Environmental Engineering Reference
In-Depth Information
during a rainy season with the probability of storms, and if the velocity is increasing, final
failure is imminent and may occur during the first heavy rain, or at some time during the
rainy season. The following two case histories relate slope movements to total failure.
Case 1
: A roadway cut made in residual soils in the coastal mountains of Brazil contin-
ued to show instability through creep, tension cracks, small slumps, and periodic
encroachment on the roadway for a period of several years. An intermediate failure stage
occurred on November 29, 1977, after a weekend of moderately heavy rain and a period
during which the highway department had been removing material from the slope toe.
Figure 9.91,
a photo taken from a helicopter about 10 days after the failure, illustrates the
general conditions. Tension cracks have opened at the base of the forward transmission
tower, a large scarp has formed at midslope, and the small gabion wall at the toe has
failed.
Figure 9.92
illustrates the tension crack and the distortion in the transmission tower
shown in Figure 9.91. The maximum crack width was about 30 cm (12 in.) and the scarp
was as high as 50 cm (20 in.). Slope movement measurements were begun immediately by
optical survey and the transmission lines were quickly transferred to a newly constructed
tower situated farther upslope.
For the first 2 weeks after the initial movement, a period of little rainfall, the vertical
drop along the scarp was about 2-3.5 cm/day (0.8-1.4 in./day). In 5 weeks, with occasional
rainy periods, the scarp had increased to 3 m (10 ft). Finally, after a weekend of heavy rains
the slide failed totally in its final stage within a few hours, leaving a scarp about 30 m
(100 ft) in height as shown in
Figure 9.93,
and partially blocking the roadway. Excavation
removed the slide debris, and 2 years afterward the high scarp still remained. The tower
in the photo was again relocated farther upslope. Although future failures will occur, they
will be too far from the roadway to cover the pavement.
Remediating the slope by permitting failure is an example of one relatively low-cost
treatment method. Cutting material from the head of the slide would be an alternate
method, but there was concern that perhaps time for construction prior to total failure
FIGURE 9.91
Rotational slide at Muriqui, Rio de Janeiro, Brazil, initiated by the road cut, had been active for 4 years. Major
movements began after a weekend of heavy rains, endangering the transmission tower. Tension crack and
distorted transmission tower shown in the inset,
Figure 9.92.
