Geotechnical Stability Assessment · MCU19/0008

The test results are reported in the following      resistance and friction ratios are only inferred
form:                                               and must not be considered as exact.

       In the case where full penetration is       Stratification can be inferred from the cone
        obtained with successive blow counts        and friction traces and from experience and
        for each 150mm of , say, 4, 6 and 7         information from nearby boreholes, etc. Where
        blows, as                                   shown, this information is presented for general
        N = 13                                      guidance, but must be regarded as
        4, 6, 7                                     interpretive.
       In a case where the test is discontinued
        short of full penetration, say after 15     Dynamic Cone Penetrometers:
        blows for the first 150mm and 30 blows
                                                    Dynamic Cone Penetrometer (DCP) tests are
        for the next 40mm, as
                                                    carried out by driving a 16mm diameter rod
        N > 30
                                                    into the ground with a 9kg sliding hammer
        15, 30/40mm
                                                    dropping 510mm and counting the blows for
Cone Penetrometer Testing (CPT): Cone
                                                    successive 100mm increments of penetration.
Penetrometer Testing with or without pore
pressure measurement (CPTu) is carried out
                                                    Logs
using a Cone Penetrometer in general
accordance with AS 1289 6.5.1, 1999.                The borehole or test pit logs are an
                                                    interpretation of the subsurface conditions, and
In the tests, a 36mm diameter rod with a
                                                    their reliability will depend to some extent on
conical tip is pushed continuously into the soil,
                                                    the frequency of sampling and the method of
the reaction being provided by a hydraulic
                                                    drilling or excavation.
ram system. Measurements are made of the
end bearing resistance on the cone and the          Interpretation of the information shown on the
fractional resistance on a separate 135mm           logs, and its application to design and
long sleeve, immediately behind the cone.           construction, should therefore take into
Pore Pressure is recovered through a pore ring      account the spacing of the boreholes or test
located either within, or more usually              pits, the method of drilling or excavation, the
immediately behind the cone/tip.                    frequency of sampling and testing and the
                                                    possibility of other than “straight line” variations
As penetration occurs (at a rate of
                                                    between the boreholes or test pits. Subsurface
approximately 20mm per second) and data is
                                                    conditions between boreholes or test pits may
recorded every 20mm of penetration, the
                                                    vary significantly from conditions encountered
results are presented graphically.
                                                    at the borehole or test pit locations.

The information     provided     on   the    plot
                                                    Groundwater
comprises:
                                                    Where groundwater levels are measured in
        Cone resistance – expressed in mPa
                                                    boreholes, there  are   several potential
        Sleeve friction – expressed in kPa
                                                    problems:
        Friction ratio – the ratio of sleeve
         friction to cone resistance expressed as          Although      groundwater        may   be
         a percentage.                                      present, in low permeability soils it may
        Pore pressure in kPa                               enter the hole slowly or perhaps not at
        Tilt of probe (in degrees).                        all during the time it is left open.
The ratios of the sleeve resistance to cone                A localised perched water table may
resistance will vary with the type of soil                  lead to an erroneous indication of the
encountered, with higher relative friction in               true water table.
clays than in sands. Friction ratios of 1% to 2%           Water table levels will vary from time to
are commonly encountered in sands and rising                time with seasons or recent weather
to 2% to as high as 8%, and higher in organic               changes and may not be the same at
soils.    Soil descriptions based on cone                   the time of construction.




October 2017                                                                                      pg. 3