ADG1304.21 Dewatering Management Plan_Rev1.1_17.12.24

                                                                                           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 interlocking steel sheet piles. Excavations to facilitate the construction of the
         basement level will extend to a maximum of approximately 2.70 m below ground level (approx.
         2.2 m AHD), which corresponds to a maximum elevation of ‐0.5 m AHD (basement floor level ‐
         0.15 m AHD; allow 350 mm for slab). Localised deeper excavations may be required for construction of
         other structural elements (i.e. footings & pile caps). Refer to Design 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 and external 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 (approx. 0.5 m AHD)
         is therefore expected to be lowered by minimum of approximately 2.0 m to around ‐1.5 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 = 23 m
               Approximate width of excavation = 20 m
               Reduction in depth of groundwater across the basement footprint/excavation = 2.1 m
               Approximate depth of saturated aquifer = 16 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

          Dewatering radius of influence (m)                                             61
          Discharge rate (L/sec)                                                         12
          Discharge rate (L/hr)                                                        43,200
          Establishment of cone of depression (hours)                                    35


         Based on the above inputs, a groundwater flow rate of approximately 12 L/sec or 720 L/min or
         43,200 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
         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


    ADG1304.21 Dewatering Management Plan_Rev1.1_17.12.24                                                     Page 9