Environmental Engineering Reference
In-Depth Information
FIGURE 9.93
Total collapse of slope shown in
Figure 9.91
left a high head scarp and temporarily closed the roadway. Failure
occurred 6 weeks later during the rainy season after several days of heavy rains.
would be inadequate. Considering the slope activity, weather conditions, costs, and con-
struction time, a retaining structure was considered as not practical.
Case 2:
Another situation is similar to that described in Case 1 although the volume of
the failing mass is greater. A 25° slope of residual soil has been moving for 20 years since
it was activated by a road cut (Garga and DeCampos, 1977). Each year during the rainy
season movement occurs at a velocity measured by slope inclinometer ranging from
0.4-2.2 cm/day (0.2-0.9 in./day). The movement causes slide debris to enter the roadway,
from which it is removed. During the dry season movement ceases, and as of the time of
the report (1977), total failure had not occurred.
9.3.4
Weather Factors
Correlations between Rainfall and Slope Failures
Significance
Ground saturation and rainfall are the major factors in slope failures and influence their
incidence, form, and magnitude. Evaluating rainfall data are very important for anticipat-
ing and predicting slope failures. Three aspects are important:
1.
Climatic cycles over a period of years, i.e., high annual precipitation vs. low
annual precipitation
2.
Rainfall accumulation in a given year in relationship to normal accumulation
3.
Intensities of given storms
Cumulative Precipitation vs. Mean Annual Precipitation
A study of the occurrence of landslides relative to the cumulative precipitation record up
to the date of failure as a percentage of the mean annual precipitation (termed the cycle
coefficient
C
c
) was made by Guidicini and Iwasa (1977). The study covered nine areas of
the mountainous coastal region of Brazil, which has a tropical climate characterized by a
wet season from January through March and a dry season, June through August.
