ADG1304.21 Dewatering Management Plan_Rev1.1_17.12.24

                                                                                        ADG Consulting Pty Ltd


      depression radius measure includes water table depressions of 1 mm depth or more, so any significant
      depression of the water table is limited to much closer to the spear point dewatering spears (generally
      within 3 to 10 m from the sheet pile wall in fine‐medium grained sands, depending on density of the
      strata).
5.3.2 Dewatering discharge rate based on stormwater system size (estimation)
      It is expected that the stormwater network on Mossman Court (designated stormwater discharge
      location) has a maximum flow capacity of approximately 81 L/sec (i.e. based on a 300 mm diameter
      stormwater network pipe and an unknown install grade, calculated using minimum grade of 0.5 % as per
      AS 3500.3, however likely to be greater).

      A conservative estimate of 12 L/sec or 43,200 L/hr has been calculated by ADG – refer to Section 5.3.1
      above.
5.3.3 Conclusions on expected extraction rates
      It is recommended that the conservative estimate of 43,200 L/hour be adopted based on the sand profile
      encountered during the sub‐surface investigations onsite. Based on previous dewatering projects
      undertaken by ADG and ADS within the area, it is anticipated that initial flow rates will reduce to between
      500 – 650 litres/minute following the initial drawdown of the water table.

      The estimated discharge rate as determined in the above section 5.3.1, is well below (14.8 %) the Council
      stormwater infrastructure capacity estimate (81 L/sec), therefore it is considered unlikely that discharge
      of extracted groundwater from the site to the stormwater network on Mossman Court will cause any
      disturbance/damage to council infrastructure or risk to the public. The likelihood of overtopping the
      stormwater network and/or adjacent gully pits is considered unlikely because any such occurrences
      would be limited to instances of extreme rainfall intensity, which are generally of short duration, and in
      such a circumstance stormwater runoff would be expected to first utilise the remaining stormwater
      network capacity (some approximately 69 L/sec), and any excess flow would then drain via the kerb and
      channel to adjacent gully pits.

      Any risks to the public associated with the placement of dewatering equipment to enable discharge to
      the designated stormwater gully pit, should be mitigated by fencing and signage, have appropriate
      lighting in place for after site hours and may need pipe over ramps to the pathway and/or driveways
      where header pipe is to be placed in roadside guttering.
5.4   Dewatering duration
      Dewatering will be necessary throughout the basement construction phase, which is expected to require
      dewatering for approximately 10 to 12 weeks. To minimise the potential impacts during construction of
      short‐term dewatering, periods of active dewatering will be minimised as much as possible. However,
      ADG notes that once active dewatering has commenced, dewatering will not be able to be ceased until
      both: a) the basement hydrostatic uplift forces have been compensated to the satisfaction of the
      consulting RPEQ engineer; and b) potential hydraulic loading on the external face of the sheet pile shoring
      system has been compensated to the satisfaction of the consulting RPEQ engineer and shoring
      contractor.
5.5   Groundwater drawdown
      Groundwater drawdown caused by dewatering has the potential to create settlement in unconsolidated
      sediments and loose sands. Groundwater drawdown may also oxidise potential acid sulfate soils within
      the cone of depression, subsequently producing acidic groundwater conditions and mobilising heavy
      metals. Monitoring of the extracted untreated groundwater will be undertaken to detect potential in‐situ
      oxidation, as required and at the frequencies stated in Section 6.5.2.




 ADG1304.21 Dewatering Management Plan_Rev1.1_17.12.24                                                    Page 10