Geology Reference
In-Depth Information
Figure 5.25 Cut
slope at Rhuallt,
North Wales,
UK. Traversing the
slope is a very
persistent narrow
stratum of weak
clay which,
combined with
provided the
mechanism for
major rock failure
in this otherwise
excellent-quality
rock mass.
Road cutting, North Wales
Note off-set by fault
Note off-set by fault
Clay-infilled bedding-parallel surface
Clay-infilled bedding-parallel surface
joint. The effect of the in
ll is a function of the relative height of
roughness asperities in the wall rock vs. the thickness of weaker
material (Papaliangas et al., 1990). If persistent and the in
ll is of
low strength, the consequences can be serious. Cut slopes on the
A55 at Rhuallt, North Wales (
Figure 5.25),
failed by sliding on
bedding-parallel thin clay in
lled discontinuities with faults acting as
release surfaces (Gordon et al., 1996). The mechanism had not been
anticipated from ground investigation prior to the failure, which
involved more than 185,000m
3
of rock.
In some slopes,
incremental movement may take place over
many years before
final detachment of a landslide and, following
each movement, sediment may be washed in to accumulate in
dilated hollows on the joint (
Figure 5.26).
The presence of such
in
ll might cause alarm during ground investigation but in many
casesiscon
ned to local down-warps and probably plays little
part in decreasing shear strength, other than in restricting drai-
nage (Halcrow Asia Partnership, 1998b). It may, however, be
taken as a warning that the slope is deteriorating and approaching
failure.
methods
Various empirical criteria have been proposed for estimating shear
strength of rock joints, based on index tests and idealised joint
shapes. The most widely used is that proposed by Barton (1973).
Frictional resistance for saw-cut or other arti
cially prepared planar
surfaces is taken as a lower bound. The limiting value is typically 28.5
to 31.5 degrees according to Barton & Bandis (1990). An additional
