RESPONSE TO FURTHER ADVICE - ADG1808.25 Dewatering Management Plan_Rev0_07.05.25 · MCU25/0054

                                                                                           ADG Consulting Pty Ltd

             Reduction in depth of groundwater across the excavation = 2.2 m (based on an approximate
              groundwater depth of 0.5 m AHD)
             Approximate depth of saturated thickness of unconfined aquifer = 10 m (conservative standardised
              approximation)

      A conservative soil profile of fine to medium grained sands, has a hydraulic conductivity of approximately
      8.2 m/day, or equivalent 0.0000949 m/sec (WA Calculator). The results of the calculation are shown in
      Table 4 below, and provided in Appendix D.

      Table 4 Dewatering extraction rate results

                                   Output                                           Result

          Effective radius of pumping well (m)                                         9
          Dewatering radius of influence (m)                                          64
          Discharge rate (L/sec)                                                       6
          Discharge rate (L/hr)                                                     21,600
          Establishment of cone of depression (hours)                                 69


      Based on the above inputs, a groundwater flow rate of approximately 360 L/min or 21,600 L/hr has been
      predicted for the development. Flow rates are predicted to be highest during initial drawdown of the
      groundwater table within the basement excavation footprint and reducing to approximately 4 – 5 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 8 to 10 L/sec, as a contingency against higher than anticipated
      flow rates exceeding the estimated 6 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 shoring wall, which will
      inhibit lateral inflows to some extent. 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 dewatering spears or wells (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
      continual dewatering for approximately eight to twelve 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 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
      are proposed to be 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.


 ADG1808.25 Dewatering Management Plan_Rev0_07.05.25                                                      Page 11