Engineering geology of British rocks and soils Mudstones of the ...

5.3 SAMPLING
The shallowest taper angle and sharpest edge practicable
should be used and maintained on the cutting shoe for
taking driven samples. However, thin wall sampling tubes,
driven by continuous pushing rather than hammering, are
unlikely to penetrate material in which there is a significant
content of lithofragments.
Samples taken by rotary coring methods may be taken
using semi-rigid plastic core liners or more sophisticated
triple tube core barrels both of which afford protection to the
core once it is cut by the drill bit. The removal of the core
from the barrel should be done in the horizontal position and
its subsequent handling, transport and storage must also be
treated with care in order to maintain the integrity and quality
of the sample. Particular care is necessary to stop the core
drying out and, for high quality testing, the logging and
selection of samples for testing should be done soon after
coring in order to minimise the effects of the flushing
medium and stress relief (Binns, 1998).
Sampling from trial pits offers the possibility of high
quality, undisturbed, block samples or tube samples in the
weaker material. Block samples may be taken by hand
excavation and, if testing is not to be done immediately,
preserved by wrapping in cling film, foil and waxing.
Transport to the laboratory should be in boxes with the
samples suitably padded and protected from mechanical
damage. In the harder material the ability to obtain block
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samples may be limited by natural discontinuities such as
bedding jointing and listric fractures.
In this study, block samples of hard, fractured material
were taken by pushing a 300 mm by 300 mm open ended
steel cutter frame 150 mm deep with its lower end
sharpened to form a cutting edge down over a slightly
oversized pre-prepared square pillar of material. The pit
was dug and the pillar prepared carefully by hand to avoid
sample disturbance. As the box was pushed down the pillar
was trimmed to size and the final cut was made by the
cutter frame. The box was separated at its base from the
base of the soil pillar and surplus material was trimmed
from the open ends of the cutter frame which were then
sealed with cling film, aluminium foil and adhesive plastic
tape. The samples so taken were transported to the laboratory
taking care to avoid mechanical damage and drying.
The samples were suitable for unconfined three dimensional
swell, swelling pressure, one dimensional swell,
index tests, scanning electron microscope examination and
mineralogical analysis to be carried out after specimen
preparation in the laboratory.
Cylindrical samples were taken in a similar manner by
lowering a plastic sample tube 100 mm in diameter over a
pre-prepared oversized cylindrical pillar with the aid of a
tripod frame which guided the tube accurately as it was
lowered (Culshaw et al. 1991; Northmore and Culshaw,
1992). The cylindrical samples were used for oedometer
testing at the sampling diameter of 100 mm.