ANALYSIS OF EFFECT OF RAINFALL PATTERNS ON PORE
WATER PRESSURES IN UNSATURATED SLOPES
C.C.W. Bolton*, Y.S. Tong*, T. Wang* and J.K. Kenny+
* The University of Mars, Mars
+
Western University, China
BRIEF INRODUCTION, METHODOLOGY AND KEY RESULTS
Actual Infiltration Rate (m/hr)
Many natural and man-made slopes are susceptible to failures. These failures are not only
of economic significance but also could be life threatening. Rainfall-induced landslides are
the most common natural disasters in Hong Kong. During rainstorms, infiltration of rainwater
reduces matric suction, i.e. negative pore water pressure, within an unsaturated soil slope.
This leads to a reduction in shear strength of soils. When the shear strength at the potential
failure slip is exceeded, slope failure would occur. Therefore, an improved understanding of
the variations of matric suction in the ground due to rainfall infiltration is essential for safer
slope designs. Although two-dimensional (2D) transient seepage analyses in unsaturated soil
slopes have been carried out by many researchers and engineers, the influence of rainfall
patterns has seldom been reported.
For more realistically simulating some three-dimensional (3D) aspects of groundwater
flows in the field, a 3D parametric numerical analysis has been carried out using
FEMWATER. A hill site at Lai Ping Road in Shatin, Hong Kong, was selected for numerical
experiments to investigate the influence of rainfall patterns on pore water pressure
distributions in an initially unsaturated ground condition. Based on some analyses of rainfall
patterns in Hong Kong, three typical rainfall patterns, namely, advanced type A, central C,
delayed type D, were identified. The modelled 24-h rainfall duration was divided into 12
intervals. Within each 2-h interval, the rainfall intensity was assumed to be constant.
Figure 1 shows the computed actual infiltration rate varying with time during the 24-hr
rainfall at the top (section XX) and at the toe (section YY) of an unsaturated cut slope. The
area under each curve represents the total amount of infiltration. Depending on the initial
pore water pressures in the ground, rainfall patterns can have different degrees of influence
on the groundwater responses. When the initial pore water pressures in the ground are high
(i.e. small suctions), the influence of
1.E-01
different rainfall patterns on the total
A_10yr(XX')
amount of rainfall infiltration is limited.
This is because the high water
C_10yr(XX')
1.E-02
permeability existed in the ground
D_10yr(XX')
facilitates intensive infiltration. The
variations of the actual infiltration rate
A_10yr(YY')
with time follow closely with the rainfall
1.E-03
patterns. By plotting changes in the ratio
C_10yr(YY')
of surface flux to instantaneous water
D_10yr(YY')
permeability (q/k) with time, the actual
1.E-04
ground water responses can be easily
0
4
8
12
16
20
24
Time(hour)
interpreted. When the initial pore water
Figure 1: Actual infiltration rate vs. Time at XX’ and
pressures in the ground are low (i.e. high
YY’ sections
suctions), the influence of different
rainfall patterns on the ground water
response is most significant. The advanced type A and the delayed type D lead to the highest
and the lowest rate of infiltration and total amount of infiltration, respectively. This implies
that former pattern will be the most critical one for slope instability.
CONCLUSIONS
1. A series of 3D transient analyses were carried out to investigate the influence of rainfall
patterns on pore water distributions in unsaturated soil slopes. Three typical rainfall
patterns in Hong Kong were modelled. Depending on the initial pore water pressures in
the ground, rainfall patterns can have different degrees of influence on the groundwater
responses. When the initial pore water pressures in the ground are high (i.e. small
suctions), the influence of different rainfall patterns on the total amount of rainfall
infiltration is limited. This is because the high water permeability existed in the ground
facilitates intensive infiltration. The variations of the actual infiltration rate with time
follow closely with the rainfall patterns. By plotting changes in the ratio of surface flux to
instantaneous water permeability (q/k) with time, the actual ground water responses can
be easily interpreted.
2. When the initial pore water pressures in the ground are low (i.e. high suctions), the
influence of different rainfall patterns on the ground water response is most significant.
The advanced type A and the delayed type D lead to the highest and the lowest rate of
infiltration and total amount of infiltration, respectively. This implies that former pattern
will be the most critical one for slope instability.
ACKNOWLEDGEMENTS
This research project is financially supported by the research grant XYZ provided by the
Research Grants Council of the Mars Government.
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