Environmental Engineering Reference
In-Depth Information
on the strength increase with depth and relict rock defects. Relatively shallow failure surfaces
are characteristic of colluvial soils.
Surface Features
During
early failure stages
tension cracks begin to form as shown in
Figure 9.22
a
nd
Figure
9.23.
After
partial failure
, in a progressive mode, the slope exists as a series of small slumps
and scarps with a toe bulge as shown in
Figure 9.24,
or it may rest with a single large scarp
and a toe bulge as illustrated in
Figure 9.25(a)
.
After
total failure
, surface features include a
large head scarp and a mass of incoherent material at the toe as shown in Figure 9.25(b)
and
Figure 9.93.
Slump landforms
remaining after total failures provide forewarning of generally unstable
slope conditions. They include spoon-shaped irregular landforms, as seen from the air
(Figure 9.26
and
Figure 9.27)
,
cylindrical scarps along terraces and water courses (see
Figure 2.17),
and hummocky and irregular surfaces, as seen from the ground
(Figure 9.28
and
Figure 9.29).
In the stereo-pair of aerial photos shown in Figure 9.26, the slump failure
mass has stabilized temporarily but probably will reactivate when higher than normal sea-
sonal rainfall arrives. A small recent failure scar exists along the road in the center of the
slide mass. The rounded features of the mass, resulting from weathering, and vegetation
growth indicate that the slide is probably 10 to 15 years old, or more. In the photo, it can be
seen that the steep highway cut on the opposite side of the valley appears stable, indicative
of different geologic conditions. In general, the geology consists of residual soils derived
from metamorphic rocks in a subtropical climate. In Figure 9.27, an old slump scar in resid-
ual soils, weathering has strongly modified the features. In the photo, the tongue lobe at the
intersection of the trails and the creep ridges are to be noted. The location is near the slide
of Figure 9.26, as shown in
Figure 2.12,
a smaller-scale stereo-pair of the area.
9.2.6
Lateral Spreading and Progressive Failure
General
Failure by lateral spreading is a form of planar failure which occurs in both soil and rock
masses. In general, the mass strains along a planar surface, such as shown in
Figure 9.3e,
represents a weak zone. Eventually, blocks progressively break free as movement retro-
gresses toward the head. The major causes are seepage forces, increased slope inclination
and height, and erosion at the toe.
Failure in this mode is essentially unpredictable by mathematical analysis, since one cannot
know at what point the first tension crack will appear, forming the first block. Nevertheless,
the conditions for potential instability are recognizable, since they are characteristic of certain
soil and rock formations. Failure usually develops gradually, involving large volumes, but can
be sudden and disastrous. Under certain conditions, it is unavoidable and uncontrollable
from the practical viewpoint, and under other conditions control is difficult at best.
Recognition
The failure mode is common in river valleys, particularly where erosion removes material
from the river banks. Characteristically, occurrence is in stiff fissured clays, in clay shales,
and in horizontal or slightly dipping strata with a continuous weak zone such as those that
occur in glaciolacustrine and glaciomarine soils. Colluvium over gently sloping residual
soils or rock also fails progressively in a form of lateral spreading.
Surface features are characterized during the early stages by tension cracks, although
failure can be sudden under certain conditions such as earthquake loadings. During the
