Page 148 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 129
the traditional approach of using a single rainfall intensity (threshold value) as a
landslide warning signal could be potentially misleading. In other words, a slope
can be perfectly stable if the initial main water table is low, even if it is subjected
to extremely intense rainfall.
Antecedent rainfalls have significant influence on the stability of the slope.
The degree of influence depends on their duration. The factor of safety decreases
as the duration of rainfall increases, until a critical duration is reached. For the
given geological and hydrological conditions of the cut slope, the critical
duration has been found to lie between 3 and 7 days. At this critical duration, the
factor of safety is the lowest. More importantly, antecedent rainfalls with
duration less than or equal to the critical duration adversely affect the stability of
the cut slope during a subsequent heavy rainstorm. On the other hand, antecedent
rainfalls with duration longer than the critical duration do not seem to affect the
stability of the slope during a subsequent heavy rainstorm.
For rainfalls lasting longer than the critical duration, the factor of safety gently
increases due to the reduction of average rainfall intensity over the entire rainfall
duration considered. The concept of the existence of a critical duration is
consistent with field measurements of groundwater response to rainfall at the
Mid-levels. High-intensity rainfall can be a triggering factor for landslides but
there are other factors such as antecedent rainfall duration, which are also
important and contributed to the occurrence of landslides.
The factor of safety of the slope decreases with water permeability, but the
rate of reduction in the factor of safety is relatively small for soils with high
permeability, with respect to the rainfall intensity. However, the factor of safety
drops sharply when the water permeability has a comparative magnitude to the
rainfall intensity and this factor is extremely sensitive to the anisotropic
permeability ratio (k /k ). The higher the ratio, the lower the factor of safety
y
x
becomes.
Acknowledgements

Reprinted from Computer and Geotechnics, Vol. 22, C.W.W.Ng and Q.Shi, A
numerical investigation of the stability of unsaturated soil slopes subjected to
transient seepage, pages 1–28, 1998, with permission from Elsevier Science.
This research was conducted under a grant awarded to the first author by the
RGC Direction Allocation Grant at the Hong Kong University of Science and
Technology. The authors would like to thank Mr K.K.S.Ho, Dr Alex Li and Dr
H.W.Sun of the Geotechnical Engineering Office for their useful discussions.

References
1 Brand, E.W. (1984) Landslides in south Asia: a state-of-art report, Proceedings of
4th International Symposium on Landslides, Toronto, 1, 17–59.