Civil Engineering Reference
In-Depth Information
6.4 Soil and rock sampling
Soil and rock samples are taken regularly during the ground investigation so that specific ground proper-
ties required in the geotechnical design can be established.
6.4.1 Soil sampling
Two types of soil sample can be obtained: disturbed sample and undisturbed sample.
Disturbed samples
The soil excavated from a trial pit, or the soil from the clay cutter or the shell from a borehole, can be
collected as disturbed samples. Such soil has been remoulded and is of no use for shear strength tests
but is useful for identification, classification and chemical tests such as liquid and plastic limit determina-
tion, particle size distribution and sulphate testing. Disturbed samples are usually collected in plastic
sampling bags or airtight tins or jars, and are labelled to give the borehole (or trial pit) number, the depth
and a description of the contents.
Undisturbed samples (cohesive soil)
Undisturbed samples can be achieved using different equipment and techniques in different situations.
There will always be an element of disturbance to any sample of soil taken from the ground, but that
disturbance can be minimal if care and appropriate methods are used.
(iii)
Trial pits
In a trial pit samples can be cut out by hand if care is taken. Such a sample is placed in an airtight
container and as a further measure to avoid change in water content, it may be sealed in paraffin wax.
(iii)
Hand excavated boreholes
In a hand excavated borehole, the hand auger can be used to obtain useful samples for unconfined
compression tests and employs 38 mm sampling tubes with a length of 200 mm (Fig.
6.2c
). The auger
is first removed from the rods and the tube fitted in its place, after which the tube is driven into the
soil at the bottom of the borehole, given a half turn, and withdrawn. Finally, the ends of the tube
are sealed with end caps.
(iii)
Rotary core drilling
During the advancement of the borehole, the cutting tool is used (Fig.
6.6a
). This process is known
as open-hole drilling. The soil cuttings are too disturbed and mixed with drilling fluid to be of any
use for sampling so the method is really only used to rapidly advance the borehole to the required
depth for sampling to commence.
To take samples of soil or rock, the cutting tool is replaced by the coring tool attached to a core
barrel (Fig.
6.6b
). Industrial diamonds are cast into the tungsten carbide cutting tool face to enable
the cutting shoe to cut through even the hardest of rocks. The core of soil or rock that is cut during
this coring process is collected in the core barrel and can be brought to the surface for labelling and
identification and transported to the geotechnical laboratory for testing. The core barrel can actually
take one of three forms: single-tube, double-tube or triple-tube.
A single-tube barrel has the same diameter as the drilling rods and is connected directly to the
cutting shoe. Samples retrieved in a single-tube core barrel experience a fair degree of disturbance,
so double and triple-tube barrels are often used in preference.
As suggested by the name, a double-tube barrel comprises two tubes: an outer one which is
attached to the coring bit and rotated by the drill rods, and a non-rotating inner one into which the
core sample passes as the cutting bit is advanced as shown in Fig.
6.6b
. An extension of the double-
tube arrangement is to include a sample liner within the inner tube. This is known as a triple-tube
core barrel.
