Environmental Engineering Reference
In-Depth Information
was unsuccessful. Movement was finally arrested by the installation of horizontal drains
as long as 400 ft, and vertical wells which were being pumped daily at the time that Noble
(1973) prepared his paper.
Colluvium on Shale Slopes
The Pennington shale of the Cumberland plateau in Tennessee and the sedimentary strata
in the Appalachian plateau of western Pennsylvania develop thick colluvial overburden,
which is the source of many slide problems in cuts and side-hill fills. The geology, nature of
slope problems, and solutions are described in detail by Royster (1973, 1979) and Hamel
(1980).
9.2.8
Debris Avalanches
General
Debris avalanches are very rapid movements of soil and rock debris which may, or may
not, begin with rupture along a failure surface. All vegetation and loose soil and rock
material may be scoured from a rock surface as shown in
Figure 9.7.
Major causes are high
seepage forces, heavy rains, snowmelts, snowslides, earthquakes, and the creep and grad-
ual yielding of rock strata.
Failure is sudden and without warning, and essentially unpredictable except for the
recognition that the hazard exists. Effects can be disastrous in built-up areas at the toes of
high steep slopes under suitable geologic conditions (see Examples below).
Occurence
Debris avalanches are characteristic of mountainous terrain with steep slopes of residual
soils where topography causes runoff concentration (see
Figure 9.86)
,
or badly fractured
rock such as illustrated in
Figure 9.47.
There is usually no initial stage, although occasionally tension cracks may be apparent
under some conditions. Total failure occurs suddenly either by a rock mass breaking loose
or by “hydraulic excavation” which erodes deep gullies in soil slopes during torrential
rains as shown in
Figure 9.48.
All debris may be scoured from the rock surface and
deposited as a terminal lobe at a substantial distance from the slope. As shown in
Figure
9.49,
the force is adequate to move large boulders, and erosion can cause the failure area
to progress laterally to affect substantial areas.
In the Andes Mountains of South America, debris avalanches are the most common
form of slope failures. They occur occasionally in natural slopes, not impacted on by con-
struction, but normally are caused by roadway cuts. Typical geologic conditions are
shown in
Figure 9.50.
Illustrated in
Figure 9.51
are the steep slopes and irregular topogra-
phy that result in the necessity for numerous cuts for roadway construction. The slope
shown in the photo,
Figure 9.52,
taken in 2002, was free of slope failures in 1995.
Investigation and treatments of slopes in the Andes is discussed in
Section 9.5.
Examples of Major Failures
Rio de Janeiro, Brazil
Event
: A debris avalanche occurred during torrential rains in 1967 in the Laranjeiras sec-
tion of Rio de Janeiro, which destroyed houses and two apartment buildings, causing the
death of more than 130 persons. The avalanche scar and a new retaining wall are shown
in
Figure 9.53.
