ADG1290.21 Dewatering Management Plan_Rev1_02.02.22 · OPW21/0315

                                                                                          ADG Consulting Pty Ltd


     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 3 to 5 L/sec as a result of varying ground conditions across the extent
     of excavation.
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 secant 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. Any such settlement is proposed to be minimised via the revised shoring
     approach of using a full secant pile (cut off) shoring wall around the basement excavation and only
     dewatering on the internal face of the secant pile wall. 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 (as per Development Approval RAP2020/0020 Conditions 27 and 28) due
     to the secant pile (cut off) walls to be located in close proximity to property boundaries and therefore
     monitoring bores cannot be installed due to limited space (refer SCG Consulting Engineers letter dated
     02/02/22, ref. 31073(L2)).

     A plan showing the proposed shoring and dewatering system and estimated cone of depression contours
     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



ADG1290.21 Dewatering Management Plan_Rev1_02.02.22                                                         Page 11