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

McInnes Wilson Lawyers



Table 2: Summary of modelling results.
 Case      Figure    Description                      FOS                Comments
                                                            The FOS value of 0.9 above the erosion cut indicates slope
                                                            failure. A vertical erosion cut will likely lead to a series of
                                                            retrogressive upslope translational failures in the loose or
            Figure   Vertical cut into slope                medium dense sand, the exact nature of which will depend
Case 1                                                0.9
              B1     caused by erosion                      on the impact of vegetation and localised variation in ground
                                                            conditions. The slip surface shown is considered a likely
                                                            outcome based on our previous experience in similar
                                                            Situations.

                     Lower slope failure
                                                            Following reprofiling of the slope as a result of the initial
                     following reprofiling of slope
            Figure                                          lower failure, retrogressive failure of the slope is likely (FOS
Case 2               due to failure from Case 1       0.8
              B2                                            values of 1 or less indicate failure).



                   Retrogressive upper slope                Following reprofiling of the slope as a result of the
                                                            downslope translational failure(s), further instability of the
            Figure failure following reprofiling
 Case 3            of slope due to lower              1.0   upper slope is likely (FOS values of 1 or less indicate
              B3
                   translational failure from               failure). Such failure is likely to extend under the proposed
                   Case 2                                   pool if founded on shallow footings.
                     Stability analysis                     If the pool is supported on appropriately designed micro-
                     following reprofiling of               piles, the potential critical slip surface takes the form of a
            Figure
Case 4               slope from Case 1 and            1.6   deep- seated failure. However the FOS is acceptable, even
              B4
                     Case 2, with the pool                  with a reprofiled slope following the erosion cut and
                     supported on micro-piles               subsequent retrogressive failures from Cases 1 and 2



7.0       RECOMMENDATIONS
It is recommended that the proposed pool is supported on micro-piles founded into the underlying natural
dense or very dense sand, as per the recommendations below. The micro-piles will also improve the overall
stability of the dune in this area and will not impact adjacent properties.
Conceptual micro-pile design provided by PCA ground Engineering is attached in Appendix C. Detailed
design will be required by the piling contractor.
The micro-pile system should comprise an ‘A-frame’ structure at the eastern edge of the pool, including one
vertical and one raked pile to resist potential lateral movement in the sands, tied together with a steel-
reinforced concrete capping beam. A row of vertical piles and capping beam is recommended along the
western edge. Grout ‘columns’ should be installed between the micro-piles to prevent flow of sand between
piles. The piles should be at least 10 m deep, with minimum preliminary spacings as shown in Appendix C.
Micro-piles have been successfully used in similar slope stabilisation/remediation projects, including the Park
Road boardwalk and slope stabilisation in Noosa Heads and for a residential development at the northern
end of Seaview Terrace. Micro-piles are able to be installed using small installation equipment to suit the
site’s access constraints.
The following additional recommendations are made for the proposed development:

    ▪ Disturbance to the surrounding slopes should be kept to a minimum (vegetation must
          be maintained below the proposed pool)
    ▪ Revegetation of any bare slopes around the development should be undertaken
    ▪ Additional fill must not be placed on the slope
    ▪ Excavations into the upper slope should be limited, with adequate
          temporary/permanent support provided. Excavations in the middle or lower portions
          of the slope must be avoided



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