1975 年 16 巻 2 号 p. 82-89
Shirasu distributed in southern Kyushu, Japan, is the great deposits caused by volcanic activities of Aira and Ata calderas. Shirasu has many problems to settle from the standpoint of engineering practice and prevention of disaster, because it is structurally unstable. The main purpose herein is to present a method of the identification and classification of Shirasu from the standpoint of soil engineering with its geological aspects in consideration.
Shirasu deposit has the hardness ranging from soft soil to weakly welded tuff, and its hardness may be used to as a criterion of the identification and classification for engineering purposes. It is proposed in the research that the hardness of Shirasu is given by the index hardness of its matrix, and is measured by applying Yamanaka's soil hardness tester with the spring pressure of 8.0 kg. Here the author investigated the hardness of Shirasu matrix measured at 145 locations in southern Kyushu and classified Shirasu deposits into five distinct groups as soft decomposed Shirasu, decomposed one, proper one, hard one, and welded tuff. The index hardness of soft decomposed Shirasu is lower than 20 mm. Decomposed Shirasu has the index hardness ranging from 20 to 26 mm, proper Shirasu has 26 to 31 mm, and hard Shirasu has 31 to 35 mm. Welded tuff is higher in the index hardness than 35 mm. The dry density of soft decomposed Shirasu is smaller than 0.80 t/m3, that of decomposed Shirasu is in the range of 0.80 to 1.10 t/m3, proper Shirasu is in 1.00 to 1.25 t/m3, hard Shirasu is in 1.15 to 1.35 t/m3, and welded tuff is larger than 1.35 t/m3. The propriety of the method proposed as the identification and classification of Shirasu for engineering purpose is ascertained statistically by means of x2, F, and t tests. There also is the correlation among hardness, dry bulk density, and moisture content. The hardness increases with an increase in the dry bulk density of Shirasu. Finally the chart of identification and classification of Shirasu is established on the basis of the relationship between index hardness and dry bulk density.