Compiled Reference Document - MCU25/0020 (As Published)

ADG Consulting Pty Ltd

 

5.4

5.5

The soil profile of comprised predominantly of fine to medium grained silty 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
Effective radius of dewatering spears (m) 41
Dewatering radius of influence (m) 193
Discharge rate (L/sec) 9
Discharge rate (L/hr) 32,400
Establishment of cone of depression (hours) 1,424

Based on the above inputs, a groundwater flow rate of approximately 9 L/sec or 540 L/min or 32,400 L/hr
has been predicted for the development, with the estimated cone of depression radius to be
predominantly limited to within 3 m of the secant pile shoring wall. Flow rates are predicted to be highest
during initial drawdown of the groundwater table within the basement excavation footprint, and
reducing to approximately 5 — 6 L/sec following establishment of the cone of depression within the
excavation. Flow rates may fluctuate due to tidal variations, generally approximately + 0.5 m, and will
also be influenced by rainfall and surface water collection within the basement excavation footprint
noting the underlying predominantly sand profile encountered during geotechnical investigations.

Regardless of the contractor employed, groundwater treatment systems should be installed with
capability for treating flow rates of up to 10 to 12 L/sec, as a contingency against higher than anticipated
flow rates exceeding the estimated 9 L/sec as a result of varying ground conditions across the extent of
excavation.

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. contiguous pile walls, 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 depression
of the water table is limited to much closer to the spear point dewatering spears (generally within 3 to
10 m from the shoring wall in fine-medium grained sands, depending on density of the strata).

Dewatering duration

Dewatering will be necessary throughout the basement construction phase, which is expected to require
dewatering for approximately 16 to 20 weeks. To minimise the potential impacts during construction of
short-term dewatering, periods of active dewatering will be minimised as much as possible. However,
ADG notes that once active dewatering has commenced, dewatering will not be able to be ceased until
both: a) the basement hydrostatic uplift forces have been compensated to the satisfaction of the
consulting RPEQ engineer; and b) potential hydraulic loading on the external face of the sheet pile or
contiguous pile shoring system has been compensated to the satisfaction of the consulting RPEQ engineer
and shoring contractor.

Groundwater drawdown

Groundwater drawdown caused by dewatering has the potential to create settlement in unconsolidated
sediments and loose sands. It is noted that dilapidation reports for surrounding structures and services
are proposed to be undertaken which were not supplied to ADG for the purposes of preparing this DMP,
and are to be provided to Council separately from this DMP.

Groundwater drawdown may also oxidise potential acid sulfate soils within the cone of depression,
subsequently producing acidic groundwater conditions and mobilising heavy metals. Monitoring of the
extracted untreated groundwater will be undertaken to detect potential in-situ oxidation, as required

 

ADG1786.25 Dewatering Management Plan_Rev1_07.10.25 Page 11