling of the slope due to the lower failure
          from Case 2.
    ▪ Case 4: Slope analysis following reprofiling of the slope from Case 1 and Case 2, with
          the proposed pool supported on micro-piles (10 m in length and with 100 kN shear
          force resistance).
The modelling was undertaken using Geostudio’s Slope/W software.
The horizontal extent of erosion adopted in Case 1 (as shown in Image 2 below) was based on information
provided by Omtrek Pty Ltd via email dated 16 August, 21 August and 11 September 2019 (60 m west of
highest astronomical tide, based on ‘100 year storm tide and 100 year wave condition with 0.3 m sea level
rise by 2050’). The horizontal extent of erosion is approximately at the western end of seismic Line 4 (refer
Section 4.1 and Figure 1).
The model ground surface profile was based on available survey information (M Design drawings 2.2 and
5.1), Noosa Council web mapping, information provided by Omtrek Pty Ltd (email dated 26 August and 11
September 2019), and site observations (including measured slope angles).
The vertical cut adopted in Case 1 assumes that erosion of the coffee rock will occur during the storm event.
This is a conservative approach as coffee rock is known to be resistant to erosion. The ‘conservative’
approach does, however, account for possible variation in coffee rock thickness/density at the toe of the
dune.
The soil strength parameters were estimated based on the subsurface conditions encountered, in-situ testing
and 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 was not included
in the model.




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