Attachment 2 - Geotechnical Stability Assessment

The borehole can be stabilised through the use       using a Cone Penetrometer in            general
of drilling mud as a circulating fluid. The term     accordance with AS 1289 6.5.1, 1999.
‘mud’ encompasses a range of products
ranging from bentonite to polymers such as           In the tests, a 36mm diameter rod with a
Revert or Biogel.                                    conical tip is pushed continuously into the soil,
                                                     the reaction being provided by a hydraulic
Continuous Core Drilling: A continuous core          ram system. Measurements are made of the
sample is obtained using a diamond tipped            end bearing resistance on the cone and the
core barrel. This technique provides a reliable      fractional resistance on a separate 135mm
(but    relatively   expensive)    method     of     long sleeve, immediately behind the cone.
investigation. In rocks, an NMLC triple tube         Pore Pressure is recovered through a pore ring
core barrel is used, which gives a core of about     located either within, or more usually
50mm diameter. The length of core recovered          immediately behind the cone/tip.
is compared to the length drilled and any
length not recovered is shown as CORE LOSS.          As penetration occurs (at a rate of
                                                     approximately 20mm per second) and data is
Standard       Penetration    Tests:     Standard    recorded every 20mm of penetration, the
Penetration Tests (SPT) are used mainly in non-      results are presented graphically.
cohesive soils, but can also be used in cohesive
soils as a means of indicating density or            The information     provided     on    the   plot
strength and also of obtaining a disturbed           comprises:
sample. The test procedure is described in
                                                            Cone resistance – expressed in mPa
Australian Standard 1289, “Methods of Testing
                                                            Sleeve friction – expressed in kPa
Soils for Engineering Purposed”, Test 6.3.1.
                                                            Friction ratio – the ratio of sleeve
The test is carried out in a borehole by driving a           friction to cone resistance expressed as
50mm diameter split sample tube with a                       a percentage.
tapered shoe, under the impact of a 63kg                    Pore pressure in kPa
hammer, with a free fall of 760mm. The sample               Tilt of probe (in degrees).
is driven in three successive 150mm increments
and the ‘N’ value is taken as the number of
                                                     The ratios of the sleeve resistance to cone
blows for the last 300mm. In dense soils, hard
                                                     resistance will vary with the type of soil
clays or weak rock, the full 450mm penetration
                                                     encountered, with higher relative friction in
may not be practicable and the test is
                                                     clays than in sands. Friction ratios of 1% to 2%
discontinued.
                                                     are commonly encountered in sands and rising
The test results are reported in the following       to 2% to as high as 8%, and higher in organic
form:                                                soils.   Soil descriptions based on cone
                                                     resistance and friction ratios are only inferred
       In the case where full penetration is        and must not be considered as exact.
        obtained with successive blow counts
        for each 150mm of , say, 4, 6 and 7          Stratification can be inferred from the cone
        blows, as                                    and friction traces and from experience and
        N = 13                                       information from nearby boreholes, etc. Where
        4, 6, 7                                      shown, this information is presented for general
       In a case where the test is discontinued     guidance, but must be regarded as
        short of full penetration, say after 15      interpretive.
        blows for the first 150mm and 30 blows
                                                     Dynamic Cone Penetrometers:
        for the next 40mm, as
        N > 30
                                                     Dynamic Cone Penetrometer (DCP) tests are
        15, 30/40mm
                                                     carried out by driving a 16mm diameter rod
                                                     into the ground with a 9kg sliding hammer
Cone Penetrometer Testing (CPT): Cone
                                                     dropping 510mm and counting the blows for
Penetrometer Testing with or without pore
                                                     successive 100mm increments of penetration.
pressure measurement (CPTu) is carried out