Appendix B - Geotechnical Investigation Report

4.2     Subsurface Conditions
Based on our review of the provided ASI “Soil Test Report” and our site observations, the pertinent aspects
of the subsurface conditions are summarised below:
     Sandy clay fill assessed to be of medium to high plasticity was encountered in BH2 and was 0.7m thick.
      Based on the location of BH2, the fill is likely backfill to the timber sleeper retaining wall. Based on the
      DCP test results the fill has been assessed to be poorly compacted. We also note that in the absence of
      formal records of placement and density test results, the fill is regarded as ‘uncontrolled’ as defined in
      AS2870-2011 ‘Residential slabs and footings’.
     Silty sand topsoil (0.03m thick) encountered at surface level (BH1) and below the fill (BH2).
     From below the topsoil, BH1 encountered gravelly sand over residual sandy clay assessed to be of low to
      medium plasticity to the termination depth of 3.0m and BH2 initially encountered a 0.5m thick band of
      residual gravelly sand overlying residual sandy clay assessed to be of medium to high plasticity which
      extends to the termination depth of 3.0m. Based on our review of the DCP blow counts we interpret:
          o The relative density of the gravelly sand in BH1 to be very loose and the clay strength to be
              initially soft then soft to firm, improving to stiff to very stiff below about 2.2m depth and hard
              strength below about 2.4m depth.
          o The relative density of the gravelly sand in BH2 to be loose to medium dense and the underlying
              clays to be generally of hard strength.
     At the termination depth of BH1, the ASI log notes “approaching weathered sandstone” and DCP refusal
      was at about 2.0m depth. In BH2, DCP refusal was at about 2.2m depth but there was no similar note
      regarding “approaching weathered sandstone”. Based on our experience in this area and the results of
      the ASI investigation, we have interpreted the DCP refusal depths to indicate the surface of extremely
      weathered sandstone bedrock surface at depths of about 2.8m (BH1) and 2.2m (BH2), although the
      presence of bands of weathered bedrock or ‘floaters’ within the residual soil profile (indicated by the
      high DCP blow counts) cannot be discounted. We note that the transition of hard residual clay to
      extremely weathered sandstone represents the weathering profile of the bedrock and the actual soil-
      rock interface can vary across the site. However, based on the topographic setting of the site overall the
      bedrock surface would be expected to step or slope down to the north.
     No groundwater was encountered within the depth of the investigation.



4.3     Summary of Observations
We recommend that the summary of observations which follows in Sections 4.3.1 to 4.3.4 be read in
conjunction with the attached Figure 2.



4.3.1 Topography
     The site is located on the crest of a hillside above the western foreshore of Lake Cootharaba and is
      situated on the northern side of a small promontory (orientated east-west) in the foreshore area. The
      upper (southern) portion of the site is relatively flat and has a southern frontage on to Woongar Street.




35891URrpt Rev1                                      3