Appendix B- Geotechnical Investigation

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The test is carried out in a borehole by driving a 50mm          resistance, fs, in kPa). These forces are measured by
diameter split sample tube with a tapered shoe, under            electrical transducers (strain gauges) within the cone
the impact of a 63 kg hammer with a free fall of 760             device. The ratio between friction resistance and cone
mm. It is normal for the tube to be driven in three              resistance is also calculated as a percentage, ie.-
                                                                                             Friction Resis tan ce,f s kPa  100
successive 150 mm increments and the 'N' value is                Friction Ratio FR             cone resis tan ce, q c kPa
taken as the number of blows for the last 300 mm, the            The friction ratio, FR, is generally low in sands (less
upper 150 mm being neglected due to possible                     than 1% or 2%) and generally higher in clays (say 3%
disturbance from the drilling method. In dense sands,            or more).    The interpretation of sandy clays, clayey
very hard clays or weak rock, the full 450 mm                    sands and material with a high silt content is more
penetration may not be practicable and the test is               difficult, but intermediate values (between 1% and 3%)
discontinued at a reduced penetration.                           would be expected. Highly organic clays and peats
In the case where full penetration is obtained with              generally have a friction ratio in excess of 5%.
successive blow counts for each 150 mm of, say 4, 6              Static cone data is recorded in the field on disc for later
and 7 blows, the record shows,                                   presentation using computer aided drafting.
          4, 6, 7                            N = 13              The equipment can be operated from any conventional
In a case where the test is discontinued short of full           drill rig. A total applied load in the range of 4 to 10
penetration, say after 15 blows for the first 150 mm             tonnes is required for practical purposes, although
and 30 blows for the next 40 mm, the record shows:               lighter loads may be used. The cone penetrometers
          15, 30/40mm                                            are available with various capacities of cone resistance
The results of the test can be related empirically to the        ranging     up   to   100   MPa     for   general      purpose
engineering properties of the soil.                              investigations, while a range of 0 to 10 MPa can be
Occasionally, the drop hammer is used to drive 50mm              used where more sensitive investigations of soft clay
diameter thin walled sample tubes (U50) in clays. In             are required.
such circumstances, it is noted on the borehole logs.            The cone resistance value provides a continuous
A modification to the SPT test is where the same                 measure of soil strength or density, and together with
                                                 0
driving system is used with a solid 60 tipped steel              the friction ratio, provide very useful indications of the
cone of the same diameter as the SPT hollow sampler.             presence of narrow bands of geotechnically significant
The solid cone can be continuously driven for some               layers such as thin, soft clay layers or lenses of sand
distance in soft clays or loose sands, or may be used            which might otherwise be missed using conventional
where damage would otherwise occur to the SPT. The               drilling methods.
results of this Solid SPT are shown as "Nc" on the               The lithology of the encountered soils is interpreted
borehole logs, together with the number of blows per             from static cone data and is generally presented on the
150 mm penetration.                                              static cone log sheets.
Cone Penetration Tests - Test Method - Cone                      It is important to note that the lithology is interpreted
Penetration Tests (CPT) are carried out in accordance            information and is based on research by Schmertmann
with AS 1289 Test 6.5.1-1977, using an electrical                (1970), Sanglerat (1972), Robinson and Campinalli
friction-cone penetrometer.                                      (1986), modified to suit local conditions as indicated by
The test essentially comprises the measurement of                borehole information and laboratory testing.
resistance to penetration of a cone of 35.7 mm                   As soils generally change gradually it is sometimes
diameter pushed into the soil at a rate of 10-20 mm per          difficult to accurately describe depths of strata
second by hydraulic force.                  The resistance to    changes, although greater accuracy is obtained with
penetration is recorded in terms of pressure on the end          the static cone compared with conventional drilling. In
area of the cone (cone resistance, qc, in MPa) and               addition, friction ratios decrease in accuracy with low
friction on the side of the 135 mm long sleeve                   cone resistance values, and in desiccated soils. As a
immediately above the top of the cone (friction                  result, some overlap and minor discrepancies may


NOTES RELATING TO THIS REPORT - SEPT 2013                                              SOIL SURVEYS ENGINEERING