Unsaturated Mechanical Properties of Shirasu

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Shirasu is the pyroclastic flow deposit distributed in the southern part of Kyushu. Shirasu sites are prone to natural disasters, like slope failure and debris flow, because Shirasu has a smaller density of soil particles and a keen erosivity. The above natural disasters occur with increasing the degree of saturation by rainfall. It is therefore necessary to investigate the mechanical properties of Shirasu on the process from dry to wet condition. In order to examine the effect of a change in saturation on the compressibility and shear strength properties, three types of tests were carried out on compacted Shirasu. The following results were obtained.1. From the results of one dimensional compression tests, the soil settles due to soaking during the test. It is the phenomenon called collapse. The compression curves of compacted soils fall to the nearby void ratio of the initially soaked soil compressed under same stress. The Incidence of collapse is the highest when the compression stress is slightly higher than the compression yield stress.2. From the results of direct shear tests, the shear strength of compacted Shirasu decreases to that of the initially soaked soil due to soaking during the process on shearing. The incidence of collapse increases while density or normal stress of the specimen decreases. The degree of saturation affects the cohesion of strength parameters, but it is not related to the internal friction angle.3. From the results of unsaturated triaxial compression tests, suction and confining stress, respectively, affect a change in water content and density.