Civil Engineering Reference
In-Depth Information
Figure 1.4 shows a range of geological condi-
tions and their influence on stability, and illus-
trates the types of information that are important
to design. Slopes (a) and (b) show typical con-
ditions for sedimentary rock, such as sandstone
and limestone containing continuous beds, on
which sliding can occur if the dip of the beds
is steeper than the friction angle of the discon-
tinuity surface. In (a) the beds “daylight” on the
steep cut face and blocks may slide on the bed-
ding, while in (b) the face is coincident with the
bedding and the face is stable. In (c) the overall
face is also stable because the main discontinuity
set dips into the face. However, there is some risk
of instability of surficial blocks of rock formed by
the conjugate joint set that dips out of the face,
particularly if there has been blast damage dur-
ing construction. In (d) the main joint set also
dips into the face but at a steep angle to form a
series of thin slabs that can fail by toppling where
the center of gravity of the block lies outside the
base. Slope (e) shows a typical horizontally bed-
ded sandstone-shale sequence in which the shale
weathers considerably faster than the sandstone
to form a series of overhangs that can fail sud-
denly along vertical stress relief joints. Slope (f)
is cut in weak rock containing closely spaced but
low persistence joints that do not form a continu-
ous sliding surface. A steep slope cut in this weak
rock mass may fail along a shallow circular sur-
face, partially along joints and partially through
intact rock.
Figure 1.3
Cut face coincident with continuous, low
friction bedding planes in shale on Trans Canada
Highway near Lake Louise, Alberta. (Photograph
by A. J. Morris.)
the orientation and characteristics (such as length,
roughness and infilling materials) of the joints,
bedding and faults that occur behind the rock
face. For example, Figure 1.3 shows a cut slope in
shale containing smooth bedding planes that are
continuous over the full height of the cut and dip
at an angle of about 50
◦
towards the highway.
Since the friction angle of these discontinuities is
about 20-25
◦
, any attempt to excavate this cut
at a steeper angle than the dip of the beds would
result in blocks of rock sliding from the face on
the beds; the steepest unsupported cut that can
be made is equal to the dip of the beds. However,
as the alignment of the road changes so that the
strike of the beds is at right angles to the cut face
(right side of photograph), it is not possible for
sliding to occur on the beds, and a steeper face
can be excavated.
For many rock cuts on civil projects, the stresses
in the rock are much less than the rock strength
so there is little concern that fracturing of intact
rock will occur. Therefore, slope design is pri-
marily concerned with the stability of blocks of
rock formed by the discontinuities. Intact rock
strength, which is used indirectly in slope design,
relates to the shear strength of discontinuities
and rock masses, as well as excavation methods
and costs.
1.2.2 Open pit mining slope stability
The three main components of an open pit slope
design are as follows (Figure 1.5). First, the over-
all pit slope angle from crest to toe, incorporates
all ramps and benches. This may be a compos-
ite slope with a flatter slope in weaker, surficial
materials, and a steeper slope in more compet-
ent rock at depth. In addition, the slope angle
may vary around the pit to accommodate both
differing geology and the layout of the ramp.
Second, the inter-ramp angle is the slope, or slo-
pes, lying between each ramp that will depend
on the number of ramps and their widths. Third,
