Avocet Pde OPW - App E Slope · OPW20/0092

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Interpretation of the
Subsurface Sheets
                                                                It is important to note that the point load strength index
The terminology included on Geotech Engineering Pty
                                                                does not take into consideration the degree of fracturing of
Ltd’s has been evolved from AS1726:2016. Typically a
                                                                a rock mass which can both lower and increase the overall
soil’s strength, colour, plasticity, grain size, other
                                                                rock strength. The strength of rock is broken down as
inclusions, and moisture will be described. A rock’s
                                                                follows:
strength, weathering, colour, grain, and other inclusions
are typically described.
                                                                 Very Low Strength (VLS)              < 2 MPa (UCS)
                                                                 Low Strength (LS)                    2 to 6 MPa (UCS)
Soil                                                             Medium Strength (MS)                 6 to 20 MPa (UCS)
                                                                 High Strength (HS)                   20 to 60 MPa (UCS)
Soil is typically described by the components that make it       Very High Strength (VHS)             60 to 200 MPa (UCS)
up by percentage or whichever has a governing behaviour.         Extremely High Strength (EHS)        > 200 MPa (UCS)
The types and sizes reproduced from AS1726 are:
 Boulder                  >200 mm
 Cobble                   63 to 200 mm                           Very Low Strength (VLS)              < 0.1 Is(50)
 Gravel                   2.36 to 63 mm                          Low Strength (LS)                    0.1 to 0.3 Is(50)
 Sand                     0.075 to 2.36 mm                       Medium Strength (MS)                 0.30 to 1.0 Is(50)
 Silty                    0.002 to 0.075 mm                      High Strength (HS)                   1.0 to 3.0 Is(50)
 Clay                     < 0.002 mm                             Very High Strength (VHS)             3.0 to 10.0 Is(50)
                                                                 Extremely High Strength (EHS)        > 10.0 Is(50)
The strength of soil is usually assessed by suitably
experienced persons through the use of laboratory test          The Is(50) values are based on a correlation between the
results, published literature, field assessments or a           unconfined compressive strength (UCS) of 20. This value
combination of the above. The strength of soil is               varies significantly and site specific testing must take
characterised into either cohesive (clays and silts) or         precedence.
cohesionless (sands and gravels) as shown below:
                                                                Rock weathering is broken down as follows:
 Cohessive Soil          Strength Term
 Very Soft               < 12.5 kPa                              Extremely weathered       Behaves as soil but rock
 Soft                    12.5 to 25 kPa                          (EW)                      structure evident
 Firm                    25 to 50 kPa                            Highly weathered          Colour and strength of original
 Stiff                   50 to 100 kPa                           (HW)                      fresh rock is not recognisable.
 Very Stiff              100 to 200 kPa                                                    Decomposition of rock evident
 Hard                    > 200 kPa                               Moderately                Defects are heavily stained with
                                                                 weathered (MW)            rock mass showing signs of
 Cohesionless Soils      Strength Term (uncorrected Qc)                                    discolouration
 Very Loose              <2 MPa                                  Slightly weathered        Some staining but minimal to no
 Loose                   2 to 5 MPa                              (SW)                      change in strength
 Medium Dense            5 to 15 MPa                             Fresh stained             Minor staining evident
 Dense                   15 to 25 MPa                            (FS)
 Very Dense              > 25 MPa                                Fresh rock                No change
                                                                 (FR)
Rock
                                                                Rock fracturing is broken down as follows and can only be
Rock is typically described by the components that make         accurately described where undamaged continuous
it up by percentage or whichever has a governing                sampling has been undertaken:
behaviour. The strength of rock is typically the most
important component. Typically, the strength of rock is          Fragmented (Fg)             Defect spacings < 20 mm
based on laboratory testing, tactile assessment and other        Highly Fractured (HF)       Defect spacings 20 to 50 mm
observations as relevant. The industry standard method           Fractured (Fr)              Defect spacings 50 to 200 mm
for assessing the strength of rock in the field is to use the    Slightly Fractured (SF)     Defect spacings 200 to 1000 mm
point load strength index (AS4133.4.1).                          Unbroken (Ub)               Defect spacings > 1000 mm