Geoscience Reference
In-Depth Information
obtain deeper disturbed but representative samples as part of the Standard Penetra-
tion Test (SPT). Screw augers also provide disturbed samples where a drill rig can be
mobilized.
2.5.2 Undisturbed sampling
As mentioned above, it is virtually impossible to obtain undisturbed samples of any
type of soil, including peat. Both physical intrusions of the sampler and the removal
of
in situ
stresses cause disturbance. However, using certain sampling techniques, dis-
turbance can be minimized. It can be said that there is a reasonably well-established
understanding of the causes of disturbance during the sampling, transport and han-
dling of inorganic clays and the corresponding accepted practices of sampling such
soils. There are additional factors that need to be considered in sampling peat. These
include compression while forcing the sampler into the ground, tensile resistance of
fibres near the sampler edge during extraction of the sampler, and drainage and internal
redistribution of water.
The thin-walled piston sampler with a fixed piston is normally the most suitable
tool for non-fibrous soils. A number of piston samplers with different sample diameters
are available, c.f. ISSMFE Subcommittee on Soil Sampling (1981).
The
piston sampler
consists mainly of sampler head, piston, piston rod, piston
extension rod or wire, sampling tube or liner and thrust equipment or machine. During
sampling, the closed sampler is pushed into the soil to a level just above the where the
sample is cut out (Figure 2.14). After a while, when part of the generated excess pore
pressure has disappeared and the sampled soil adheres to the inside of the walls, the
sampler can be extracted. To obtain high-quality samples it is, among other things, very
important to have a very sharp edge on the sampler and to have the piston carefully
fixed to the ground or thrust machine. It is also important to have the sample cut out
steadily and slowly (Kallstenius, 1963). Samplers with small inner clearances and small
area ratios should be used. One difficulty with the small-diameter sampler is taking
good samples in fibrous peat. Because of the force applied to cut off the fibres, the
sample may be compressed at sampling.
A
peat sampler
, Ø100mm, has been developed in Sweden. This sampler consists
of a sharp wave-toothed edge mounted on a plastic tube with a driving head at the
upper end (Figure 2.15). Samples are taken from the ground surfaces or the bottom
of pre-bored holes. After extraction of the sampler, the cutting edge and driving head
are removed and the sample in the plastic tube is sealed. Laboratory tests show that
samples of fibrous soils taken with this peat sampler are of better quality than samples
taken with a small-diameter piston sampler. Practical experience has also shown good
correlation between laboratory test data from this kind of sample and measured field
behaviour under embankment on fibrous peat.
The NGI 54mm fixed piston sampler (composite version) was developed and
designed by NGI and it is the most common sampler used in Norway. It is a composite
piston sampler using plastic inner cylinders to prevent corrosion and avoid practical
difficulties with the production and use of steel cylinders. Pre-augering through the dry
crust is usually done before starting the sampling. Beneath the dry crust the displace-
ment method is used, wherein the sampler (with the piston in front of the sample tube)
is pushed down to the desired sampling depth without preboring. During sampling
