Geology Reference
In-Depth Information
Figure 5.24
Characterising
discontinuity
roughness using
plates of different
diameter. Skipton
Quarry, West
Yorkshire, UK.
from one area to another, as of course also might the mean dip of
the plane. At one location, a blockmight be prevented from sliding by a
wave in the joint surface causing a reduction in the effective down-dip
angle along the sliding direction; elsewhere, a slab of perhaps several
metres length may have a dip angle steeper than the mean angle for
the joint as a whole because it sits on the down-slope section of one of
the major waves. De
ning the scale at which roughness will force
dilation during sliding, rather than being sheared through, requires
considerable judgement. Some assistance is provided by Schneider
(1976) and by Goodman (1980) who indicate that for typical rough
joint surfaces, where slabs are free to rotate during shear, as the
length of the slab increases (at
field scale), the dilation angle controlling
lifting of the centre of gravity of the upper block will reduce. The
problem cannot be
finessed by improved analytical methodology.
There is no substitution to careful engineering geological inspection,
investigation, characterisation of the ground model and judgement
based on experience of similar joints and geological settings, and an
appreciation of the fundamental mechanics controlling the potential
failure.
The two walls of a joint might be separated by a layer or pockets
of weaker material which may reduce shear strength. A similar
situation arises from preferential weathering along a persistent
