Attachment 12 - Geotechnical Investigation Report

                                PROPOSED RETIREMENT DEVELOPMENT
                                MYALL STREET, COOROY

At the time of the investigation, groundwater seepage was recorded in isolated locations, generally indicating
seepage could be expected at around 0.5 m to 0.75 m BGL close to the toe of the immediate flanks to the
higher ground comprising the rounded knoll to the central-south parts of the site. Standing groundwater was
only recorded in borehole BH5, close to the creek in the lower lying ground, at a shallow depth of 0.65 m
BGL. Groundwater seepage may occur more extensively across the site, within permeable horizons such as
the sandy layers and fissures within the clay/weathered rock, particularly following periods of heavy or
prolonged rainfall.

5.3       Geotechnical Laboratory Test Results
Table 2 below summarises the results of laboratory tests conducted on soil samples recovered from select
boreholes. Laboratory test certificates are presented in Appendix B.
Table 1: Summary of Atterberg Limits, Percent Fines, and Shrink-Swell Index Laboratory Results
                                        Atterberg Limits
                                                                                                       Shrink-Swell
                               Liquid         Plastic        Plasticity       Percent Fines
 BH No.      Depth (m)                                                                                    Index
                               Limit           Limit           Index               (%)
                                                                                                         (%, SSI)
                                (%)             (%)             (%)
  BH2          1.0-1.17             -             -               -                   -                    2.3
  BH4          1.0-1.17             -             -               -                   -                    2.0
  BH5          1.0-1.15             -             -               -                   -                    1.9
  BH12         2.0-2.25          77              27               50                 91                     -
    Notes:        1.        Percent fines is the percentage of clay/silt fraction in the soil sample
                  2.        “-“ denotes not scheduled


Laboratory testing indicates that the residual materials present on site are silty clays of high plasticity,
generally featuring very high ‘fines’ content. Shrink-swell index testing indicates that the residual clay
samples tested had a moderate reactivity to moisture content variations, however based on the liquid limit
and linear shrinkage shown for the borehole BH12 sample, it is possible that the material may be highly
reactive.

Table 3 below summarises the result of laboratory testing conducted on bulk soil samples recovered from
the likely pavement subgrade materials at boreholes BH3 and BH7 (based on information supplied in the
original project brief).
Table 2: Summary of CBR Laboratory Results
                                                                                       Optimum
                                         Maximum Dry          Field Moisture
      BH No.           Depth (m)                                                       Moisture             CBR (%)
                                         Density (t/m3)        Content (%)
                                                                                      Content (%)
       BH6                0.5-1.0              1.55                    30.3                25.1                  2.5
      BH10             0.25-0.75               1.78                    22.2                15.5                  4.0
      BH13                0.5-1.0              1.65                    28.4                20.4                  2.6

Testing conducted on the residual clay samples from boreholes BH6 and BH13 indicates low CBR values of
2.5 % and 2.6 %, respectively. Possible alluvial material, comprising reworked residual clays, gleaned a CBR
value of 4 % in borehole BH10. It is noted that the materials had moderate to high swell values, being around
2 % in all samples tested. Field moisture contents of the samples were all about 5 % to 8 % ‘wet’ of optimum
moisture content (OMC).




17 May 2019
Report No. 19071-001-Rev0                                     7