Compiled Reference Document - MCU25/0020 (As Published)

6.3.2        Temporary and Permanent Retention Systems
Given that the planned basement footprint extends close to the site boundaries, excavations will cover
most of the site. Allowing for the footprint extents some form of temporary shoring will be required to
support the excavations. Suitable shoring methods for the ground conditions encountered could include
secant (over-lapping) piles and mud diaphragm walls.
Temporary shoring should penetrate into the underlying dense (or denser) indurated sands/coffee rock,
to minimise flows up through the base of the excavation. Deeper ‘key in depths’ may be required,
depending on detailed design of the temporary shoring system. Care would need to be taken with
shoring to manage potential vibration impacts on nearby structures. It would be prudent to conduct a
dilapidation survey of such adjacent structures prior to commencement of the site works.
Considering the potential for adverse impacts on nearby structures due to settlement, and potential
environmental impacts due to significant dewatering of the groundwater table; the proposed temporary
retention system should be designed to support at least partial hydrostatic pressure. Given our
observations of groundwater levels and ASS conditions, we recommend that the external groundwater
level should not be dewatered deeper than 1.5 m BGL.
An internal dewatering system (e.g. a series of sand spears or wells connected to an extraction pump) will
likely be required to pick up any seepage that migrates through the shoring system, and up through the
sands across the basement excavation floor. This would be to prevent ‘boiling’ of the basement
subgrade. It should be possible to manage the groundwater extraction system such that the impact on
the surrounding, external groundwater table is negligible. To minimise the volumes of extracted
groundwater and the associated treatment requirements, it would be prudent to adjust the depths of
wells/spears and pumping rates such that the stabilised groundwater level below the excavation floor is
optimised (i.e. dewatering is not carried out to an excessive depth).
To minimise inflow rates and reduce the risk of external dewatering care must be taken to ensure the
temporary shoring wall is ‘water-tight’. Special attention would need given to the detailing such as
ensuring that the shoring elements overlap.
Subject to the recommendations outlined above, and the advice of a suitably experienced
shoring/dewatering contractor (including hydrogeological assessment of the proposed dewatering),
negligible impacts are anticipated on surrounding buildings and infrastructure due to the following:
   Temporary retention will be utilised around the entire excavation perimeter.
   External dewatering will not be undertaken below 1.5 m BGL, with the internal dewatering system
    optimised such that dewatering is not undertaken to an excessive depth.
   The upper soil layers primarily consist of sands, and groundwater levels are expected to fluctuate in
    response to weather conditions and tidal cycles. As such, the internal dewatering system is not
    anticipated to cause significant changes in the surrounding groundwater levels or induce soil
    shrinkage or swelling.
Due to the relatively shallow depth that groundwater was encountered during the investigation, the
proposed basement will need to be designed as a ‘tanked’ structure. The design of the proposed
basement walls and floor will therefore need to include allowance for hydrostatic pressures. This would
need to include allowance for the highest astronomical tide and potential flood/storm surge levels,
together with predicted rises due to climate change. It may be possible to incorporate a permanent
pressure relief system for the basement; however, due to ongoing requirements for treatment of
extracted groundwater, such a system is not likely to be feasible.



Geo-environmental Investigation Proposed Residence
4 June 2025 | 25040-001-Rev1                                                                            13