Compiled Reference Document - MCU25/0020 (As Published)

8.2.2        Record Keeping
To help demonstrate that appropriate quantities of lime have been used as specified to neutralise the
ASS, quantities of lime delivered to the site, verified by delivery dockets, will be maintained by the
Contractor. The lime usage shall quantify volumes of soil treated.

8.3          Management of Groundwater & In Situ ASS
Excavation below the groundwater table, such as that proposed for this site, has the potential to result in
localised lowering of the water table caused by dewatering. This process is referred to as groundwater
drawdown and can create a time variable cone of depression around the excavation, the scale of which is
dependent on the type and method of dewatering used, and the nature of the aquifer containing the
groundwater. The cone of depression is the volume of soil around the disturbance which was previously
saturated (below the water table), but due to the dewatering becomes unsaturated (at least for a short
time period during construction).
This lowering of the water table may lead to two potential problems:
   Settlement of the land adjacent to the excavation which may lead to subsidence; and
   Exposure of in situ PASS within the cone of depression, thereby causing oxidation of these soils and
    subsequent generation of acidity, having potential adverse effects on groundwater conditions.
The proposed construction is expected to utilise a engineered shoring retention system around the full
basement perimeter to provide temporary batter support for construction, and to minimise groundwater
inflows. So that the retention system can achieve an effective groundwater ‘cut-off’, it is recommended
that the retention system penetrates into the underlying dense (or denser) sands/coffee rock at around 7
m BGL. Furthermore, the retention system shall be designed to allow for the development of some
hydrostatic pressure to minimise external (e.g. spear point) dewatering.
Design and construction of a retention system as described above should result in negligible drawdown
of the groundwater table outside the excavation and mitigate the risk of ground settlement and/or
oxidation of possible in situ ASS in areas adjacent to the works.
It is anticipated that groundwater flows would be encountered through the base of the excavation during
construction, however such inflows should be handled using shallow well or spear points. These
groundwater inflows shall be treated and discharged together with any surface water runoff
accumulating during the construction period.
To further mitigate potential for risk associated with short-term dewatering, the following management
strategies should be undertaken to mitigate any potential issues:
   Depth of dewatering shall be limited to within 1 m from the base of proposed excavation.
   Quality of any groundwater inflows into excavations shall be monitored to ensure in situ acidification
    is not occurring. Monitoring will involve pH testing of inflows daily during dewatering periods. It
    should be noted that groundwater in the area may be naturally acidic (pH 5.0 to 6.0). If the
    monitored pH levels fall more than 1.0 unit over the course of construction monitoring, or is
    consistently below pH 4.0, then an ASS specialist should be engaged to determine whether there has
    been any significant environmental impact, why the pH has dropped and how the works may
    progress without undue environmental risk.




Geo-environmental Investigation Proposed Residence
4 June 2025 | 25040-001-Rev1                                                                             23