Geology Reference
In-Depth Information
Society for Rock Mechanics provides guidance on many
field and
laboratory tests (Ulusay & Hudson, 2006). Recommendations for
the same test sometimes differ, for example regarding sample dimen-
sions and testing rate, so care has to be taken that an appropriate
method is being adopted and referenced. Furthermore several different
techniques or different equipment can sometimes be used ostensibly to
measure the same parameters but inevitably with different results.
For example, small strain dynamic tests may give very different
values for soil stiffness compared with large-scale loading tests but
each might be appropriate to some aspect of numerical analysis and
design within a single project (Clayton, 2011). It should also be
remembered that, however much they are standardised, all tests on
soil and rock are experiments. There will be many variables, not least
the geological nature and moisture content of the sample to be tested,
so interpretation is always necessary. Further judgement is required
before attempting to apply small-scale results at the larger scale
(e.g. Cunha, 1990).
Intact rock, clay and concrete are generally classi
ed in shorthand by
their uncon
ned (or uniaxial) compressive strength (UCS) as discussed
in
Chapter 4.
Compressive strength is not a relevant concept for purely
frictional materials such as sand, which must be con
ned to develop
shear resistance. Indicative UCS values for various materials are pre-
sented in
Table 5.1;
fresh rock is often considerably stronger than the
highest strength concrete. For concrete, UCS is used as a quality
assurance test on construction sites.
In a UCS test the axial stress is
σ
1
and the con
ning stresses (
σ
2
and
σ
3
) are zero. Despite the apparent loading condition, the sample does
not actually fail in compression but either in tension or in shear or in
some hybrid mode. If the sample contains adverse weak fabric such as
incipient joints or cleavage, then the sample will fail at lower strength
than it would without the
flaws. UCS is really essentially an index test
used especially in rock mass classi
cation. In practice strength can
often be estimated quite adequately using index tests such as hitting
with a geological hammer (see
Box 5-1).
UCS can also be measured
using point load testing, which is quick and easy, but correlation with
UCS from laboratory testing may be imprecise. The Schmidt hammer
is sometimes used to estimate strength using standard impact energy to
measure rebound from a rock or concrete surface. It is sensitive to
surface
finish and any fractures behind the impact location will cause
low readings. It is also insensitive to strength over about 100 MPa. It is
generally unsuitable for testing rock core
its main use in engineering
geology is as an index test to help differentiate between different
degrees of weathering as discussed in
Chapter 4.
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