ADG Dewatering Management Plan · OPW21/0315

                                                                                       ADG Consulting Pty Ltd


     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 Wyuna Drive (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 Wyuna Drive 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.
5.6 Groundwater treatment
     All extracted groundwater will be treated on-site using available technologies prior to discharge to the
     stormwater network. The following treatment measures will be adopted, and will be installed and
     operational prior to the commencement of dewatering:



ADG1290.21 Dewatering Management Plan_Rev0_12.10.21                                                      Page 10