Civil Engineering Reference
In-Depth Information
is that of the rock mass. In contrast, the bench
height is about equal to the joint length so sta-
bility could be controlled by a single joint, and
the appropriate rock strength to use in design of
the benches is that of the joints set that dips out
of the face. Finally, at a scale of less than the
joint spacing, blocks of intact rock occur and the
appropriate rock strength to use in the assessment
of drilling and blasting methods, for example,
would be primarily that of the intact rock.
Based on this relationship between sample size
and rock strength characteristics, this chapter
examines methods of determining the strength of
the following three classes of rock:
the strength could diminish over the design
life of the slope due to weathering.
In this topic, the subscript “i” is used to designate
intact rock, and the subscript “m” to desig-
nate the rock mass, for example, the respective
compressive strengths are
σ
ci
and
σ
cm
.
4.1.2 Examples of rock masses
Figures 4.2-4.5 show four different geological
conditions that are commonly encountered in the
design and analysis of rock slopes. These are
typical examples of rock masses in which the
strength of laboratory-size samples may differ
significantly from the shear strength along the
overall sliding surface. In all four cases, instabil-
ity occurs because of shear movement along a
sliding surface that either lies along an existing
fracture, or passes partially or entirely through
intact rock. Figures 4.2-4.5 also show that, in
fractured rock, the shape of the sliding surface
is influenced by the orientation and length of the
discontinuities.
Figure 4.2 shows a strong, massive limestone
containing a set of continuous bedding surfaces
that dip out of the slope face. Because the near ver-
tical cut face is steeper than the dip of the bedding,
the bedding surfaces are exposed, or “daylight”
on the face and sliding has occurred with a tension
(i)
Discontinuities
—Single bedding planes,
joints or faults. The properties of dis-
continuities that influence shear strength
include the shape and roughness of the sur-
faces, the rock on the surface which may be
fresh or weathered, and infillings that may
be low strength or cohesive.
(ii)
Rock mass
—The factors that influence the
shear strength of a jointed rock mass include
the compressive strength and friction
angle of the intact rock, and the spacing of
the discontinuities and the condition of their
surfaces.
(iii)
Intact rock
—A factor to consider in meas-
uring the strength of the intact rock is that
(a)
(b)
Figure 4.2
Plane failure on continuous bedding plane dipping out of the slope (strong, blocky limestone,
Crowsnest Pass, Alberta, Canada).
