210533 - 108 Black Mountain Range Road, BLACK MOUNTAIN - Slope Stability Report

Total Concept Homes                                                                                            20332-003-Rev0
                                                                                                                  16 June 2022

2.0         DESCRIPTION OF EXISTING CONDITIONS
2.1       Geology and Subsurface Conditions
The 1:100,000 scale Gympie Special Geological Map 1 (Ref. Text Figure 3 below) indicates that the site is
underlain by materials derived of the early to middle Triassic age Kin Kin Beds (green shading), which are
described as “commonly cleaved and commonly kinked mudstone…” The Australian Stratigraphic Units
Database (www.asud.gov.au) describes the Kin Kin Beds as “arenite, argillite and phyllite.”




                                                                                       Approx. site location




Text Figure 3: Extract of Gympie Surface Geology Map
The Ground Drill investigation comprised drilling of two boreholes (designated BH1 and BH2) to the
respective downslope (fill side) and upslope (cut side) of the building pad prepared at the granny flat site.
Borehole BH1 encountered silty clay fill, in a firm condition to 1.2 m below ground level (BGL); then the
original topsoil (firm clayey silt) to 1.3 m BGL, followed by residual soils comprising high plasticity, stiff, silty
clay to the borehole termination depth of 2 m BGL. Borehole BH2 encountered distinctly weathered ‘rock’ at
surface, resulting in auger drilling refusal at 0.1 m BGL.
Three additional shallow boreholes (taken to 0.6 m BGL) were also drilled to the north of the prosed granny
flat, to provide information for the on-site effluent disposal design. These encountered topsoil to 0.1 m BGL,
then residual soils of stiff, high plasticity, silty clay to 0.5 m BGL, underlain by distinctly weathered ‘rock’ to
the termination depth at 0.6 m BGL.
All boreholes were drilled using a 4WD utility mounted drill rig, with solid flight augering equipment. In situ
strength testing comprised the dynamic cone penetrometer (DCP) method, taken from surface adjacent to
the boreholes at the site of the proposed granny flat. Pocket Penetrometer testing was also undertaken in
selected disturbed auger cuttings.
Groundwater seepage was not encountered during the drilling of the investigation boreholes, with all soils
being described as “moist” and weathered ‘rock’ materials as “dry”.



1
    State of Queensland, Dept. of Natural Resources, Mines and Energy, Gympie Surface Geology, Sheet 9445, 2018




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