ABSTRACT:

In the southern part of Kyushu Island, Japan, slope failures often occur due to heavy rainfalls in the rainy season (from June to September). Most of these slope failures are classified into the surface slip failure whose depth is commonly less than 1 m. The phase change in pore fluid in unsaturated soil with the change in temperature, humidity and pressure in atmosphere is investigated to make clear the mechanism of surface slip failure because the phase change is deeply related to the change in effective stress in unsaturated soil near the surface. A numerical model is proposed to simulate the mechanical behavior of unsaturated soil. The results of numerical experiments are compared with the data obtained by the field measuring of suction and rainfall to test the validity of the proposed model.

INTRODUCTION

The non-welded part of pyroclastic flow deposits which is locally called Shirasu, are widely distributed in the southern part of Kyushu Island, Japan. The slope failures have been often caused on the Shirasu slopes by heavy rainfall. Most of these slope failures are classified as the surface slip failures whose depths are commonly less than 1 m. The qualitative mechanism of surface slip failure is well known, the equilibrium of force and moment is lost due to the infiltration of heavy rainfall. The stable Shirasu slope has experienced the cycle of sunny and rainy days, change in temperature and atmospheric pressure, which means that the effective stress in unsaturated soil repeatedly changes. Such a process must be quantitatively clarified to analyze the mechanism of surface slip failure on the Shirasu slope. Therefore it is necessary to investigate the heat transfer and infiltration of rainwater in unsaturated soil. In this paper a numerical model is proposed to simulate the mechanical behavior of unsaturated soil.

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