Compiled Reference Document - MCU25/0020 (As Published)

ADG Consulting Pty Ltd

 

5.2

5.3

Dewatering methodology
Basement excavation and shoring

It is understood that the excavation/basement perimeter will be stabilised along the perimeter of the
basement excavations using a contiguous soldier pile wall. Excavations to facilitate the basement
construction will extend to a maximum of approximately 7.4 m below ground level (bgl) to approximately
RL-5.0 m Australian Height Datum (AHD; assuming local existing surface levels of around 2.4 m AHD and
a basement slab in the order of 0.4 m thickness). Localised deeper excavations will also be required for
the construction of the car stacker overrun pit and other structural elements (i.e. footings and pile caps),
where required.

Groundwater seepage was previously recorded at a depth of approximately 1.8 to 2.0 mbgl (equating to
an approximate elevation of approximately 0.6 to 0.4 mAHD), during geotechnical investigations on the
site (Tectonic, 2025). No further recent data has been supplied for review in preparing this DMP, however
based on ADG’s experience on nearby sites in the locality in recent years, it is anticipated that
groundwater should be expected at or around 0.6 m AHD, +/- 0.5 m, depending on tidal and seasonal
rainfall conditions. Dewatering will therefore be necessary to allow the basement to be constructed in
dry conditions.

It is expected that the development will include a ‘tanked’ basement to prevent the requirement for a
permanent dewatering facility at the site.

Groundwater extraction

Dewatering is anticipated to be undertaken using a series of 8.0 m length, 40 mm diameter PVC spears
installed to 8.0 m in depth from approx. RL1.5 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 approximately 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 a minimum of approximately 6.6 m to around -6.0 m AHD. It is expected that
the cone of depression in the standing groundwater table will be predominantly limited to within the site
boundaries due to the low permeability secant pile wall.

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 0.3 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.

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. The
following is applicable to the basement excavations spanning across the lot:

— Approximate length of excavation = 23 m

— Approximate width of excavation = 17 m

— Reduction in depth of groundwater across the basement footprint/excavation = 6.6 m

— Approximate depth of saturated aquifer = 10 m (standardised conservative value applied)

 

ADG1786.25 Dewatering Management Plan_Rev1_07.10.25 Page 10