Reference Document - OPW23 0189

                                                                                         ADG Consulting Pty Ltd


      excavation footprint based on area and depth, but does not take account of the method of perimeter
      piling i.e. sheet pile/concrete secant wall, 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 pile 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 8 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 piled 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 available or 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).

      A plan showing the proposed basement extent is provided at Appendix C.
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:

      1.    A groundwater pH buffering treatment system (primary treatment)
      2.    A sedimentation tank to provide additional residence time and sedimentation (secondary
            treatment).
      3.    Installation of fine media filtration to retain suspended sediments and/or heavy metals (tertiary
            treatment), if required.

      Details of the treatment systems and methodologies are provided in Section 6 of this report.

      Note: Consideration should be taken, regarding placement of treatment gear on site to enable easy
      cleaning and maintenance throughout the dewatering phase for the project. Consider the potential
      requirement for regular de‐sludging of sedimentation tank/s to suitable area onsite, if available (cost
      minimisation).
5.7   Nominated Hold Points
      In accordance with anticipated DA Conditions, nominated hold points to be enacted during construction
      have been proposed to facilitate for certification by the suitably qualified person experienced with Acid
      Sulfate Soils, as well as Council Development Engineer to ensure all possible avenues for continual
      dewatering below permanent groundwater level, including around pipework bedding that outlet to



 ADG1580.23 Dewatering Management Plan_Rev0_28.09.23                                                      Page 10