Geology Reference
In-Depth Information
'basic', 'residual' or 'smooth' friction
Limit is about tan
φ
= 0.85
(40°) for coarse silicate
rocks
40
τ
= tan (
φ
b
+
i
)
σ
30
Textural component
-ploughing/deformation
(non-dilational)
20
10
Adhesional friction component
0
Natural joints
(non-dilational)
Naturally
polished
mudstones
Chlorite
coated rock
joints
Rough, interlocking
coarse silicate rocks-
corrected for dilation
Range
Artificial
surfaces
Probable
minimum
Repeated
sliding tests
Typical values for
saw cut/sand blasted/
tilted core
Roughness of flat surface
ne,
polished or coated with low-friction minerals, as illustrated by a case
example in
Box 5-2.
The author has measured values of only 10 to 15
degrees for naturally polished joint surfaces through Coal Measures
mudstones of South Wales, UK, and such low values are lower than
measured for saw-cut surfaces through the parent rock. The range of
variation for basic friction, measured for natural joints with different
surface textures and for arti
where they are planar and where the surface texture is very
cially prepared (saw-cut and lapped)
joints, is indicated in
Figure 5.23.
Box 5-2 Yip Kan Street landslide
-
an example of use of direct shear testing.
The Yip Kan Street landslide occurred in July 1981 on a dry Sunday night. It mainly involved large
blocks of rock of up to 10m
3
, which slid on persistent joint planes dipping at only 22 degrees out of the
slope (Hencher, 1981b). The total failure volume was estimated to be 1,235m
3
. The 8m high, near-
vertical slope was cut in very strong, slightly decomposed, coarse-grained igneous rock (quartz-
syenite). The upper part of the slope was in saprolite. The failure occurred next to a construction
site where blasting had been carried out recently, before the failure but not over the weekend. There
had been intense rainfall a week before the failure. The slope had been deteriorating in the days
preceding the failure, with cracks in chunam cover in the weathered part having been repaired
ve days
before failure.
