Reference Document - Dewatering Management Plan (For Stamping) MCU25/0053 & OPW25/0074

                                                                                                                   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




Document Set ID: 23735664
Version: 1, Version Date: 12/05/2025