Slope Stability Report - Site 3

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Interpretation of the
Subsurface Sheets
                                                                  It is important to note that the point load strength index
The terminology included on Arcos Unit Group Pty Ltd’s
                                                                  does not take into consideration the degree of fracturing of
has been evolved from AS1726:2016. Typically a soil’s
                                                                  a rock mass which can both lower and increase the overall
strength, colour, plasticity, grain size, other inclusions, and
                                                                  rock strength. The strength of rock is broken down as
moisture will be described. A rock’s strength, weathering,
                                                                  follows:
colour, grain, and other inclusions are typically described.
                                                                   Very Low Strength (VLS)               < 2 MPa (UCS)
Soil                                                               Low Strength (LS)                     2 to 6 MPa (UCS)
                                                                   Medium Strength (MS)                  6 to 20 MPa (UCS)
Soil is typically described by the components that make it         High Strength (HS)                    20 to 60 MPa (UCS)
up by percentage or whichever has a governing behaviour.           Very High Strength (VHS)              60 to 200 MPa (UCS)
The types and sizes reproduced from AS1726 are:                    Extremely High Strength (EHS)         > 200 MPa (UCS)
 Boulder                  >200 mm
 Cobble                   63 to 200 mm
 Gravel                   2.36 to 63 mm                            Very Low Strength (VLS)               < 0.1 Is(50)
 Sand                     0.075 to 2.36 mm                         Low Strength (LS)                     0.1 to 0.3 Is(50)
 Silty                    0.002 to 0.075 mm                        Medium Strength (MS)                  0.30 to 1.0 Is(50)
 Clay                     < 0.002 mm                               High Strength (HS)                    1.0 to 3.0 Is(50)
                                                                   Very High Strength (VHS)              3.0 to 10.0 Is(50)
The strength of soil is usually assessed by suitably               Extremely High Strength (EHS)         > 10.0 Is(50)
experienced persons through the use of laboratory test
results, published literature, field assessments or a             The Is(50) values are based on a correlation between the
combination of the above. The strength of soil is                 unconfined compressive strength (UCS) of 20. This value
characterised into either cohesive (clays and silts) or           varies significantly and site specific testing must take
cohesionless (sands and gravels) as shown below:                  precedence.
 Cohessive Soil           Strength Term                           Rock weathering is broken down as follows:
 Very Soft                < 12.5 kPa
 Soft                     12.5 to 25 kPa                           Extremely weathered       Behaves as soil but rock
 Firm                     25 to 50 kPa                             (EW)                      structure evident
 Stiff                    50 to 100 kPa                            Highly weathered          Colour and strength of original
 Very Stiff               100 to 200 kPa                           (HW)                      fresh rock is not recognisable.
 Hard                     > 200 kPa                                                          Decomposition of rock evident
                                                                   Moderately                Defects are heavily stained with
 Cohesionless Soils       Strength Term (uncorrected Qc)           weathered (MW)            rock mass showing signs of
 Very Loose               <2 MPa                                                             discolouration
 Loose                    2 to 5 MPa                               Slightly weathered        Some staining but minimal to no
 Medium Dense             5 to 15 MPa                              (SW)                      change in strength
 Dense                    15 to 25 MPa                             Fresh stained             Minor staining evident
 Very Dense               > 25 MPa                                 (FS)
                                                                   Fresh rock                No change
Rock                                                               (FR)

Rock is typically described by the components that make           Rock fracturing is broken down as follows and can only be
it up by percentage or whichever has a governing                  accurately described where undamaged continuous
behaviour. The strength of rock is typically the most             sampling has been undertaken:
important component. Typically, the strength of rock is
based on laboratory testing, tactile assessment and other          Fragmented (Fg)             Defect spacings < 20 mm
observations as relevant. The industry standard method             Highly Fractured (HF)       Defect spacings 20 to 50 mm
for assessing the strength of rock in the field is to use the      Fractured (Fr)              Defect spacings 50 to 200 mm
point load strength index (AS4133.4.1).                            Slightly Fractured (SF)     Defect spacings 200 to 1000 mm
                                                                   Unbroken (Ub)               Defect spacings > 1000 mm