Jae-Hyeon Park

Damage process of intact granite under uniaxial compression: microscopic observations and contact stress analysis of grains

Understanding the damage mechanism in materials is one of the very important subjects in the science and engineering field. The microstructural change, microcracking, which causes material strength deterioration, is usually termed as damage. We observed micro-damage localization and propagation, in a coarse-grained granite specimen under uniaxial compressive stress to better understand the fundamental problems of the true damage process at a micro to macro scale. With the use of an experimental system, the continuous observation of the damage process also enabled us to clarify micro-damage in great detail. The results indicate that the mechanisms of micro-damage initiation in a granite specimen under uniaxial compressive stress may be separated into two cases; the first in which two grains, such as quartz and feld-spar, contact each other in the same direction as the axial stress, and the second in which a biotite grain inclined to the axial stress direction is contained within a feldspar grain. The damage is strongly localized for both cases and some shear zones are found in the specimens.

Changes of permeability characteristics dependent on damage process in granites

Fracture properties are closely related to the permeability characteristics of massive crystalline rocks. In order to understand the changes of rock permeability related to the damage process, this study performed characterization on changes of hydraulic properties with both in-situ tests and laboratory tests. The results of in-situ tests showed that permeability coefficients have differences on each section from the distance of tunnel wall. The differences are related to the conditions of fracture distribution due to rock damage in the process of tunneling. This study performed uniaxial compressive tests (UCT) and water injection tests in order to understand the relationship between the damage process and changes of permeability characteristics by the laboratory tests. Crack distribution on each damage grade was observed by the acetate peel. According to the results of the tests, the strongly damaged specimens with crack length density of more than 0.6 cm−1 showed higher values of permeability coefficients than the weakly damaged specimens. Moreover, the former specimens have persistent cracks and good connectivity throughout the rock specimens. It indicates that rock damage influences on the permeability characteristics in rocks.