三軸変形下の岩石中の微小割れ目の発達について 序報

Abstract

The development of microcracks in the deformation of rocks was studied with microscopic method.A coarse sandstone was deformed under the confining pressure of 500kg/cm2, 1000kg/cm2 and 1500kg/cm2 at a strain rate 2.7×10-6/sec. The locality of the sandstone is Oshima in Sakito-Matsushima coal field, northwestern Kyushu, Japan. The age is oligocene. Most of the phenocrysts are quartz and the matrix contains a lot of calcareous materials.The mode of deformation is brittle under 500kg/cm2 and transitional under 1500kg/cm2. The experimentation was stopped at several stages of deformation as shown in fig. 4. The deformed specimens were cut into thin sections parallel to the axis of cylinder and perpendicular to the fractures. When the microcracks have opening, they are dark under crossed nicols and clear under uncrossed nicols.Microcracks appear at very early stages. Most of microcracks have zigzag patterns. Average length of the straight parts of this zigzag lines ranges from 0.01 to 0.03mm. The density of microcracks increases approximately in proportion to strain. It's opening begins to spread at about the yield point. The preferred orientation of microcracks are visible in early stages. They concentrate to the direction parallel into the maximum compression and two conjugate directions which have an angle of 30°-40° with it. However under the confining pressure of 500kg/cm2, they concentrate relatively more to the direction parallel to the maximum compression than under 1500kg/cm2. Microcracks concentrate in some particular bands at early stages. The band is called “deformation band” in this paper. The direction of deformation band has an angle of 45° or a little less degrees with the direction of maximum compression. The deformation bands grow at later stages. The occurrence of macroscopic cracks are associated with the distribution of deformation bands.