Environmental Engineering Reference
In-Depth Information
Cumulative precipitation increases the ground saturation and a rise in the water table.
A rainstorm occurring during the dry season or at the beginning of the wet season will
have a lesser effect on slope stability than a storm of the same intensity occurring near the
end of the wet season. A plot by month of the occurrence of failures as a function of the
cycle coefficient is given in
Figure 9.94.
It is seen that the most catastrophic events occur
toward the end of the rainy season, when the cumulative precipitation is higher than the
mean annual.
Regarding rainfall intensity, Guidicini and Iwasa concluded that:
Extremely intense rainfalls, about 12% greater than the mean annual rainfall (300
mm in 24-72 h) or more, can cause natural slope failures in their area, regardless
of the previous rainfall history.
●
Intense rainfalls, up to 12% of the mean annual, where the precipitation cycle is
normal or higher, will cause failures, but if the preceding precipitation level is
lower than the mean annual, failures are not likely even with intensities to 12%.
●
Rainfalls of 8% or less of annual precipitation will generally not cause failures,
regardless of the preceding precipitation, because the gradual increase in the sat-
uration level never reaches a critical magnitude.
●
A danger level chart
(Figure 9.95)
was prepared by Guidicini and Iwasa for each
study area, intended to serve communities as a guide for assessing failure haz-
ard in terms of the mean cumulative precipitation for a given year.
●
Such data can be used for temporarily closing mountain roadways, subject to slope fail-
ures, when large storms are expected during the rainy season. At the least warnings signs
can be placed.
Evaluating Existing Cut Slope Stability
General
It is often necessary to evaluate a cut slope that appears stable to formulate judgments as
to whether it will remain so. If a cut slope has been subjected during its lifetime to condi-
2.0
Plot of the ''cycle coefficient'' (C
c
) for all the selected rainfall events
x
Legend
Displacement
Serra of caraguatatuba
Imigranates highway
Anchieta highway
Serra of maranguape
Riode janeiro
Sea of Araras
South of minas gerais
1.5
Santos lowlands
60 days
15 days
30 days
1.0
Tubarao valley
Land slides
Catastrophic events
0.5
Number of
rainfall
events
Number of
landslide
events
Percent-
age
Zone
A
B
C
D
6
27
52
14
6
48
16
0
100
67
31
0
+
0
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May.
Jun.
FIGURE 9.94
Comparison of landslide events and rainfall cycle coefficient
C
c
for coastal mountains of Brazil. (From
Guidicini, G. and Iwasa, O.Y.,
Bull. Int. Assoc. Eng. Geol.,
16, 13-20, 1977. With permission.)
