RFI - ADG1451.22 Dewatering Management Plan_Rev0_28.02.23

                                                                                      ADG Consulting Pty Ltd


5    Dewatering methodology
5.1 Basement excavation and shoring
     It is understood that the excavation/basement perimeter will be stabilised along the perimeter of the
     basement excavations using a contiguous secant pile wall (refer to supplied indicative basement design
     at Appendix C). Excavations to facilitate the construction of the basement will extend to approximately
     2.3 m below ground level, which corresponds to a maximum elevation of ‐0.7 m AHD. Localised deeper
     excavations may be required for construction of other structural elements (i.e. footings & pile caps).

     All excavated spoil from depths between 1.5 to 2.5 mbgl within the excavation footprint will be
     appropriately neutralised with lime and validated in accordance with recommendations by PG in the ASS
     Investigation Report (PG, 2022).

     It is expected that the development will include a ‘tanked’ basement to prevent the requirement for a
     permanent dewatering facility at the site.
5.2 Groundwater extraction
     Dewatering is anticipated to be undertaken using a series of 6.0 m length, 40 mm diameter PVC spears
     installed to below maximum excavation depth at 1.5 m intervals around the internal
     perimeter of the basement excavation (refer to supplied indicative shoring design at Appendix C).

     The basement excavation must be dewatered to a minimum of approximately 1.0 m below the deepest
     point of excavation to allow construction to be undertaken in dry conditions. The water table is therefore
     expected to be lowered by a minimum of approximately 2.3 m to around ‐1.7 m AHD. It is expected that
     the cone of depression in the standing groundwater table will be predominantly limited to within the site
     boundaries.

     It is proposed that the development will include a ‘tanked’ basement to prevent the requirement for a
     permanent dewatering facility at the site. Similarly, no permanent groundwater dewatering is proposed
     or likely to occur as a result of the development, with all services and pipework proposed to be situated
     above standing groundwater elevations (that is, at elevations above 0.6 mAHD) and subject to the
     nominated hold points at Section 5.7. Where any deviation is proposed from this arrangement, all
     pipework within the saturated zone, that outlets direct to waterways must be fitted with buried
     headstocks at suitable intervals to prevent ongoing dewatering of groundwater along pipework bedding.
5.3 Extraction/discharge rates and volume
5.3.1 Dewatering extraction calculation based on subsurface soil profile
     The dewatering extraction rate, area of influence and cone of depression has been estimated using the
     Western Australian Department of Water and Environmental Regulation Cone of Depression Calculator.

     The calculation results are based on the following parameters (as provided by SCG) and known site‐
     specific conditions. The following is applicable to the basement excavations spanning across the lot:

           Approximate length of excavation = 35 m
           Approximate width of excavation = 30 m
           Reduction in depth of groundwater across the basement footprint/excavation = 2.3 m
           Approximate depth of saturated aquifer = 10 m (standardised conservative value applied)

     The soil profile of comprised predominantly of fine to medium grained silty sands, which has a hydraulic
     conductivity of 8.2 m/day, or equivalent 0.0000949 L/sec (WA Calculator). The results of the calculation
     are shown in Table 4 below, and a copy of the output is provided in Appendix D.




ADG1451.22 Dewatering Management Plan_Rev0_28.02.23                                                      Page 9