Civil Engineering Reference
In-Depth Information
Figure 17.3
Methods of sampling in boreholes (schematic).
pushed into the bottom of a borehole. There are many different tube samples; two
designs used in the United Kingdom are shown in Fig. 17.3(a) and (b).
The tube sampler most often used in practice in the United Kingdom is the U100
illustrated in Fig. 17.3(a). The tube, nominally 100 mm in diameter, is screwed to
a cutting shoe and a sampler head. The thickness of the wall of the cutting head is
6 to 7 mm, which is relatively large. A thin wall sample tube like that illustrated in
Fig. 17.3(b) has a wall thickness of 1 to 2 mm and a cutting edge formed by machining.
Both samplers are capable of taking samples in many soft and stiff clays. Intact samples
may be recovered by coring (see Fig. 17.3c), where a rotary drill cuts an annulus around
the core sample. In the past this method was used exclusively for rocks but is now also
used in stiff clays.
17.5
In situ
testing
Laboratory tests to determine soil strength, stiffness and permeability are described
in Chapter 7, but there are also a number of
in situ
tests. These can be grouped into
probing tests, loading tests and permeability tests.
(a) Probing tests
In these tests a tool, usually cone-shaped, is hammered or pushed into the ground and
the resistance to penetration recorded. This gives some measure of the strength and
stiffness of the ground. In the standard penetration test (SPT) shown in Fig. 17.4(a) a
solid cone or thick-wall tube is hammered, with a standardized blow, into the bottom
of a borehole. The result is given as
N
, the number of blows to achieve a standard
penetration; values increase from about 1 to more than 50 with increasing relative
density or overconsolidation ratio.
In the static cone, or Dutch cone, penetration test shown in Fig. 17.4(b) the instru-
ment is steadily pushed into the ground from the surface and the resistance recorded
