Page 142 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 123

Figure 4.10 Factor of safety vs rainfall duration.

the main water table following a 2-hour rainstorm is shown in Figure 4.9 for
various antecedent rainfall duration, and the influence of the 2-hour rainstorm on
the factor of safety of the slope is illustrated in Figure 4.10. As expected, the 2-
hour rainstorm affects the factor of safety with and without an antecedent
rainfall. However, the degree of influence of the 2-hour rainstorm on the factor
of safety depends on the duration of antecedent rainfalls. For an antecedent
rainfall duration shorter than the critical duration (7 days), the drop in the factor
of safety (≥ f ) caused by the 2-hour rainstorm increases with the duration. On the
2
other hand, however, (≥ f ) becomes smaller when the duration of an antecedent
2
rainfall event is longer than the critical value. This suggests that the risk of a
landslide is highest during a short-duration rainstorm when the slope has been
subjected to an antecedent rainfall event of the critical duration. This is because
an antecedent rainfall event of critical duration not only causes an increase in
water permeability due to the advancement of the wetting front, but it also causes
a rise in the main water table. A subsequent 2-hour rainstorm of high intensity
leads to a sharp fall in the factor of safety and causes a major landslide,
especially for those slopes with a marginal factor of safety.

Influence of saturated water permeability
Making use of equations (4.2) and (4.3), Pradel and Raad [13] studied the effect
of permeability on surficial stability of homogenous slopes in Southern
California and concluded that soils with a low permeability such as clayey and
silty soils are more prone to slope failure than granular materials. For given