Compiled Reference Document - MCU25/0020 (As Published)

Although relatively good trafficability should be encountered during ‘dry’ weather conditions and with
effective groundwater dewatering on the expected natural sand subgrades, the soils could be subject to
strength loss from repetitive vehicle loading (particularly from rubber tyred plant), mainly if they are
saturated. Thus, the measures outlined below may be required:
   Grade the base of the excavation (minimum 1V:100H) to achieve positive drainage to a dewatering
    point(s) (e.g. sump-pump), with collected waters then pumped to a suitable discharge outlet (after
    treatment to a suitable quality).
   Place non-woven geosynthetic filter fabric (e.g. Bidim A34 or equivalent) across the subgrade to
    prevent ‘pumping’ of ‘fines’ (silt materials) into the rock fill working platform. The grade of
    geosynthetic to be used will be dependent upon the particle size of the selected granular material
    used (see below).
   Construct a minimum 150 mm thick granular working platform to the design subgrade level. We
    suggest the use of ‘150 minus’ rock fill with a thin capping layer of CBR 15% material to provide a
    smooth working surface.
Specific design will need to be carried out to determine working platform requirements for any heavy
plant (e.g. mobile cranes or piling rigs).
Construction areas external to the basement should be graded to enable flow of stormwater to a control
point(s) for pumping out and/or treatment, if necessary, prior to off-site release.

6.2.2        Excavations
Excavations for the basement to depths in the order of about 7 m to 8 m BGL are expected to encounter
mostly loose to medium dense sands, with some firm clays. Excavations to the design level should be
achievable using conventional earthmoving plant (e.g. excavators with toothed buckets).
Groundwater seepage and inflow was encountered during the investigation at depths between 1.8 m and
2 m BGL, with the standing groundwater table recorded in the standpipe installed in borehole BH1 at a
depth of 1.8 m BGL. Based on the close proximity of the site to the Noosa River, groundwater levels will
likely fluctuate with tidal levels on a damped cycle and may rise closer to the surface following periods of
prolonged or heavy rainfall. Due to the presence of groundwater at shallow depth in natural sands and
considering that the basement excavations will extend up to most site boundaries, battering is generally
not expected to be possible. Temporary shoring of the excavation will be required and is discussed in
Section 6.3.2.

6.2.3        Material Re-Use
Soils excavated for the basement where free of any deleterious materials (e.g. organics, building debris
etc.) are expected to comprise mostly silty sands. The sandy soils should generally be suitable for re-use
on or off site as structural fill if spread out in thin layers and allowed to dry out to optimum moisture
content for compaction (noting the limits on filling described above). Internal dewatering prior to bulk
excavation would be prudent to enable easier handling of excavated materials for off-site transport and
re-use, particularly if groundwater is shallow at the time of construction.
It should be noted that compaction of such relatively poorly graded natural sands as structural fill
generally requires the use of vibratory compaction equipment (typically a smooth drum roller).




Geo-environmental Investigation Proposed Residence
4 June 2025 | 25040-001-Rev1                                                                              11