OPW052025 Report - Part 2

6.2           Earthworks
6.2.1         Site Preparation and Trafficability
Any organic rich topsoil, firm/loose (or weaker) soils, overly wet materials must be stripped and removed
from the proposed building footprint and pavement subgrades. Topsoil materials must not be used in
structural fill; however, may be placed across landscaping areas. It is recommended that following
removal of unsuitable materials all building/pavement subgrades be proof rolled using a minimum 4
tonne rubber tyred vehicle to identify any ‘soft spots’ in the presence of a geotechnical RPEQ. Any such
soft spots should be excavated and replaced with select fill material, constructed as recommended in
Section 6.2.2. Based on visual appraisal and the subsurface investigation findings, excavations to remove
unsuitable materials are anticipated to be minimal; however, deeper pockets of unsuitable materials may
be present in isolated locations.
The subgrade materials exposed after site stripping are expected to comprise silty clays with varying
gravel content. These soils could be subject to strength loss from repetitive vehicle loading (particularly
from rubber tyred plant) or if they become wet. Due to the potential for poor trafficability during and
after wet periods, placement of a minimum 150 mm thick granular working platform may be required,
particularly along access ways and any laydown areas.
The basaltic soils in the proposed building area for the residence can variably grade to clayey silts, which
are sensitive to repeated tracking and moisture variation. Therefore, wherever possible during
construction, light plant should be utilised and trafficking of subgrades should be minimised.
Subgrades should be graded and ‘sealed off’ at the end of each working day using a smooth drum roller
to prevent ponding of water and aid shedding of runoff, so that the risk of subgrade softening is
minimised. Earthworks would likely be particularly prone to disruption due to wet weather, which should
be considered accordingly during project planning.
The site should be graded to shed surface water runoff away from construction areas. It should be noted
that during wet weather, the natural soils may cause turbid stormwater run-off. Appropriate erosion and
sediment control measures must be installed prior to commencement of earthworks at this site.

6.2.2         Filling
Site soils, where free of deleterious materials (e.g. vegetation, oversize etc.), are expected to be suitable
for re-use as structural fill, subject to appropriate moisture adjustment. Due to the mostly high plasticity
and potentially highly reactive nature of the site soils, it is recommended that the high plasticity clays
and high-liquid limit silts should not be reused as structural fill onsite; however, they could be used as
non-structural fill (e.g. in landscaping areas, batters etc.). Reuse of high plasticity clays and silts will have
adverse implications for site classification, pavements (low CBR values) and footing design.
Should importation of fill be required, it is recommended that such fill should comply with the following:
▪    Minimum CBR of 10%
▪    Maximum plasticity index of 15%
▪    Shrink-swell index ≤ 1%
▪    Maximum particle size of 100 mm
Any new fill materials should be compacted at moisture contents within the range of -2% to +2% of
optimum moisture content for Standard Compaction.



Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                  19