Page 138 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 119
rainfall intensity (e.g., 70 mm/hr adopted in Hong Kong) as a landslide warning
signal could be potentially very misleading. In other words, a slope can be perfectly
stable if the initial main water table is low, even if subjected to extremely high
rainfall intensity.

Influence of rainfall duration

The relevance of long periods of rainfall to slope stability has attracted
considerable attention and debate over the years. Based on empirical correlation
found between rainfall and landslide data, researchers such as Peck [30] and
Lumb [8] have suggested the significance of 10-day and 15-day antecedent
rainfall on slope stability respectively. By making use of automatic rain gauges
to obtain better and more data of rainfall distribution and intensity, Brand [1] and
Premchitt et al. [3] concluded that the majority of landslides are induced by
localised short duration rainfall events of high intensity. Antecedent rainfall is not
of major significance. However, all these traditional correlations between rainfall
data and slope failure events ignore local geological and hydrological conditions.
To investigate the effects of rainfall duration on slope stability, 1 in 10-year
rainfall records collected by the Hong Kong Royal Observatory between 1980
and 1990 (Lam and Leung [23]) were adopted for parametric studies. The
rainfall events considered are shown in Figure 4.8. In the parametric analyses, it
is assumed that the rate of infiltration is equal to the rainfall intensity. The
hydraulic heads at HI and FG (see Figure 4.4) were specified at 62 mPD and 6 mPD
respectively, and other relevant mean values from Table 4.1 were adopted for the
parametric analyses. At the end of each prolonged rainfall, a 2-hour rainstorm of
high intensity (74 mm/hr) is also included in each analysis to investigate the
influence of antecedent rainfall on the subsequent performance of the slope
subjected to the intensive 2-hour rainstorm.
Figure 4.9 shows the locations of the main water table between sections JJ and
KK of the natural slope under various rainfall conditions. It can be seen from
Figure 4.9a that the intensive 2-hr rainstorm cause a significant rise of the main
water table at the toe of the cut slope (near to Section A-A). This can probably
explain why many slopes with a marginal factor of safety fail after an intensive
rainstorm. For slopes with a higher factor of safety, a substantial amount of rain-
water may be needed to cause landslides.
Figures 4.9b and 4.9c show a typical groundwater profile after 7-day and 15-
day prolonged rainfall events with an average intensity of 82 mm/day and 46 mm/
day respectively. Because of limited space, only some computed results are
included. The main groundwater table rises globally. By comparing with
Figure 4.9a, the influence of the 2-hr intensive rainstorm on the level of the main
water table is more significant after the 7-day and 15-day prolonged rainfall events
in terms of the amount and extent of the rise of the main water table. This is
probably due to the increase of water permeability (hydraulic conductivity) as a