Civil Engineering Reference
In-Depth Information
(a)
(b)
Figure 4.5
Shallow failure in very weak, massive rock containing no discontinuities (volcanic tuff, Trans
European Highway, near Ankara, Turkey).
from residual soil in the upper part (right) of
the slope to slightly weathered rock at greater
depth. For these conditions, the sliding surface
will lie predominantly in the weaker materials in
the upper part of the slope, and in the stability
analysis it is necessary to use different strength
parameters for the upper and lower portions of
the sliding surface. Because the degree of degrada-
tion of weathered rock tends to be highly variable,
the strength of the rock mass will also be variable
and can be difficult to measure. Consequently, a
means of determining the strength of weathered
rock is to carry out a back analysis of slopes
in similar material; this approach is described in
Section 4.4.
A fourth geological condition that may be
encountered is that of a very weak but intact
rock containing essentially no discontinuities.
Figure 4.5 shows a cut face in tuff, a rock formed
by the consolidation of volcanic ash. A geolo-
gical hammer could be embedded in the face with
a few blows, indicating the low strength of this
rock. However, because this rock contains no dis-
continuities, it has a significant cohesive strength
in addition to a moderate friction angle. There-
fore, it was possible to cut a stable, vertical face
to a height of tens of meters in this material,
provided
4.1.3 Classes of rock strength
Based on the scale effects and geological condi-
tions discussed in the previous sections, it can be
seen that sliding surfaces can form either along
discontinuity surfaces, or through the rock mass,
as illustrated in Figure 4.6. The importance of the
classification shown in Figure 4.6 is that in essen-
tially all slope stability analysis it is necessary to
use the shear strength properties of either the dis-
continuities or of the rock mass, and there are
different procedures for determining the strength
properties as follows:
•
Discontinuity shear strength
can be measured
in the field and the laboratory as described in
Sections 4.2 and 4.3.
•
Rock mass shear strength
is determined by
empirical methods involving either back ana-
lysis of slopes cut in similar geological con-
ditions, or by calculation involving rock
strength indices as described in Sections 4.4
and 4.5.
As a further illustration of the effects of geology
on shear strength, relative strength parameters
for three types of discontinuity and two types
of rock mass are shown on the Mohr diagram
in Figure 4.7. The slope of these lines repres-
ents the friction angle, and the intercept with
water
pressures
and
erosion
were
controlled.
