Microscopic Study of Rock for Estimating Long-Term Behavior

Ichikawa, Yasuaki*

Rock mass is a complex material including several classes of discontinuities and inhomogeneous/anisotropic minerals. If observing rock samples, we know that crystalline rock is a complex of minerals, grain boundaries and microcracks. Deformation and failure mechanism may be understood if we characterize correctly the microscale composition and grain boundary properties. However rock minerals, especially quartz and biotite of granite, involves a lot of microcracks. A microcrack propagation is initiated by these existing crack tips, and runs into feldspar domain beyond the grain boundary. This suggests difficulty of its theoretical treatment. On the other hand sandstone which is a sedimentary rock mainly consists quartz and feldspar crystals, and bonding materials fills the interbrain space, so the mechanical behaviors may be similar to the crystalline rock. In order to clarify a long-term behavior of the crystalline rock we here treated the following two subjects:(1)Observation of microcrack initiation and propagation by Confocal Laser Scanning Microscope(CLSM)under stress relaxation tests(before loading and at each loading stage),(2)Stress-dissolution analysis of rocks with silicate minerals and characterization of the time-dependent deformation/failure behaviors. First, CLSM was used to acquire clearly focused three-dimensional images of granite specimens, and observed the change of microscale structure including the mineral configuration under applying compression stress. Then though microcracks have ever thought to be initiated and propagated on intergranular boundaries, we understand through the CLSM observation that new microcracks are generated from the ends of pre-existing cracks which are distributed in quartz and biotite. Second, in order to simulate the experimental results which indicate that initiation and propagation of microcracks control the stress-relaxation phenomenon, we introduce a coupled analysis method of stress-dissolution phenomena.