Geology Reference
In-Depth Information
Shear and normal stiffness of rock joints are not parameters that are
normally required for civil engineering design but are needed as inputs
when carrying out numerical simulations of jointed rock masses where
each joint is modelled discretely using software such as UDEC. Guidance
is given in the UDEC manuals (Itasca, 2004). Permeability of joints
depends on their openness, tortuosity and connectivity. It is a very
dif
cult but important subject area, especially for nuclear waste disposal
considerations and tunnel in
ow assessments (Black et al., 2007).
When dealing with rock slopes, often any discontinuity that appears
that could be persistent, is treated as so (ignoring potential cohesion
from rock bridges). This is a conservative thing to do (see discussion in
Chapter 6)
but there is little alternative. It is generally agreed that the
shear strength of persistent joints is derived from some basic frictional
resistance offered by an effectively planar natural joint, plus the work
done in overriding the roughness features on that joint. This is
expressed by the following equation (after Patton, 1966):
τ ¼ σ tanðϕ
b
þ i
Þ
where
b
° is a basic friction angle
for a planar joint and i° is a dilation angle that the centre of gravity of the
sliding slab follows during shear, i.e. the deviation from the direction that
the shearing would have followed if the plane had been
τ
is shear strength,
σ
is normal stress,
ϕ
flat and sliding
had occurred along the mean dip direction of the joint. Despite the
apparent simplicity of the Patton equation, derivation of the parameters
can be dif
cult, especially for judging the effective roughness angle. The
available international standards and codes deal with this inadequately.
5.7.3.1 Basic friction,
ϕ
b
Basic friction of natural joints can be measured by direct shear testing
on rock joint samples, but samples taken from different sections of the
same joint and joint set can be highly variable, particularly in terms of
roughness. Furthermore, it is found that any rough rock joint sample
will give different values for peak strength, depending on the direction
of shear under the same normal load. Tests need very careful set up,
instrumentation, analysis and interpretation, if they are to make sense.
A series of tests on different samples of a joint will often yield very wide
scatter, which is meaningless without correcting for sample-speci
c
dilation, as described by Hencher & Richards (1989) and Hencher
(1995). Dilation re
ects work being done in overriding asperities. The
dilation angle measured during a shear test will vary, especially accord-
ing to the original roughness of the sample and the stress level. It is test-
speci
c, will vary throughout a test and with direction of testing. It is
