003 ECM_22828362_v1_Reference Document - OPW23 0189 · OPW26/0094

                                                                                                                 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




Document Set ID: 22828362
Version: 1, Version Date: 26/02/2024