D13

Physical modeling of rainfall in the geotechnical centrifuge〇Jiawei XU, Ryosuke UZUOKA, Kyohei UEDA

Introduction
Due to rain infiltration into soil, both the increase in
the positive water pressure and the decrease in thematric suction cause the reduction in the shear
strength of soil and finally lead to slope failure(Rahardjo et al., 2007). Influenced by the intensity andduration of the rainfall, slopes show various
deformation patterns (shallow failure or deep-seated
failure, debris flows, and minor soil movement). InFig. 2 Centrifuge model test geometry and instruments

this study, physical modeling of rainfall-induced slope

Results

failure was carried out in one geotechnical centrifuge

The recorded progressive failure of the slope andexperiment where a rainfall simulator was applied to

the final slope shape were shown in Figs. 3 and 4.

provide rainfall above a small-scale slope model in theDisplacement in each section (Fig. 5) was obtained

hyper-gravitational field.

through image analyses and the results were in Fig. 6.

Inflight rainfall simulator

Pore pressure pressures in the slope were in Fig. 7.

Rainfall simulator (in Fig. 1), which was made with
18 nozzles, was mounted on top of the container. Air
pneumatic nozzles were used where air was to break
the water into smaller droplets and water supply was(a) 0s(b) 53.9s(c) 60.6s(d) 67.0s(e) 80.7s(f) 113.1s

to provide the nozzles with water. A remote control
was used to control the starting and ending of rainfall
could be realized inflight.

Fig. 1 Rainfall simulator
Test program

Fig. 3 Inflight slope photos (time in the model scale)

One test where an infinite slope (the length of slope

The slope failure was characterized by two flow

is as long as possible) was subject to rainfall (with anslide. Displacement in A1 displaced the largest value

intensity of 5 mm/h in the prototype scale) provided

both in the first and second slide, which were 15.3

by the inflight rainfall simulator. The centrifuge modelmm and 12.3 mm. As rainfall continued, soil below

test geometry and instrument were shown in Fig. 2.

the slide plane didn’t mobilize, instead, soil above
A-A section including soil between B-B section andE-E section started moving and the second surface

slope surface, however, showed quite small growth.

failure occurred and ended at 113.1 s. The maximum
traveling velocities of soil in A-A section in the firstand second slide were 34.7 mm/s and 20.6 mm/s.

Fig. 4 Final soil displacement (model scale)Fig. 7 Pore water pressures in the slope (model scale)Fig. 5 Sections selected for displacement analyses

Conclusions
Surface failure occurred above the phreatic line andthe slide was mobilized along a plane 2 cm at mostbelow the slope surface. The first slide was initiated at
the lower part of the slopes and travelled at fast speed,
resulting in a following second slide away from thecollapsed soil took place. The slope failure took place
during the transient flow state when pore waterpressures within the soil were still increasing. Slopestability depends on the rainfall intensity and a more
intense rainfall is needed to cause large landslides.
Acknowledgments
The authors gratefully acknowledge the funding
provided by the joint research (2019-K-05) between
Earthquake Research Institute, the University ofTokyo and Disaster Prevention Research Institute,
Kyoto University.

Fig. 6 Displacement in various sections (model scale)Considerately large displacement was mainly in thelower part of the slope where rainfall was evenly
distributed to the slope surface and pore waterpressures increased to high levels when the steadystate flow was achieved. Pore water pressures near theReference
Rahardjo, H., Ong, T. H., Rezaur, R. B., and Leong, E.
C. Factors Controlling Instability of Homogeneous
Soil Slopes under Rainfalls. Journal of Geotechnicaland Geoenvironmental Engineering, 2007, 133 (12),1532-1543.