Geology Reference
In-Depth Information
Although rocks actually usually fail in tension rather than compres-
sion, tensile strength is rarely measured directly or used in analysis or
design, compressive strength being the preferred parameter for rock
mass classi
cations and empirical strength criteria (see later). Tensile
strength of rock and concrete is relatively low, typically about 1/10
th
of
UCS. It is because of the weakness of concrete in tension that reinfor-
cing steel needs to be used wherever tensile stresses are anticipated
within an engineering structure.
Shear strength is a very important consideration for many geotechnical
problems, most obviously in landslides where a volume of soil or rock
shears on a slip surface out of a hillside. It is also important for the
design of foundations and in tunnelling (
Chapter 6).
There are two
main types of test used to measure shear strength in the laboratory
-
direct shear and triaxial testing. There are also many other in situ tests
used to measure shear strength parameters, either directly (e.g. vane
test) or indirectly (e.g. SPT and static cone penetrometer tests), and
these have been introduced in
Chapter 4.
For persistent (continuous) rock discontinuities, direct shear
testing is the most appropriate way of measuring shear strength.
Details of testing and interpretation are given in Hencher & Richards
(1989) and Hencher (1995). Because of the inherently variable
roughness of different natural samples, dilation needs to be measured
and results normalised, as discussed later. If this is not done then, in
the author
s opinion, the tests are usually a total waste of time.
The details of a shear box capable of testing rock discontinuities and
weak rocks with controlled pore pressures is described by Barla et al.
(2007).
Direct shear tests are also carried out on soil and are much easier to
prepare and conduct than tests on rock discontinuities, although the
stress conditions are not fully de
'
ned in the test, which can cause some
dif
culties in interpretation (Atkinson, 2007). This is one reason why
triaxial testing is preferred for most testing of soils. Other advantages
are that factors like drainage and pore pressure measurement can be
carefully controlled. A disadvantage is that the soil may well become
disturbed during trimming and preparation for the test as well as
during back saturation and loading/unloading, but that is a problem
for all testing. In a triaxial test, the cylindrical sample is placed inside a
cell and then an all-around
3
). This remains
the constant minimum principal stress throughout the test. Some tests
are carried out drained, in that water is allowed to seep out of the
sample as it is compressed; in others drainage is prevented, the water
fluid pressure applied (
σ
