Predict Failure of Shirasu Slope in Consideration of Patterns of Rainfall and Unsaturated Seepage

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Shiras is a pumic-flow deposit soil widely distributed in southern Kyushu and oilier districts in Japan, and is counted to be a typical collapsible soil for its frequent damages mainly owning to rainfall and earthquakes. Such a soil seems to be similar to the yellow-brown pumice soil in North Island, New Zealand, the cemented-sand in the U. S. A., and the volcanic ash soil in Central America. The strength characteristics of the undisturbed Shirasu are different from those of the disturbed Shirasu, since undisturbed samples have tensile strengh due to a geological cause.It is well known that Shirasu becomes very collapsible after increasing the moisture content. Many studies on failure of slope by rainfall have been done by many laboratories. The failure mechanism of Shirasu has not been clarified yet, because the mechanisum of it by rainfall is very complicated.The conclusion drawn were as follows ;(1) The patern of rainfall in Southern Kyushu was become clear.(2) This study make it clear about the relation between unsaturated coefficent of permeability, water content by volume and suction of Shirasu.(3) The triaxial test results make clear characteristics of soil mechanics and liquefaction in Shirasu.(4) This paper refers to characteristics of unsaturated seepage of Shirasu slopes in raifall.