Soil tests and fluidization tests for the soils sampled from debris flows at the Gamahara River, at the Harihara River and at the Hachimantai Area were conducted to investigate characteristics of soils and to research the fluidity of the soils. Grain size distribution, saturated water content, liquid limit and plastic limit were measured by the soil tests. A small flume, 1.6m long, 10cm wide and 15 cm deep, with 2 different gradient sections was used for the fluidization tests. The gradient of the upper section of the flume was 45°, and lower section was chaged 10°, 15° and 20°. The soils were settled in the upper section, then the soils slid down in the upper section and flowed into the lower section. The lengths of flow-down in the lower section were measured. We judged fluidization occurred when the flow-down length in the lower section exceeded 110 cm. The water content when the fluidization occurred was measured and de-fined as “fluidity limit”.
Results obtained from the soil tests can be summarized as follows.
The order of A.M.I. of the soils sampled from the slopes where the debris flows generated is Hachimantai Area<Harihara River<Gamahara River. The order of percentage of fine material, saturated water content, liquid limit and plastic limit is same. The A.M.I, of the soils sampled from the slopes where the debris flows generated and from the deposits under the slopes ranged from 0.8 to 0.9, and those of debris flow deposits were about 1.0 in the Harihara River. These data indicate that the water contents of collapsed soils increased on the way of flow-down in the Hari-hara River.
Results obtained from the fluidization tests can be summarized as follows.
The order of the fluidity limit of soils sampled from the deposits of debris flows is Hachimantai Area<Harihara River<Gamahara River like A.M.I, and liquid limit. The fluidity limits of the soils contained coarse material were smaller than those of the soils without coarse material. The fluidity limits were larger than the liquid limits in any soils, and it suggests that liquid limit does not always indicate the threshold of fluidization of collapsed soils which change into debris flows.

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