Reference Document - OPW25 0085 (AS Published)

All excess stormwater collected around the proposed development area, including the effluent disposal
area, must be directed by pipes or surface channels (e.g. rock lined ‘dry creek’ beds) to discharge at least
10 m away from and downslope of any structure or embankment, and not towards the effluent disposal
areas. Discharge should be directed towards the natural drainage lines, where possible. A surface
dissipation device such as rock weir or rock lined contour drain must be installed at the stormwater
outlet(s).
Subsurface disposal of stormwater must not be undertaken (including infiltration pits).


6.6          Pavement Design Parameters
Laboratory testing of the clayey silt from test pit TP8 indicated a CBR value of 6 %. Pavement design will
be dependent upon the subgrade materials present after earthworks. Following preparatory earthworks,
we anticipate the pavement subgrades in the centre and southern end of the site will likely comprise
mostly stiff (or stiffer), high plasticity silty clays or high liquid limit clayey silts; or weathered siltstone in
areas of deeper cuts. Pavement subgrades at the northern end of the site are likely to comprise clayey
sands/sandy clays or weathered sandstone in deeper cuts.
Due to the high silt/clay content, the natural soil subgrades would be susceptible to poor engineering
qualities if they become significantly wet of OMC. Earthworks planning would be prudent to avoid
saturation of the natural subgrades. Provided removal of any unsuitable materials is undertaken as
outlined within this report, preliminary pavement design could be carried out adopting a design CBR
value of 3% for the natural clay and silt subgrades and design CBR value of 5 % for the natural clayey
sands.
Should significant ‘wetting’ of the natural subgrade occur, over-excavation of the silts and replacement
with select, granular materials may be required during construction to achieve a design CBR value.
Where future fill provides at least 500 mm deep coverage across the natural soils, pavement design may
be based upon the CBR value of those materials. In sections of the alignment where very low strength
(or stronger) weathered rock is exposed consistently over the full width of the road, a presumptive CBR
value of 10 % could be adopted for design.
All pavement subgrades must be ‘proof rolled’ and visually assessed by an experienced RPEQ engineer to
ensure no heaving or unacceptable subgrade deflection occurs. Should heaving or deflection occur,
excavation of ‘soft spots’ may be required, followed by reinstatement with compacted select granular fill.
Subgrades should be compacted to 100% DDR (standard) at optimum moisture content with
confirmatory testing carried out prior to pavement construction.
The recommended design CBR values are applicable to properly drained subgrades only and must be
confirmed by subgrade inspection and testing (if required), following earthworks. ‘Properly drained
subgrades’ require open graded drains, site grades that shed water and prevent ponding, and subsurface
drains adjacent to pavements.


6.7          Geotechnical Certification of Operational Works
NC generally requires subdivision Operational Works on hillside sites to be certified by a geotechnical
RPEQ prior to Plan Sealing. Should Tectonic be required to certify the Operational Works, inspections
must be completed by Tectonic at various stages during construction (e.g. following site stripping, post
completion etc.) to ensure the recommendations provided within this report have been adhered too.
Tectonic should be contacted prior to commencement of Operational Works to confirm specific
inspection requirements.


Geo-Environmental Investigation for Proposed Subdivision
7 February 2025 | 24386-001-Rev0                                                                                 22