OPW052025 Report - Part 2

     weathered, very low strength phyllite was logged to the termination depth of 3.2 m BGL (limit reach
     of machine).
Test pits TP4 and TP5 were positioned to the edges of the proposed building area, with a relatively
similar subsurface profile logged; of topsoil up to 0.2 m depth, followed by high plasticity, residual silty
clay to between 0.4 m and 0.7 m BGL; underlain by highly fractured, medium strength basalt. It was
noted that the weathered basalt ranged to high strength in more intact portions, and hard digging
conditions resulted in termination at depths of between 0.9 m and 1.2 m BGL.
A veneer of localised slopewash was encountered in TP3 close to the transition between the basalt and
the underlying country rock, however no colluvium was encountered in the test pits, or observed on the
slopes within or adjacent to the proposed building or pavement areas.

4.2.3         Groundwater
The development of groundwater over the majority of the driveway alignment, given the position along
the crest if the ridgeline, would be unlikely. However, it is acknowledged that the variable nature of the
soil mass (residual soils, gravelly XW material and fractured upper surface of the phyllite) and the
direction of surface flows towards the site, groundwater seepage could be possible. This would be more
likely at distinct breaks in slope along the driveway (e.g. below the steeper slopes on the edge of the
localised knob flanks).
Groundwater would be rarely encountered other than ‘perched’ flows following periods of heavy or
prolonged rainfall over the proposed house site, being very close to a regional drainage divide and local
high point within the site.
No signs of groundwater seepage (‘springs’) were observed in the vicinity of the proposed construction,
however, slightly wetter soils were logged below the topsoil in test pit TP1.
Groundwater conditions are expected to be reflective of prevailing weather conditions and seepage may
develop, especially at key material boundaries (e.g. soils to rock), and/or rise to shallower depths;
particularly following periods of prolonged or heavy rainfall.


4.3           Laboratory Test Results
The shrink-swell index testing on a sample of the residual basaltic soil in the area of the proposed
residence recorded a value of 6.9 %. This indicates that the soil material is likely to be highly reactive to
normal seasonal moisture variation.
Table 1 below provides a summary of the CBR testing.

Table 1: Summary of CBR Laboratory Results

                                                           Optimum
                                          Maximum Dry
 Test Location        Depth (m)                            Moisture       CBR Swell (%)     CBR (%)
                                          Density (t/m3)
                                                           Content (%)
 BH1                  0.0 – 0.5           1.5              28             5                 1.5




Geotechnical Investigation & Slope Stability Assessment
9 April 2025 | 25050-001-Rev0                                                                                   10