RESPONSE TO FURTHER ADVICE - ADG1808.25 Dewatering Management Plan_Rev0_07.05.25 · MCU25/0054

                                                                                             ADG Consulting Pty Ltd


5        Dewatering methodology
5.1      Basement excavation and shoring
         Based on preliminary information provided by SCG, existing ground levels within the proposed basement
         footprint are approximately RL 1.35 m Australian Height Datum (AHD). Excavations to facilitate the
         basement construction are proposed to extend to an approximate depth of 2.05 m below existing ground
         level (bgl), corresponding to an approximate elevation of ‐0.7 mAHD (allowing for up to 325 mm raft slab
         and 75 mm blinding layer for basement finished floor level of ‐0.3 mAHD). Localised deeper excavations
         may also be required for other structural elements (i.e. pile caps and footings) across the site. It is
         understood the excavation/basement perimeter will generally be stabilised using a temporary sheet pile
         wall (or similar retention system), 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 reported at the time of drilling to a depth of 3.0 mbgl during the
         Geotechnical Investigations on the site in 2025 (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 2.2 m in the locality of the basement
         excavation footprint to a maximum of approximately ‐1.7 mAHD throughout the basement footprint.
         Deeper localised dewatering may be required for the installation of 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.

         The calculation results are based on the following parameters and known site‐specific conditions:
             Approximate length of excavation = 21m
             Approximate width of excavation = 11 m

    ADG1808.25 Dewatering Management Plan_Rev0_07.05.25                                                        Page 10