Abstract
There is an extensive distribution of organo-volcanic ash soils (Kuroboku soils) and a partial distribution of organic clayey soils (non-volcanic ash soils) in Japan. These soils have particular characteristics of physical and engineering properties.
In this paper, the authors have described the physical characteristics and problems regarding the measurements of these soils, and proposed several new measuring methods. However, we have described mainly the particular characteristics of the physical properties for organo-volcanic ash soils from the viewpoint of their relationship with organic content and changes due to the drying procedure in this area.
(1) Dry bulk density in the field is very small and it decreases with an increase in the organic content, but increases with air-drying.
(2) Natural water content is very high and it increases with an increase in the organic content.
(3) Specific gravity is small and it decreases with an increase in the organic content.
(4) Water retention is very large and it increases with organic content. But there is a large decrease in water retention after air-drying.
(5) Liquid and plastic limits have very large water contents, and these increase with organic content.
(6) Liquid and plastic limits vary with initial water content. In the first step, the values of the liquid and plastic limits are constant in spite of the initial water content of a soil sample, then those values decrease with a decrease in the initial water content. We defined the decreasing points of liquid and plastic limits as the first critical point and the second critical point of the initial water contents, respectively. The first critical point of the initial water content is always higher than the second critical point of the initial water content. Also, liquid limit decreases more sharply by air drying than in the case of the plastic limit.
(7) The first critical point of the initial water content is equivalent to about pF 4. 2, and it corresponds to the plastic limit and the turning point of the shrinkage curve for a fresh soil sample. Moreover, the plastic limit corresponds to the turning point of the shrinkage curve at any initial water content of a soil sample.
