Appendix G - Dewatering Management Plan · MCU25/0053.01

                                                                                             ADG Consulting Pty Ltd


5        Dewatering methodology
5.1      Basement excavation and shoring
         Based on preliminary information provided by Seymour Group, existing ground levels within the
         proposed basement footprint are approximately RL 2.4 m Australian Height Datum (AHD). Excavations to
         facilitate the basement construction are proposed to extend to an approximate depth of 3.5 m below
         existing ground level (bgl), corresponding to an approximate elevation of ‐1.1 mAHD. Localised deeper
         excavations may also be required for other structural elements (i.e. lift pit and footings) across the site.
         It is understood the excavation/basement perimeter will generally be stabilised using a contiguous secant
         pile wall, except in proximity of the proposed pool on the northern side (facing the vacant parkland to
         the north of the site) and as such will limit the potential for ground movement and instability of
         surrounding lands, services and structures (refer to Appendix C).

         Groundwater seepage was not encountered at the time of drilling to a depth of 4.5 mbgl during the
         Geotechnical Investigations on the site in 205 (PTG Consulting, 2025). However, ADG notes that, typically,
         the groundwater levels in the area are encountered at levels of approximately RL 0.5 m to RL 1.0 m.
         Fluctuations of +/‐ 0.5 m are typically noted in the area. Groundwater levels are also likely to be subject
         to fluctuations associated with the seasonal and climatic conditions, as well as tidal variations and
         significant rainfall events.

         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 40 mm diameter PVC spears installed at
         1.5 m intervals around the internal and external perimeter of the excavation. Alternatively, well point
         dewatering may be appropriate within the internal face of the shoring wall. The final dewatering method
         is subject to engagement of a shoring and dewatering contractor, as different contractors may specialise
         in different methods of achieving the required dewatering drawdown. Preference should be given to
         dewatering primarily on the internal face of the shoring wall, with preference for minimal dewatering
         external to the shoring system to assist in minimising the extent of the cone of depression in the water
         table (unless expressly required to limit hydrostatic force on the shoring system) and thereby limit
         potential ground movement that may affect surrounding structures and services.

         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 (approx.
         0.5 mAHD) is therefore expected to be lowered by at least 2.6 m in the locality of the basement
         excavation footprint to a maximum of approximately ‐2.1 mAHD throughout the basement footprint.
         Deeper localised dewatering may be required for the installation of a lift overrun pit and/or other
         structural elements.

         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 0.5 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.




    ADG1777.25 Dewatering Management Plan_Rev0_01.05.25                                                       Page 10