Attachment 4 - Geotech Report · MCU23/0030

                             50 NOOSA PARADE, NOOSA HEADS
                             PROPOSED RESIDENCE

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.
To minimise dewatering and mitigate impacts to groundwater outside of the construction area, construction
will utilise a retention system around the full basement perimeter to provide temporary batter support for
construction, and to minimise groundwater inflow. This should enable construction activities occurring inside
the basement to have a negligible impact on the environmental values of the groundwater and soil outside of
the basement perimeter.
To achieve this, the retention system must be designed to incorporate the following:
       Development of full hydrostatic pressure so that external dewatering is minimised.
       For sheet piles, the design incorporates an efficient sheet interlock method so that seepage
        between individual sheets is minimal, and ingress of fine soils is negated.
       So that the perimeter wall can achieve an effective groundwater ‘cut-off’, piles should
        penetrate into the very dense, indurated sand layers present at depths below 7 m BGL (about
        4 m below the proposed excavation level).
Proposed pool excavation is also expected to extend below groundwater levels and temporary retention may
also be required for the pool excavation, however a less intensive method may be adopted considering that
excavations would only likely penetrate just below the groundwater table.
Design and construction of a basement retention system as described above should result in negligible
drawdown of the groundwater table outside the excavation and minimise oxidation of in situ ASS in areas
adjacent to the works. This methodology should also significantly reduce the volumes of groundwater
ingress, which would reduce difficulty in managing construction waters prior to discharge.
It is anticipated that some groundwater flows may be encountered through the base of the excavation and
through the perimeter piles during construction, however such inflows should be handled using shallow
sump-pumps. These groundwater inflows shall be treated and discharged together with any surface water
runoff accumulating during the construction period.
Some minor dewatering may be required to facilitate construction of footings and lift overrun pits and may be
undertaken with minimal risk as these are typically located in the centre of the basement zone. Where minor
dewatering or likely drawdown may be occurring, passive control measures such as lime application and/or
surface watering may be used to mitigate risk of soil oxidation or leachate transport.
To further mitigate any potential for risk associated with minor/short-term dewatering, the following
management strategies should be undertaken to mitigate any potential issues:
       Periods of any active dewatering shall be limited as much as possible.
       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 (5.0-6.0). If the monitored pH
        levels fall more than 1.0 unit over the course of construction monitoring, or is consistently below pH


2 February 2023
Report No. 22500-001-Rev0                              17