Civil Engineering Reference
In-Depth Information
the shear stress axis represents the cohesion (see
Figure 1.8(a)). A description of these conditions
on Figure 4.7 is as follows:
the cohesion can be higher than that of a strong
intact rock that is closely fractured.
The range of shear strength conditions that may
be encountered in rock slopes as illustrated in
Figures 4.2-4.5 clearly demonstrates the import-
ance of examining both the characteristics of the
discontinuities and the rock strength during the
site investigation.
Curve 1 Infilled discontinuity:
If the infilling
is a weak clay or fault gouge, the infilling fric-
tion angle (
φ
inf
) is likely to be low, but there may
be some cohesion if the infilling is undisturbed.
Alternatively, if the infilling is a strong calcite
for example, which produces a healed surface,
then the cohesive strength may be significant (see
Section 4.2.5).
Curve 2 Smooth discontinuity:
A smooth,
clean discontinuity will have zero cohesion, and
the friction angle will be that of the rock sur-
faces (
φ
r
). The friction angle of rock is related
to the grain size, and is generally lower in fine-
grained rocks than in coarse-grained rocks (see
Section 4.2.2).
Curve 3 Rough discontinuity:
Clean, rough
discontinuity surfaces will have zero cohesion,
and the friction angle will be made up of two com-
ponents. First, the rock material friction angle
(
φ
r
), and second, a component (
i
) related to the
roughness (asperities) of the surface and the ratio
between the rock strength and the normal stress.
As the normal stress increases, the asperities are
progressively sheared off and the total friction
angle diminishes (see Section 4.2.4).
Curve 4 Fractured rock mass:
The shear
strength of a fractured rock mass, in which
the sliding surface lies partially on discontinuity
surfaces and partially passes through intact rock,
can be expressed as a curved envelope. At low
normal stresses where there is little confinement
of the fractured rock and the individual fragments
may move and rotate, the cohesion is low but the
friction angle is high. At higher normal stresses,
crushing of the rock fragments begins to take
place with the result that the friction angle dimin-
ishes. The shape of the strength envelope is related
to the degree of fracturing, and the strength of the
intact rock (see Section 4.5).
Curve 5 Weak intact rock:
Rock such as the
tuff shown in Figure 4.5 is composed of fine-
grained material that has a low friction angle.
However, because it contains no discontinuities,
4.2 Shear strength of discontinuities
If geological mapping and/or diamond drilling
identify discontinuities on which shear failure
could take place, it will be necessary to determine
the friction angle and cohesion of the sliding sur-
face in order to carry out stability analyses. The
investigation program should also obtain inform-
ation on characteristics of the sliding surface
that may modify the shear strength parameters.
Important discontinuity characteristics include
continuous length, surface roughness, and the
thickness and characteristics of any infilling, as
well as the effect of water on the properties of the
infilling.
The following sections describe the relationship
between the shear strength and the properties of
the discontinuities.
4.2.1 Definition of cohesion and friction
In rock slope design, rock is assumed to be a
Coulomb material in which the shear strength of
the sliding surface is expressed in terms of the
cohesion (
c
) and the friction angle (
φ
) (Coulomb,
1773). The application of these two strength
parameters to rock is discussed in the following
paragraphs.
Assume a number of test samples were cut from
a block of rock containing a smooth, planar dis-
continuity. Furthermore, the discontinuity con-
tains a cemented infilling material such that a
tensile force would have to be applied to the two
halves of the sample in order to separate them.
Each sample is subjected to a force at right angles
to the discontinuity surface (normal stress,
σ
),
and a force is applied in the direction parallel to
