Environmental Engineering Reference
In-Depth Information
Conditions with a High Failure Incidence
Jointed rock masses on steep slopes can result in falls, slides, avalanches, and
flows varying from a single block to many blocks.
●
Weakness planes dipping down and out of the slope can result in planar failures
with volumes ranging from very large to small.
●
Clay shales and stiff fissured clays are frequently unstable in the natural state
where they normally fail by shallow sloughing, but cuts can result in large rota-
tional or planar slides.
●
Residual soils on moderate to steep slopes in wet climates may fail progres-
sively, generally involving small to moderate volumes, although heavy runoff
can result in debris avalanches and flows, particularly where bedrock is
shallow.
●
Colluvium is generally unstable on any slope in wet climates and when cut can
fail in large volumes, usually progressively.
●
Glaciolacustrine soils normally fail as shallow sloughing during spring rains,
although failures can be large and progressive.
●
Glaciomarine and other fine-grained soils with significant granular components
can involve large volumes in which failure may start by slumping, may spread
laterally, and under certain conditions may become a flow.
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Any slope exposed to erosion at the toe, particularly by stream activity;
cut too steeply; subject to unusually heavy rainfall; or experiencing defor-
mation.
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Some Examples
A general summary of typical forms of slope failures as related to geologic conditions is
given in
Table 9.3.
Dipping beds of sedimentary rocks in mountainous terrain are often the source of disas-
trous slides or avalanches (see
Figure 9.18)
.
Very large planar slides failing along a major
discontinuity occur where the beds incline in the slope direction. On the opposite side of
the failure in Figure 9.18 the slope is steeper and more stable because of the bedding orien-
tation. Failures will generally be small, evolving under joint sets, although disastrous ava-
lanches have occurred under these conditions, such as the one at Turtle Mountain, Alberta.
Orientation of joints with respect to the rock slope face controls stability and the form of
failure. The near-vertical slope in the 40-year-old railroad cut illustrated in
Figure 9.80
is
stable in decomposed amphibolite gneiss because of the vertical jointing. The cut shown
in
Figure 9.81
is near that of Figure 9.80 but at a different station and on the opposite side
of the tracks. Here, the slope is much flatter, approximately 1:1, but after 40 years is still
experiencing failures such as that of the wedge shown in the photo that broke loose along
the upper joints and slid along a slickensided surface. These examples illustrate how joint
orientation controls slope stability, even in “soft” rock. The cuts were examined as part of
a geologic study for 30 km of new railroad to be constructed in the same formation but
some distance away.
Sea erosion undercutting jointed limestone illustrated in
Figure 9.82
was causing concern
over the possible loss of the roadway, which is the only link between the town of Tapaktuan,
Sumatra, and its airport. A fault zone may be seen on the right-hand side of the photo. For
the most part, the joints are vertical and perpendicular to the cliff face, shown as plane
a
in
Figure 9.83,
and the conditions are consequently stable. Where the joints are parallel to the
face and inclined into it, as shown by plane
b
in the figure, a potentially unstable condition
exists. This condition was judged to prevail along a short stretch of road beginning to the
