SIMULATION OF A SHALLOW LANDSLIDE FROM OCCURRENCE TO MOVEMENT

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12^th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at

SIMULATION OF A SHALLOW LANDSLIDE

FROM OCCURRENCE TO MOVEMENT

Takahisa Mizuyama¹, Yoshiiku Musashi², Miho Yamasaki³ and Ken’ichiro Kosugi⁴

INTRODUCTION

Shallow landslides are a major cause of sediment-related hazards as well as debris flows in Japan.

Given that many shallow landslides occur with heavy rainfall every year causing damage and the loss of human lives, it is desirable to predict the occurrence of shallow landslides and their areas of influence. A two-dimensional model of rainfall infiltration was applied to a slope where a shallow landslide had actually occurred. The model well simulated the occurrence time. In addition, an extended distinct element method was applied to the landslide and well calculated the distance travelled.

A SLOPE WHERE SHALLOW LANDSLIDE OCCURRED Heavy rainfall caused a small shallow landslide in

Kyoto University’s experimental forest on June 29, 1999. As shown in Fig.1 the landslide had a horizontal length of 20 m and an average slope angle of 29 degrees. The upper part of the slope had a horizontal length of 55 m and an average angle of 19 degrees.

Fig.1 The slope of the studied landslide TWO-DIMENSIONAL SIMULATION OF A SHALLOW LANDSLIDE

A two-dimensional Richard’s infiltration equation was applied to the slope using the finite element method. On the basis of a survey with a penetration meter, the soil was divided into three layers. The surface layer was 47 cm deep. Soil water characteristics parameters were derived from the results of

Fig. 2 Longitudinal profile of the study slope

landslide Fig. 3 Change in slope stability with time the

study slope landslide

1 Prof. Takahisa Mizuyama, Corresponding author, Kyoto University, Graduate School of Agriculture, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: Mizuyama@kais.kyoto-u.ac.jp)

2 Yoshiiku Musashi, Yachiyo Engineering Co., Ltd, 18-12, Nishiochiai 2-chome, Shinjuku-ku, Tokyo 161-8575, Japan

3 Yamasaki Miho, Graduate student, Kyoto University, Graduate School of Agriculture, Oiwake-cho, Kitashirakawa, Sakyo- ku, Kyoto 606-8502, Japan

4 Ken’ichiro Kosugi, Prof. Kyoto University, Graduate School of Agriculture, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

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laboratory tests on soil sampled from the site. The total rainfall on the day of the landslide (from 5 am on June 29 to 8 am on June 30) was 198.5mm and the maximum hourly rainfall was 45.5 mm/hr.

Slope stability was calculated using a two-dimensional simplified Bishop method. The slip shape of the landslide was assumed and the change in the safety factor (Fs) with time was computed. The calculated result matched the actual occurrence time well as shown in Fig.3.

SIMULATION OF THE MOVEMENT OF THE LANDSLIDE MASS

The following equations were used in our extended distinct element method. (An explanation of the parameters is omitted here.)

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The forces between particles are expressed using the rebound spring and a pore-spring as shown in Fig. 4. The calculated result shows good agreement with the actual deformed shape of the landslide mass.(Fig.5).

k_n

h_n k_pn h_pn k_s

h_s

k_ps h_ps

Slider m=tanf

Normal Direction Tangential Direction

Pore-spring and dashpot transmits compressive and tensile force Element

i Element

i

Element

j Element

j Pore-spring gets to be

broken when the distance of elements or shearing force excess the limit .

Fig. 4 Schematic diagram of the extended Fig. 5 Calculated result for the deformed shape of distinct element method (EDEM) the landslide mass.

REFERENCES

Kosugi K. (1999). General model for unsaturated hydraulic conductivity for soils with lognormal pore size distribution, Soil Sci. Am. J. 63: 270-277

Musashi Y., Mizuyama T., Yamasaki M., (2009). Study on collapsed soil movement by flume experiments and application of distinct element method corresponding to fluidized conditions, Asia Pacific Symposium on New Technologies for Prediction and Mitigation of Sediment Disasters, JSECE Pub. 55: 66-67

Keywords: shallow landslide, simulation, two-dimensional infiltration model, slope stability, modified distinct element method

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