Reference Document - OPW25 0085 (AS Published)

Based on the results of the investigation, it is likely that excavation depths up to about 0.2 m to 0.6 m will
be required to remove the existing topsoil and loose clayey sands across the proposed pavement areas;
however, deeper pockets may be present.
After stripping and excavation to design levels, the subgrade is expected to comprise mostly medium
dense/stiff (or stronger) residual soils or very low strength (or stronger) sedimentary rock (areas of
deeper excavations). These materials could be subject to strength loss from repetitive vehicle loading
(particularly from rubber tyred plant) or if they become overly 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 laydown areas.
There is a risk that elevated soil moisture conditions could occur if the works are undertaken during a
period of wet weather and shallow groundwater could become perched in the sands above the
underlying less permeable clay. The upper sands could then become particularly unstable and very
difficult to work, proof roll or compact under elevated moisture contents. Draining water from these
materials can be very difficult and subgrade removal might be required if the materials become wet.
Although the CBR strength is moderate at close to OMC, it can be cost prohibitive to re-work, dry out
and re-compact such materials. Measures to divert water away from work areas and using lighter
earthmoving equipment should be considered to minimise the potential for development of unstable
subgrades. It would also be prudent to plan for earthworks to be conducted during the typically ‘drier’
months of the year.
Bridging layers may be needed if weak subgrade conditison are and encountered and this could
comprise:
    Placement of a non-woven geosynthetic filter fabric (e.g. Bidim A39, or equivalent) across t subgrade
     to prevent ‘pumping’ of ‘fines’ (silt/clay materials) into the rock fill.
    Placement of a minimum 300 mm thick rock mattress. We suggest the use of 75 mm to 150 mm,
     angular, durable rock fill.
    Fully wrapping the rock fill with non-woven geosynthetic filter fabric (Bidim A39, or equivalent), prior
     to placement of select controlled fill.
Suitably sized plant should be used for earthworks, and heavy vehicles should avoid trafficking directly
on the subgrades. Care must be taken with the use of vibratory equipment to ensure that there are no
adverse impacts on surrounding structures, and that vibration does not cause groundwater to ‘pump’
into and weaken the subgrade. The impacts from ground vibration must be closely monitored and
managed during construction.
The site should be graded to shed surface water runoff away from all construction areas. Swale drains
should be constructed along the toes of any cuts, and above cuts where possible, with the drains graded
to discharge collected waters to a control point before release off-site. Appropriate erosion and
sediment control measures must be installed prior to commencement of earthworks at this site.
Specific design would need to be carried out to determine working platform requirements for any heavy
plant (e.g. mobile cranes and piling rigs), and a thicker rock fill platform may be required than suggested
above.

6.2.2        Excavations
Excavations in the natural soils and very low to low strength sedimentary rock to the test pit termination
depths should be achievable using conventional earthmoving plant, such as medium to large (8 to 20
tonne) excavators with toothed buckets and/or ripper attachments.


Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                           17