19009-004-Rev0 RIP Advice · OPW22/0166

CBD Settlers Cove Pty Ltd                                                                         19009-004-Rev0
                                                                                                1 September 2022

It is noted that the proposed swimming pool will be supported by piles adjacent to the RIPs so any potential
adverse impact on the pool foundations/stability from the RIP operation would be mitigated such that it
would be negligible risk.

Noosa Council contour mapping indicates that the ground surface below the toe of the boulder wall dips at
less than 10° to the south for a distance of about 15 m, then becomes relatively flat for some distance
across a broad natural terrace at about RL 5 m. This area is also densely vegetated. Sand dunes are
typically marginally stable where the face slope is about 30°. Given the gentle to relatively flat gradients
extending away from the toe of the retaining wall, it is assessed that the proposed RIP operation would
present a Low Risk of causing global instability to the boulder wall and adjacent area.
Regarding potential impact c), we note anecdotal advice which indicates that the RIPs are unlikely to
overflow. The design calls for a grouted rock spillway fed by an open channel with sides battered at 1V:2H
from the RIP overflow grates. There would be increased potential for overflow to develop if the basal
geosynthetic filter fabric in the RIPs becomes blocked with fines. Migration of RIP overflow waters into the
retaining wall backfill could also occur, however this will be prevented in the long-term by the proposed
sheet piles.

Based upon the above, to achieve a Low Risk of adverse impact to the existing/proposed structures and
general area from the operation of the RIPs, we recommend the following:

         Sheet piles should extend at least 3 m beyond the ends of the RIP zone to provide effective
          stabilisation during temporary works, and to increase the distance that infiltrated water needs to
          travel to outflow under the retaining wall.

         The basal geosynthetic filter fabric in the RIPs should be regularly checked and replaced if it is found
          to have become blocked with fines. We understand that Alta has developed a management plan to
          deal with maintenance of the RIPs.

5.0       CLOSURE

Your attention is drawn to the document Limitations, which is attached to this letter report.

Please contact the undersigned should you wish to discuss any of the above matters.

Yours faithfully

TECTONIC GEOTECHNICAL PTY LTD




Ashley Davey BEng (Geological) RPEQ 8159
Principal Geotechnical Engineer

Attachments:       Figure 1 – Site Plan
                   Report for Borehole BH9 & Explanatory Notes
                   Limitations




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