Civil Engineering Reference
In-Depth Information
There are also laboratory tests for determining the abrasiveness of rocks
in order to gauge the wear on cutting tools. One such test is the CERHAR
Abrasiveness Test (CAI Test) developed at the Centre d'Étude et de
Recherche du Charbon (Büchi
et al.
1995). This test provides an index
value that can be used as a gauge for the abrasiveness of different rock
types. The index value ranges from 0.3 for very low abrasiveness to 6.0
for extremely abrasive. Using this test, basalt has an abrasiveness index of
2.7, gneiss 4.4 and granite 4.9.
Further details on laboratory tests can be obtained from standards, such
as Eurocode 7: Geotechnical Design - Part 2: Ground investigation and testing
(BSI 2007). This document includes guidance on both soil and rock testing.
In addition, details on identification and description, and classification of
soft ground can be obtained from BSI (2002a) and BSI (2004a) respectively.
For the identification and classification of rocks the reader is referred to
BSI (2003). Details of shear strength tests can be obtained from BSI (1990).
In addition, Head (1997 and 2008) and Head and Keeton (2010) provide
extensive descriptions of all the main soil laboratory tests.
In order to determine the strength parameters for rock and soil, and the
modulus, E, uniaxial and triaxial tests can be conducted. These tests are
briefly described below.
UNIAXIAL TEST
This is a standard experiment for rock cores in order to obtain failure
parameters (unconfined compressive strength UCS,
. For
the uniaxial test the core sample is loaded in one direction. Laboratory
samples are made up of cylindrical shaped cores with diameters of at least
90-100 mm. During the test, load is applied to the end of the sample
(Figure 2.16a). Some considerations for sample preparation include:
u
,
u
), E and
•
the end surfaces must be flat and even;
•
the ends must be parallel and at right angles to the sample axis in order
to avoid bending stresses being induced into the sample (which will
give a reduced value of strength);
•
during the test, friction is generated between the end surfaces of the
sample and the end loading platens. This has the potential for increasing
the failure load of the sample as it restricts the sample expansion. This
is negligible if the height (h) to diameter (d) ratio is greater than or
equal to 2 (h/d
2), for example if
there is not enough intact material in the core sample, then equation
2.6 can be used to adjust the stress,
2). If h/d is less than 2 (h/d
u,adj
.
8
⋅
σ
u
σ
=
uadj
,
d
h
72
+ ⋅
(2.6)
