19009-004-Rev0 RIP Advice · OPW22/0166

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

Appendix D of the Alta SWMP indicates that the maximum infiltration rate across the five RIPs will be
265 L/s, or 53 L/s per pit, with the pits full of water.

As indicated by borehole BH9, the RIP bases at about 3 m to 3.5 m below existing ground level will be
founded in medium dense to dense, fine to medium grained sand/silty sand. The medium dense to dense
sand extends to borehole termination at 9 m depth. Groundwater seepage was not encountered during
drilling, with the sands being moist and becoming moist to wet at about 8 m depth (approx. RL 1 m). Similar
foundations and deeper subsurface conditions are expected under the adjacent boulder wall.
A field test carried out by an Alta engineer (see Appendix C of the Alta SWMP) indicates that the infiltration
rate under 2700 mm of head (when the RIPs are full) would be approximately 40 mm/sec. Anecdotal
evidence indicates that existing RIPs in earlier stages of the development have never been seen to
overflow, except for one occasion where the basal geosynthetic filter fabric became clogged with fines.

Design drawings (Ref. Alta Dwg. No. A6107/22/L9/EW02, Rev A) indicate that the proposed pool adjacent
to the RIPs will be supported by screw piles.

To enable the RIP construction adjacent to the existing boulder wall, sheet piles will be installed within the
approximate 1 m wide space between the back of the retaining wall and the proposed RIPs. ASP has
indicated that the sheet piles will be 4 mm thick and will be installed to depths of 6 m to 8 m below existing
ground level (behind the wall crest). The sheets will be interlocking with a clutch mechanism so that there
would be minimal opportunity for fine grained sand and/or water to penetrate through the joints.

Based on AS2159-2009 Piling - Design and installation, the sandy soil conditions at this site (allowing for
regular saturation by ‘groundwater’) would be classified as non-aggressive. From Table 6.5.3 of AS2159,
corrosion allowance for steel piles would therefore be <0.01 mm/year. However, if we conservatively
assumed a Mild exposure and corrosion at 0.02 mm/year, for a design life of 50 years, the sheets could lose
1 mm from both sides (2 mm from total thickness) so there should be at least 2 mm still in place after 50
years. Therefore, although the sheet piles are proposed to maintain temporary stability of the existing
boulder wall, the would remain in place sufficiently to provide protection to the wall from infiltrated water for
the long-term.
3.0       ASSESSMENT & ADVICE

As demonstrated by previous field testing at Settlers Cover by others, and visual observations of the long-
term performance of existing RIPs within Settlers Cove by Alta, the local sands have sufficiently high
permeability to enable the proposed rapid infiltration of stormwater. Such sands were identified to at least
9 m depth in a borehole at the proposed location of the RIPs at the subject site. It is noted that groundwater
could affect the performance of the RIPs, however the adjacent borehole indicates that groundwater is at
about RL 1 m, almost 5 m lower than the underside of the proposed RIPs. It is therefore concluded that the
soils are suitable to meet the design intent of the RIPs at this site.

It is acknowledged that the proposed RIP operation could potentially have the following adverse effects on
the existing boulder retaining wall:

      a) Infiltration causing densification of foundation sands and settlement of the wall foundations.

      b) Saturation of the subsurface reducing the angle of friction in the sands beneath the wall and
         impacting global slope stability.

      c) Uncontrolled overflow from the RIPs (>Q100 events) causing scour erosion of the wall backfill
         materials.
With regard to potential impact a), the existing wall foundations are in a medium dense to dense condition,
thus there is negligible potential for the foundation sands to densify due to water infiltration. From our
extensive experience with densification of the natural sands at Settlers Cove, it is necessary to impart
significant vibration (typically by an excavator mounted vibrating plate) in combination with ample water to
effect sand densification (from loose to medium dense, or better). The foundations would not be subject to
such concentrated vibration during operation.




                                                        3/4