RFI - ADG1451.22 Dewatering Management Plan_Rev0_28.02.23

                                                                                            ADG Consulting Pty Ltd



     Table 4 Dewatering extraction rate results

                               Output                                                Result

      Effective radius of dewatering spears (m)                                        18
      Dewatering radius of influence (m)                                               67
      Discharge rate (L/sec)                                                            9
      Discharge rate (L/hr)                                                          32,400
      Establishment of cone of depression (hours)                                      76


     Based on the above inputs, a groundwater flow rate of approximately 9 L/sec or 540 L/min or 32,400 L/hr
     has been predicted for the development, with the estimated cone of depression radius to be
     predominantly limited to within 3 m of the secant pile shoring wall. Flow rates are predicted to be highest
     during initial drawdown of the groundwater table within the basement excavation footprint, and
     reducing to approximately 6 L/sec following establishment of the cone of depression within the
     excavation. Flow rates may fluctuate due to tidal variations, generally approximately ± 0.5 m, and will
     also be influenced by rainfall and surface water collection within the basement excavation footprint
     noting the underlying predominantly sand profile encountered during geotechnical investigations.

     Regardless of the contractor employed, groundwater treatment systems should be installed with
     capability for treating flow rates of up to 10 to 12 L/sec, as a contingency against higher than anticipated
     flow rates exceeding the estimated 6 to 9 L/sec as a result of varying ground conditions across the extent
     of excavation.

     However, it should be noted that the WA Calculator provides a conservative assessment of inflows to an
     excavation footprint based on area and depth, but does not take account of the method of perimeter
     piling i.e. steel sheet pile, which will inhibit lateral inflows. Also note that the cone of 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 shoring wall in fine‐medium grained sands, depending on density of the strata).
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. It is noted that dilapidation reports for surrounding structures and services
     have been undertaken which were not supplied to ADG for the purposes of preparing this DMP, and are
     to be provided to Council separately from this DMP.

     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. Groundwater monitoring bores for groundwater level
     gauging external to the excavation footprint are unable to be established and monitored prior to and
     during construction on the site, due to limited space (SCG Consulting Engineers).


ADG1451.22 Dewatering Management Plan_Rev0_28.02.23                                                         Page 10