22244-002-Rev1 Barina Ct

Mark Kussrow                                                                                               22244-002-Rev1
C/- CadCon Surveying & Town Planning                                                                      26 February 2024

3.3       Geotechnical Laboratory Test Results

Full geotechnical laboratory test certificates are attached at the end of the report text. Table 1 summarises
the results of geotechnical laboratory testing conducted on soil samples recovered from the boreholes.

Table 1: Summary of Geotechnical Laboratory Results

                                                                  Atterberg Limits
                    Depth                                                          Linear
   BH ID                              Liquid Limit           Plasticity                                         ECN
                   (m BGL)                                                        Shrinkage   Fines (%)
                                          (%)                Index (%)
                                                                                     (%)
    BH1            0.3 – 0.6                 42                   17                 8.5         47               4
    BH1            3.4 – 3.6                 42                   19                 9.5         78               4
Notes: Fines Content is the percentage of clay/silt fraction in the soil sample

Laboratory test results indicate that the soils tested from borehole BH1 were gravelly and silty clays of
medium plasticity. ECN testing of disturbed samples from borehole BH1 returned values of 4. This indicates
that there was slaking but no dispersion before remoulding and no dispersion after remoulding. The results
suggest that the existing dam wall fill materials, and natural soil samples tested, have a low dispersion
potential.


4.0        STABILITY ASSESSMENT
4.1       Existing Embankment Construction
No records have been sighted regarding certification and testing of the fill placed during the construction of
the earthen dam embankment, and no formal construction records regarding site stripping, foundation
preparation, fill placement, moisture conditioning and compaction were provided prior to our assessment.
However, based on the results of borehole logging and in situ testing completed as part of the investigation,
we make the following comments:

          The embankment appears to be of homogenous construction.
          Materials forming the embankment mostly comprised silty and gravelly clays that could generally be
           considered acceptable for dam construction. Rootlets were encountered at the fill/natural soil
           interface but topsoil materials were not apparent.
          From in situ soil strength testing in the boreholes, it is inferred that the embankment fill has been
           compacted to some degree (rather than being end tipped and loosely placed). However, it cannot
           be stated whether compaction levels meet the minimum standard of compactive effort (i.e. typically a
           dry density ratio of 95 % of the Maximum Standard Dry Density Ratio [DDR]).
          The foundation subgrade beneath the crest of the dam appears to comprise stiff to very stiff clays.

4.2       Stability Assessment (Quantitative)
A global stability assessment was carried out for the main dam wall, based on our investigation results, and
our understanding of the construction. Combined with our site observations, we have completed a
quantitative assessment of slope stability, comprising numerical analysis using Slope/W. This has been
used to calculate a factor of safety (FOS) against slope instability, and to check that a FOS ≥ 1.5 can be
achieved based on the model developed as part of this report. FOS values of ≥ 1.3 and ≥ 1.5 are the
general industry minimums for short- and long-term conditions, respectively.

Our model was developed based on the following assumptions:
          The dam embankment profile across the upstream, downstream and crest areas was developed by
           field mapping as described in Section 1.1. The analysed section considered the highest
           embankment observed, this being about 4.5 m high, with a crest width of 3.5 m and a downstream
           face slope of about 27°.




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