Civil Engineering Reference
In-Depth Information
20
10
5
2
1
0.5
0.2
0.1
1
2
3
4
5
6
friction ratio R
f
(%)
Figure 2.12
Simplified version of a friction ratio cone resistance plot used to
obtain an approximate description of the ground
shearing resistance, but local experience and correlation with laboratory
tests is essential. There are also penetrometer tips with pore water pressure
measurement (piezocone) and geophysical testing (seismic cone and resistiv-
ity cone). Further details on cone penetration testing can be found in Mayne
(2007).
Dilatometer/pressuremeter
A dilatometer (or a pressuremeter, the terms
seem interchangeable for rock testing) is a borehole deformation device.
It is used as a rock/soil loading test in boreholes with a defined diameter.
The aim of the dilatometer test is to determine the deformation modulus of
the ground (see Figure 2.17, 'Definitions of different modulus values') and
horizontal stress. The dilatometer consists of a cylindrical pressure cell
containing strain arms within a cylindrical rubber membrane, which is
pressed hydraulically against the borehole wall (Figure 2.13a). The borehole
walls are loaded and then unloaded (cyclically) causing the borehole walls
to deform (measured by the strain arms), thus allowing an estimate to be
made of the deformation modulus of the material for an associated change
in radii. By conducting the test in different directions within the ground, it
is possible to determine the deformation modulus in different directions
and hence obtain information on anisotropy within the ground. As with all
in situ
experiments the validity of the results are potentially limited because
only small areas/sections of the ground are tested. Problems in validation
can occur if the ground has many fractures, the borehole wall is not even
or when the borehole is not stable and rock is collapsing into the bore.
