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

      GEOTECHNICAL INVESTIGATION AND POOL SUPPORT DESIGN

195   An agreed assessment of the geotechnical investigation and proposed pool design is as follows:

      Core Consultants Pty Ltd provided an undated geotechnical report on 24 September 2019 (included
      in Attachment 2 to this joint report), which includes an assessment of Slope Stability in Chapter 6,
      taking account of coastal erosion. The initially assumed coastal erosion extent was based on the
200   joint advice by NC and PP, which was for a cut to 60m west of the toe of the frontal dune (assumed
      to align with the location of the highest astronomical tide). This cut was based on a 100 year storm
      tide and 100 year wave condition with 0.3m sea level rise by 2050.

      The analysis considered slope failure / erosion and slope reprofiling subsequent to the initial coastal
205   erosion cut, comprising upslope failure and lower slope failure, with and without micro‐piling to
      support the proposed pool.

      Case 3 results from the Core report show that, without piling, the pool is at risk of failure due to
      failure of the slope to the east of the pool, following the assumed coastal erosion.
210   Case 4 shows that, with the 10m long micro‐piling, that an adequate factor of safety against failure is
      achieved, and that the pool structure would remain in place. A consequence of the upper and lower
      slope failure is that a near vertical erosion scar approximately 5m high would result immediately
      adjacent to the pool.

215   The micro‐piles proposed on the eastern side of the pool are spaced at approximately 0.5m
      minimum centres, with 49 piles in total distributed over two rows. An additional 7 micro‐piles are
      proposed to support the western side of the pool.

      Grout columns are also proposed between the micro‐piles to prevent sand flow, such that a
220   continuous wall is created under the eastern side of the pool.

      RESPONSES TO KEY COASTAL ENGINEERING QUESTIONS

      In terms of the 4 questions raised above, taking account of our investigations and the latest
225   geotechnical report by Core consultants, we provide the following agreed responses.

      1.    The extent of erosion associated with a storm event equivalent to a 100 year return period is
            conservatively assessed as being a vertical cut extending 60m shoreward of the highest
            astronomical cut line. This cut would result in subsequent upper and lower slope failures and
230         reprofiling. Without piling under the proposed pool, failure as shown in Case 3 could occur,
            resulting in the collapse of the pool. With the proposed 10m deep, 56 micro‐pile
            arrangement, the pool structure will remain intact, and the possible erosion scar profile as
            shown in Core Consulting’s Case 4 could occur.
            This addresses questions 1 to 3.
235   2.    In relation to question 4, whilst some coffee rock has been encountered in geotechnical
            investigations, in the orange zone shown on Core Consultants four Cases, there is insufficient
            information to assume that a continuous and homogenous layer of coffee rock exists. Hence
            the need for the deep micro‐piles.



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