ADG Dewatering Management Plan · OPW21/0315

                                                                                         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 interlocking steel sheet piles. Excavations to facilitate the construction of the
     basement will extend to approximately 3.1 m below ground level, which corresponds to a maximum
     elevation of -0.8 m AHD. Localised deeper excavations may be required for construction of other
     structural elements (i.e. footings & pile caps).

     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 6.0 m length, 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 is therefore
     expected to be lowered by minimum of approximately 1.8 m to around -1.8 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 = 26 m
       −    Approximate width of excavation = 20 m
       −    Reduction in depth of groundwater across the basement footprint/excavation = 1.8 m
       −    Approximate depth of saturated aquifer = 16 m (standardised conservative value applied)

     The soil profile of comprised predominantly of fine to medium 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.

     Table 4 Dewatering extraction rate results

                               Output                                                Result

      Dewatering radius of influence (m)                                               53
      Discharge rate (L/sec)                                                           12
      Discharge rate (L/hr)                                                         43,200
      Establishment of cone of depression (hours)                                      25


     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 steel 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


ADG1290.21 Dewatering Management Plan_Rev0_12.10.21                                                         Page 9