Reference Document - OPW23 0189

                                                                                               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 excavation using a contiguous secant pile wall. Excavations to facilitate the construction of the
         basement level will extend to a maximum of approximately 2.4 m below ground level, which corresponds
         to a maximum elevation of ‐0.7 m AHD (assuming 400 mm basement slab thickness). Localised deeper
         excavations may be required for construction of other structural elements (i.e. footings & pile caps). Refer
         to Preliminary Concept Plans, Appendix C.

         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 will be undertaken using a series of 40 mm diameter PVC spears installed to below maximum
         excavation depth at 1.5 m intervals around the internal perimeter of the basement excavation.

         The basement excavation must be dewatered to a minimum of 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 minimum of approximately 2.7 m to around ‐1.7 m AHD.
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 ADS) and known site‐
         specific conditions. The following is applicable to the basement excavations spanning across the lot:

               Approximate length of excavation = 18 m
               Approximate width of excavation = 16 m
               Reduction in depth of groundwater across the basement footprint/excavation = 2.7 m
               Approximate depth of saturated aquifer = 10 m (standardised conservative value applied)

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

         Table 4 Dewatering extraction rate results

                                   Output                                               Result

          Effective pumping radius of spears (m)                                          10
          Dewatering radius of influence (m)                                              79
          Discharge rate (L/sec)                                                          7
          Discharge rate (L/hr)                                                         25,200
          Establishment of cone of depression (hours)                                    111


         Based on the above inputs, a groundwater flow rate of approximately 7 L/sec or 420 L/min or 25,200 L/hr
         has been predicted for the development.

         However, it should be noted that the WA Calculator provides a conservative assessment of inflows to an




    ADG1580.23 Dewatering Management Plan_Rev0_28.09.23                                                        Page 9