片状岩類の物理的性質と岩石構造の関係について

Specimens of schistose rocks such as crystalline schists, schistose green-rocks, and slightly schistose Palaeozoic rocks were collected along the Dosan Line of JNR across the Sambagawa and the Chichibu belts of Central Shikoku. The elastic wave velocity (Vp), the Young's modulus, the compressive strength, and the tensional strength were measured on each specimen in the directions of fabric axes a, b, and c, respectively. The specimens are classified into the following four types after the characteristics of the rock structure: the type A having plane surface of schistosity, the type B characterized by the development of distinct transversal cleavage, the type C having folded surface of schistosity accompanied with distinct cleavage, and the type D of massive rocks. The change of the elastic wave velocity in different fabric directions is shown in the diagram of fig. 2, in which a/b represents the ratio of Vp in the direction of a to that of b. Rocks of different structural types are marked with open circles (the type A), solid circles (the types B and C), and crosses (the type D). In rocks of the type A no essential difference is shown between a and b, while Vp in the direction of c, namely, per-pendicular to the schistosity plane, is distinctly lowered, that suggesting the effect of lowering the wave velocity of the schistosity. On the other hand, in rocks of the types B and C, both characterized by the presence of distinct transversal cleavage across the schistosity surface, Vp in the direction of a is lowered, that suggesting the effect of the cleavage. The similar relation is shown for the Young's modulus in fig. 3. The influence of the cleavage to the mechanical behaviour of rocks is also shown in figs. 4 (the compressive strength) and 5 (the tensional strength): the strength is distinctly lowered in the rocks of the types B and C, when the axial stress is exerted in the direction of c. The author stresses the importance to recognize the cleavage in schistose rocks in the work of engineering geology.