Page 140 - Numerical Analysis and Modelling in Geomechanics

STABILITY OF UNSATURATED SOIL SLOPES 121

Figure 4.8 Relations between rainfall intensity and duration.
reported in the literature. Based on extensive field observations on failures,
Vaughan [31] reported that slope failures occurred on the third day of prolonged
heavy rainfall. Costa Nunes et al. [32] discussed how in slope failures in Rio de
Janeiro, landslides only developed after 18 days of continuous rainfall.
Based on the field data and the current numerical investigation, it is
anticipated that a critical duration is likely to be governed by the ratios Q/k and
m /k for given geometry and boundary conditions and rainfall patterns (or
w
characteristics), as shown in Figure 4.8.
The relative importance of rainfall duration and short-duration rainstorms with
high intensity on factor of safety is clearly illustrated in Figure 4.10. For rainfalls
with duration shorter than the critical duration, an antecedent rainfall results in a
larger fall in the factor of safety (≥ f ) during the subsequent 2-hr rainstorm. This
2
finding is contradictory to the conclusions drawn by Brand [1] and Premchitt et
al. [3]. It is clear from Figures 4.7, 4.9 and 4.10 that high intensity rainfall may
be a triggering factor for landslides in those slopes with a marginal factor of
safety. However, there are other factors such as duration which are also important
and contribute to the occurrence of landslides. Senanayaka et al. [33]
investigated slope failures in Sri Lanka and provided convincing evidence that the
chance of a slope failure is not only affected by intensity but it is also determined
by duration. The rule of thumb that they developed is “if more than 200 mm
rainfall had occurred on the hill slopes in a period of 3 days and if wet weather was
continuing, then the possibility of landslides looms ahead.”
A prolonged period of rainfall will certainly lead to an increase of water
permeability in unsaturated soils. From equation (4.8), it can be deduced that the