Civil Engineering Reference
In-Depth Information
infilling. For example, for a clay-filled fault zone
in granite, it would be assumed that the shear
strength of the discontinuity would be that of the
clay and not the granite. In the case of a healed,
calcite-filled fracture, a high cohesion would be
used in design, but only if it were certain that the
discontinuity would remain healed after any dis-
turbance caused by blasting when excavating the
slope.
The presence of infillings along discontinuity
surfaces can have a significant effect on stabil-
ity. It is important that infillings be identified
in the investigation program, and that appropri-
ate strength parameters be used in design. For
example, one of the contributing factors to the
massive landslide into the Vaiont Reservoir in
Italy that resulted in the death of about 3000
people was the presence of low shear strength clay
along the bedding surfaces of the shale (Trollope,
1980).
The effect of the infilling on shear strength will
depend on both the thickness and strength prop-
erties of the infilling material. With respect to the
thickness, if it is more than about 25-50% of the
amplitude of the asperities, there will be little or
no rock-to-rock contact, and the shear strength
properties of the fracture will be the properties of
the infilling (Goodman, 1970).
Figure 4.14 is a plot of the results of direct
shear tests carried out to determine the peak
friction angle and cohesion of filled discontinu-
ities (Barton, 1974). Examination of the test
results shows that the infillings can be divided
approximately into two groups, as follows:
(a)
i
1
=13
°
p
0.5-2 m
(b)
i
2
=26
°
i
2
=28
°
p
50-100 mm
Figure 4.13
Effect of asperities on stability of sliding
blocks: (a) shear strength of displaced block
controlled by first-order asperities (
i
1
); (b) tensioned
rock bolts prevent dilation along potential sliding
surface and produce interlock along second-order
asperities
(i
2
)
.
design of stabilization measures. This demon-
strates the value of using construction measures
that minimize relaxation and dilation of rock
masses.
•
Clays
: montmorillonite and bentonitic clays,
and clays associated with coal measures have
friction angles ranging from about 8 to 20
◦
and cohesion values ranging from 0 to about
200 kPa. Some cohesion values were measured
as high as 380 kPa, which would probably be
associated with very stiff clays.
4.2.5 Discontinuity infilling
The preceding section discussed rough, clean
discontinuity surfaces with rock-to-rock contact
and no infilling, in which the shear strength
is derived solely from the friction angle of the
rock material. However, if the discontinuity con-
tains an infilling, the shear strength properties of
the fracture are often modified, with both the
cohesion and friction angle of the surface being
influenced by the thickness and properties of the
•
Faults, shears and breccias
: the material
formed in fault zones and shears in rocks
such as granite, diorite, basalt, and limestone
may contain clay in addition to granular frag-
ments. These materials have friction angles
