Attachment 3 - Revised Slope Stability Assessment

Brett Hage                                                                                            22328-001-Rev1
                                                                                                    17 November 2022

The boreholes encountered slightly different subsurface profiles as follows:
Borehole BH1 was drilled from the crest of the earthworks cut/fill pad present on site at the time of our
fieldwork, and the following profile was logged:

          Fill: Firm to stiff, high plasticity, silty clay to 0.4 m BGL, then becoming stiff to 0.8 m BGL;
           underlain by
          Residual soil: stiff, silty clay, high plasticity to 1.3 m BGL; overlying
          Extremely weathered (XW) material: described as very stiff to hard silty clay, with fine to medium
           size angular gravel fragments; then below 2.3 m BGL grading to
          Meta-Mudstone, being distinctly weathered and very low strength; recovered during augering as
           grey, silty ‘rock flour’, tending to be dry below 3 m depth.
          Borehole BH1 was terminated at 4.3 m BGL (target depth).

Borehole BH2, drilled downslope from the natural ground surface, featured a subsurface profile described as
follows:

          Residual soil: stiff to very stiff, high plasticity, silty clay with medium to coarse size gravel and fine
           grained sand, to 1.3 m BGL; then
          XW material: of very stiff to hard, gravelly silty clay to 3 m BGL; overlying
          Meta-Mudstone, being distinctly weathered and very low strength; possibly becoming slightly
           stronger (up to low strength below 4.3 m BGL), where auger drilling refusal then occurred at
           4.5 m BGL.
Groundwater seepage was not encountered during the drilling of either borehole BH1 or BH2, with all fill
and residual soils being moist; and XW materials generally slightly drier.

Along the western edge of the existing building pad, a cut of between about 0.5 m to 0.6 m depth below
the original ground surface was present. This exposed an XW material comprising pale yellow brown to
grey, hard, silty clay with meta-mudstone gravels. The nature of material in the cutting tended to allow the
gravel particles to fritter from the face, with some minor erosion debris located at the toe.

Soil samples were dispatched to Construction Sciences, a NATA accredited laboratory located in Kunda
Park for geotechnical testing. Laboratory testing was undertaken in accordance with Australian Standard
AS1289 Method for Testing Soils for Engineering Purposes and comprised one Atterberg Limits and Particle
Size Distribution test to allow an assessment of soil classification. The laboratory test certificate is attached.

The following Table 1 summarises the results of laboratory testing conducted on a selected soil sample
recovered from borehole BH1.
Table 1: Summary of Laboratory Results
                                                                        Atterberg Limits
                 Depth          Fines
  TP No.                                         Liquid Limit        Plasticity Index      Linear Shrinkage
                  (m)         Content (%)
                                                     (%)                   (%)                    (%)

   BH1          0.4 – 0.6           82                 52                    20                  9.5

‘Fines Content’ is the percentage of clay/silt fraction in the soil sample
Laboratory testing indicates that the sample tested from borehole BH1 (residual soil) may be described as a
high plasticity, silty clay with trace gravel and sand.

2.2       Topography
As illustrated in Text Figure 5 on the following page, the wider area around the site consists of a series of
approximately parallel ridgelines and gullies, aligned east-west. The ridgelines within the subject allotment
lead east from Black Mountain Range Road, which traces a crest forming a drainage divide between
eastward and westward draining surface water regimes.




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