Geotechnical Investigation

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                                      Interpretation of the Subsurface Sheets


The terminology included on Arcos Unit Group Pty            the most important component. Typically, the strength
Ltd’s has been evolved from AS1726:2016. Typically          of rock is based on laboratory testing, tactile
a soil’s strength, colour, plasticity, grain size, other    assessment and other observations as relevant. The
inclusions, and moisture will be described. A rock’s        industry standard method for assessing the strength of
strength, weathering, colour, grain, and other              rock in the field is to use the point load strength index
inclusions are typically described.                         (AS4133.4.1). It is important to note that the point load
                                                            strength index does not take into consideration the
                                                            degree of fracturing of a rock mass which can both
Soil                                                        lower and increase the overall rock strength. The
Soil is typically described by the components that          strength of rock is broken down as follows:
make it up by percentage or whichever has a                 Very Low Strength (VLS)              < 2 MPa (UCS)
governing behaviour. The types and sizes reproduced         Low Strength (LS)                    2 to 6 MPa (UCS)
from AS1726 are:                                            Medium Strength (MS)                 6 to 20 MPa (UCS)
Boulder >200 mm                                             High Strength (HS)                   20 to 60 MPa (UCS)
Cobble 63 to 200 mm                                         Very High Strength (VHS)             60 to 200 MPa (UCS)
Gravel 2.36 to 63 mm                                        Extremely High Strength (EHS)        > 200 MPa (UCS)
Sand    0.075 to 2.36 mm                                    Very Low Strength (VLS)              < 0.1 Is(50)
Silty   0.002 to 0.075 mm                                   Low Strength (LS)                    0.1 to 0.3 Is(50)
Clay    < 0.002 mm                                          Medium Strength (MS)                 0.30 to 1.0 Is(50)
The strength of soil is usually assessed by suitably        High Strength (HS)                   1.0 to 3.0 Is(50)
experienced persons through the use of laboratory           Very High Strength (VHS)             3.0 to 10.0 Is(50)
test results, published literature, field assessments or    Extremely High Strength (EHS)        > 10.0 Is(50)
a combination of the above. The strength of soil is         The Is(50) values are based on a correlation between
characterised into either cohesive (clays and silts) or     the unconfined compressive strength (UCS) of 20.
cohesionless (sands and gravels) as shown below:            This value varies significantly and site specific testing
Cohessive Soil Strength Term                                must take precedence.

Very Soft            < 12.5 kPa                             Rock weathering is broken down as follows:
Soft                 12.5 to 25 kPa                         Extremely weathered (EW) Behaves as soil but rock structure
Firm                 25 to 50 kPa                                  evident
Stiff                50 to 100 kPa                          Highly weathered (HW) Colour and strength of original fresh rock
                                                                    is not recognisable. Decomposition of rock evident
Very Stiff           100 to 200 kPa
                                                            Moderately weathered (MW)    Defects are heavily stained with
Hard                 > 200 kPa
                                                                   rock mass showing signs of discolouration
Cohesionless Soils Strength Term (uncorrected Qc)           Slightly weathered (SW)     Some staining but minimal to no
Very Loose           <2 MPa                                          change in strength

Loose                2 to 5 MPa                             Fresh stained       (FS) Minor staining evident

Medium Dense         5 to 15 MPa                            Fresh rock        (FR) No change

Dense                15 to 25 MPa                           Rock fracturing is broken down as follows and can only
Very Dense           > 25 MPa                               be accurately described where undamaged
                                                            continuous sampling has been undertaken:
                                                            Fragmented (Fg)      Defect spacings < 20 mm
Rock
                                                            Highly Fractured (HF)    Defect spacings 20 to 50 mm
Rock is typically described by the components that          Fractured (Fr) Defect spacings 50 to 200 mm
make it up by percentage or whichever has a                 Slightly Fractured (SF) Defect spacings 200 to 1000 mm
governing behaviour. The strength of rock is typically      Unbroken (Ub) Defect spacings > 1000 mm




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