Civil Engineering Reference
In-Depth Information
and a plastic liner system where the liner fits inside a larger steel tube and cutting shoe assembly,
During driving any entrapped water, air or slush can escape through a non-return valve fitted in
the driving head at the top of the tube. After collection, the sample is sealed with end caps at both
ends. If the sample is to be stored for a long time, a paraffin wax coating can be applied to each
end of the soil in the tube to prevent long-term changes in water content.
Despite their popularity in the UK, with the implementation of EN ISO 22475-1:2006 (
International
Standard for Geotechnical Investigation and Testing - Sampling and groundwater measurements
),
U100 samples are likely to become less used in the site investigation industry in the coming years.
This standard recognises that U100 samples are not wholly appropriate for use for certain geotechni-
cal tests (see Section
6.4.3)
and shows that other methods of soil sampling retrieve better quality
samples.
(v)
Thin-walled tube sampler
For soils such as soft clays and silts that are sensitive to disturbance, a thin-walled sampling tube can
be used. Because of the softness of the soil to be collected, the tube is simply machined at its end
to form a cutting edge and does not have a separate cutting shoe. The thin-walled sampler is similar
in appearance to the sample tube shown in Fig.
6.7a
but can have an internal diameter of up to
about 200 mm.
These sampling techniques involve the removal of the boring rods from the hole, the replacement
of the cutting edge with the sampler, the reinsertion of the rods, the collection of the sample, the
removal of the rods, the replacement of the sampler with the cutting edge and, finally, the reinsertion
of the rods so that boring may proceed. This is a most time-consuming operation and for deep bores,
such as those that occur in site investigations for offshore structures, techniques have been developed
to enable samplers to be inserted down through the drill rods so that soil samples can be collected
much more quickly.
(viii)
Piston sampler
A piston sampler is a specific thin-walled sampler for use in weak soils such as soft clays and slurry
materials. A hydraulically powered piston sits neatly within the sampling tube and the assembly, with
the piston locked in place at the cutting end of the tube, is carefully lowered to the bottom of the
already formed borehole. The piston is connected by piston rods which pass through a sliding joint
in the sampler head assembly so that the sample tube and the piston can move vertically independ-
ently of each other (Fig.
6.8)
.
The sample is taken from the undisturbed zone of soil beneath the borehole by releasing the lock
between piston and tube and pushing the sampling tube into the ground. The piston remains still
during this operation.
Once the tube is fully driven into the soil, the undisturbed sample fully occupies the sampling tube
and the assembly is rotated slightly to shear the sample from the ground. The tube and piston are
then locked together again and raised to the surface. A suction pressure is imparted within the
sampling tube, which ensures that the sample is delicately held intact.
(viii)
Continuous sampler
In some cases, particularly where the soil consists of layers of clay, separated by thin bands of sand
and silt and even peat, it may be necessary to obtain a continuous core of the soil deposits for closer
examination in the laboratory. Such sampling techniques are highly specialised and require the elimi-
nation of friction between the soil sample and the walls of the sampler. A sampler which reduces
side friction by the use of thin strips of metal foil placed between the soil and the tube was developed
by Kjellman
et al
. (
1950
) and is capable of collecting a core 68 mm in diameter and up to 25 m in
length.
(viii)
Window sampler
Window sampler tubes, about 1 m in length, are driven into the ground using the percussive or jacking
methods described earlier. The sampler possesses a slot, or window, cut on one side through which
the soil can be inspected and sampled.
