OPW052025 Report - Part 2

6.            Geotechnical Recommendations
General recommendations to help maintain the stability of the site are given in the document Some
Guidelines for Hillside Construction, which is attached in Appendix C.


6.1           Site Layout and Geotechnical Constraints
It is noted that detailed building and pavement design plans were not available at the time of preparing
this report, however building works should be limited to the areas described in Section 2, and finalised
plans should be reviewed by Tectonic prior to construction.
In general, it is expected that for the driveway and future house, significant earthworks would not be
required, other than about 0.5 m depth/height of cut/filling to achieve drainage gradients etc. Deeper
isolated excavations may be required for underground services and footings.
While there would generally be no limitations around earthworks over the proposed house site (provided
the offsets described in Section 4.1 are maintained), or over the majority of the driveway; where the
driveway is sidelong across the ridgeline flank to the west, fill should be limited to no more than 0.5 m
height.
In the sidelong area, which extends over a distance of about 60 m, it is recommended that the soil
materials are trimmed back to 1V:2H near the crest, however the weathered phyllite towards the toe
could be maintained at 1V:1H in the long term. Diversion drainage should be installed above the cut
batter slope, which could take the form of a suitably sized swale, preferably taken to discharge away from
the driveway to the south. In this area, where possible, surface waters should not discharge in an
uncontrolled fashion to the gully head downslope (within about 3 m distance). A rock lined dissapator
would be recommended for the gully head to prevent future erosion, or alternatively surface waters
could be directed around this, to disperse over other slopes to the north-east.
Additionally, to minimise the downslope effects of surface waters, channel bars could be installed at key
breaks in slope along the alignment, to intercept flows incrementally. This concept is shown in Text
Figure 13 below.




                                                                                     Indicative channel bar
                                                                                     locations



                                                          Diversion drainage above
                                                          cut

                                 Text Figure 14: Concept drainage arrangements


Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                 18