Geotech 95141.00.R.001

Sampling                                                   reliability, due to the remoulding, possible mixing
Sampling is carried out during drilling or test pitting    or softening of samples by groundwater.
to allow engineering examination (and laboratory
testing where required) of the soil or rock.
                                                           Non-core Rotary Drilling
Disturbed samples taken during drilling provide            The borehole is advanced using a rotary bit, with
information on colour, type, inclusions and,               water or drilling mud being pumped down the drill
depending upon the degree of disturbance, some             rods and returned up the annulus, carrying the drill
information on strength and structure.                     cuttings. Only major changes in stratification can
                                                           be determined from the cuttings, together with
Undisturbed samples are taken by pushing a thin-           some information from the rate of penetration.
walled sample tube into the soil and withdrawing it        Where drilling mud is used this can mask the
to obtain a sample of the soil in a relatively             cuttings and reliable identification is only possible
undisturbed state. Such samples yield information          from separate sampling such as SPTs.
on structure and strength, and are necessary for
laboratory determination of shear strength and
compressibility. Undisturbed sampling is generally         Continuous Core Drilling
effective only in cohesive soils.                          A continuous core sample can be obtained using a
                                                           diamond tipped core barrel, usually with a 50 mm
                                                           internal diameter. Provided full core recovery is
Test Pits                                                  achieved (which is not always possible in weak
Test pits are usually excavated with a backhoe or          rocks and granular soils), this technique provides a
an excavator, allowing close examination of the in-        very reliable method of investigation.
situ soil if it is safe to enter into the pit. The depth
of excavation is limited to about 3 m for a backhoe
and up to 6 m for a large excavator. A potential           Standard Penetration Tests
disadvantage of this investigation method is the           Standard penetration tests (SPT) are used as a
larger area of disturbance to the site.                    means of estimating the density or strength of soils
                                                           and also of obtaining a relatively undisturbed
                                                           sample.     The test procedure is described in
Large Diameter Augers                                      Australian Standard 1289, Methods of Testing
Boreholes can be drilled using a rotating plate or         Soils for Engineering Purposes - Test 6.3.1.
short spiral auger, generally 300 mm or larger in
diameter commonly mounted on a standard piling             The test is carried out in a borehole by driving a 50
rig. The cuttings are returned to the surface at           mm diameter split sample tube under the impact of
intervals (generally not more than 0.5 m) and are          a 63 kg hammer with a free fall of 760 mm. It is
disturbed but usually unchanged in moisture                normal for the tube to be driven in three
content. Identification of soil strata is generally        successive 150 mm increments and the 'N' value
much more reliable than with continuous spiral             is taken as the number of blows for the last 300
flight augers, and is usually supplemented by              mm. In dense sands, very hard clays or weak
occasional undisturbed tube samples.                       rock, the full 450 mm penetration may not be
                                                           practicable and the test is discontinued.

Continuous Spiral Flight Augers                            The test results are reported in the following form.
The borehole is advanced using 90-115 mm                   •   In the case where full penetration is obtained
diameter continuous spiral flight augers which are             with successive blow counts for each 150 mm
withdrawn at intervals to allow sampling or in-situ            of, say, 4, 6 and 7 as:
testing. This is a relatively economical means of                                      4,6,7
drilling in clays and sands above the water table.                                     N=13
Samples are returned to the surface, or may be             •   In the case where the test is discontinued
collected after withdrawal of the auger flights, but           before the full penetration depth, say after 15
they are disturbed and may be mixed with soils                 blows for the first 150 mm and 30 blows for
from the sides of the hole. Information from the               the next 40 mm as:
drilling (as distinct from specific sampling by SPTs                                  15, 30/40 mm
or undisturbed samples) is of relatively low




                                                                                                       July 2010