OPW052025 Report - Part 2

The recommended design CBR value is applicable to properly drained subgrades only, and must be
confirmed by subgrade inspection and testing, if necessary, by an experienced engineer following
earthworks. ‘Properly drained subgrades’ require open graded drains, site grades that shed water and
prevent ponding, and subsurface drains adjacent to pavements.


6.6           Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area.
Permanent site drainage must be installed to ensure that stormwater runoff is directed away from
structures and footings around the residence (e.g. positive gradients and/or channel drains), and also not
allowed to flow over fill embankments along the driveway and agitate steeper slopes close by on the
downslope side. Uncontrolled flow of stormwater over cut/fill batters has the potential to saturate and
weaken the soil material, potentially leading to erosion and slumping failure; and where allowed to
impact the existing natural slopes may initiate retrogressive failures.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in
filter ‘sock’ placed in gravel backfill). Surface water should be prevented from infiltrating the subsurface
drainage by placement of a minimum 300 mm thick clay ‘plug’ above the drainage media (separated by
geosynthetic filter fabric), or construction of a concrete swale, or similar. Depending on matters such as
the extent of drainage installed and site-specific groundwater conditions, the designer must assess
hydrostatic pressure to be accounted for in the design of the retaining structure.
All excess stormwater collected around the proposed development area must be directed by pipes or
surface channels (e.g. rock lined ‘dry creek’ beds) to discharge at least 10 m downslope and away from
any structure or embankment. 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).


6.7           On-Site Effluent Disposal
Effluent disposal land application areas will require investigation and design but must be located
downslope (or oblique across the slope) and not less than 10 m from any structures, retaining walls,
cut/fill batters or the proposed driveway.



7.            Limitations
Your attention is drawn to the document Limitations, which is included in Appendix D of this report. The
statements presented in this document are intended to advise you of what your realistic expectations of
this report should be, and to present you with recommendations on how to minimise the risks associated
with the services provided by Tectonic for this project.




Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                              24