P14

Infiltration Analysis of Unsaturated Soil Slopes〇Yeshey CHODEN, Sanchitha JAYAKODY, Ryosuke UZUOKA, Kyohei UEDA

1.

Introduction

and is a fractional parameter between 0 and 1. As theLandslides are often catastrophic, posing significant
threats

tocommunities,

rainfall infiltrates the soil layers, it brings about awithrainfall

inducedsignificant rise in m parameter which indicates soil

themostprevalent.

saturation. However, the intricate processes causing

Researchers often study the intricate relationshipwater loss are simplified into a negative exponential

between rainfall infiltration and slope failures

function governed by KT which represents the global

considering a wide range of crucial factors. Moreover,drainage capability of the slope. The representative

early warning systems heavily rely on rainfall as it isequation describing infiltration in this original model isimpractical to incorporate many other boundary

given by Eq. 1.

occurrences being

among

conditions such as heterogeneous soil profiles andsubterraneous flows if slopes are not instrumented and𝑚(𝑡) = 𝛴𝑖 ⅇ

sin 𝛽.(𝑡−𝑡0𝑖 )𝑛⋅𝐿⋅(1−𝑆𝑟 )−𝑘𝑇 ⋅⋅ℎ(𝑡0𝑖 )𝑛⋅𝐻⋅(1−𝑆𝑟 )(1)monitored properly. Current methods utilize theintegration of GIS-based susceptibility mapping, asatellite-based precipitation monitoring and forecasting
system,

along

with

rainfall-induced

landslideprediction model. The most crucial determinant in thisβcontext is the soil's infiltration capacity. As soil
saturation leads to a loss of shear strength andsubsequent failure, an accurate prediction of early
warning relies heavily on assessing this criticalparameter. Therefore, this study aims to review existingmodels on rainfall infiltration in unsaturated soil slopes
and validate by comparing the analytical solution with

Valentino and Montrasio, 2008 developed amathematical model that translates the physicalphenomenon of rainfall triggering process through asimplified water-flow model. It directly relates thesafety factor of a slope with rainfall amountparameter

introduced the SLIDE model whereby Eq. 1 wasrevised and rewritten differentially to better evaluate

Eq. 2. Konagai (2022) further modified the SLIDE

2. Review of Existing Infiltration ModelstheIn line with the preceding study, Liao et al. (2010)

the effects of single rainfall discrepantly as shown in

the experimental result.

incorporating(Liao et al., 2010)
Fig. 1. Schematic illustration describing the phenomenon.

mthat

describesinfiltration. m represents the dimensionless thickness ofthe saturated part of the soil layer as depicted in Fig. 1

model (which will be referred to as the modified SLIDEmodel) by incorporating the variation of mt with respect
to the slope length and flow divergence, as depicted inEq. 3. The KT parameter that signifies global drainage
capability of the slope lacked clear definition inprevious literatures. However, Eq. 3 specificallyconsiders Kt as the hydraulic conductivity of the soil,
thereby simplifying its application in the analysis.

experimental and analytical results is shown below.
1

Experimental: P3
Experimental: P4
Experimental: P5
Experimental: P6
Analytical: LE
Analytical: P3
Analytical: P4
Analytical: P5
Analytical: P6
Analytical: UE

0.8

0.6

(3)

m(2)

0.4

0.2

3.

Comparison of Infiltration Models
0The comparison between the original model and the050100150200-0.2t (sec)modified SLIDE model was studied using a set ofFig. 3. Comparison of analytical and experimental values

parameters for the analysis to generate m parameter.
The comparison result of m vs. t is depicted in Fig. 2.
There is an exponential growth in the soil saturation
1

Original Model
0.8

Lower slope end
Middle0.6

Modified SLIDE modelm

Upper slope end
0.4

Fig. 4. m vs. t plot for analytical and experimental data

0.25.

Result and Conclusion

001020304050t (hr)Fig. 2. m vs. t plot for original and modified SLIDE models

as the rainfall infiltration progresses in the originalmodel with a peak value of m = 1. This indicates that
there is a complete soil saturation in 48 hours, which isimpractical and could mean that the model does notadequately capture the outflow. On the other hand, themodified SLIDE model observes a peak value of m =0.62 at the lower end of the slope, which is well within
the bounds. It indicates that 62% of the unsaturated soil
layer gets saturated as a result of rainfall infiltration.
Moreover, the saturation is not uniform within the slope
length and exhibits lower levels at the upper end of theslope.
4.

Validation with experimental result
The modified SLIDE model (analytical model),given its better representation of the infiltration
phenomenon was used for validation against theexperimental results. Data of centrifuge experiments(Jayakody, 2023) conducted on unsaturated soil slopes
subjected to rainfall application were used. The
comparison of m values for the corresponding porepressure transducers in the experimental setup for

Comparing m values for varying rainfall intensities
in the analytical model with the experimental resultshows discrepancies observed in the values. Thisimplies that m parameter in the analytical model ishighly sensitive to rainfall intensity and is important toconsider effective rainfall that infiltrates into the soil
layers resulting in soil saturation. Fig. 4 shows the m vs.
t plot between the two models. The analytical model
assumes immediate saturation upon rainfall infiltrationneglecting the infiltration time needed for soil
saturation, as demonstrated by the experimental results.
To address this limitation, incorporating a simple onedimensional infiltration model in the modified SLIDEmodel could effectively address this issue.
References
1) Montrasio, L., & Valentino, R. (2008). A model for
triggering
mechanisms
ofshallow
landslides. Natural Hazards and Earth SystemSciences, 8(5), 1149-1159.
2) Liao, Z., Hong, Y., Wang, J., Fukuoka, H., Sassa, K.,
Karnawati, D., & Fathani, F. (2010). Prototyping anexperimental early warning system for rainfallinduced landslides in Indonesia using satellite
remotesensingand
geospatial
datasets. Landslides, 7, 317-324.
3) Jayakody, S. H. S. (2023). Hydro-MechanicalAnalysis of Unsaturated Slope Subjected to Rainfall
and Groundwater Flow (Doctoral dissertation).
Kyoto University, Japan.