Gyo-Cheol Jeong

Microscopic observation and contact stress analysis of granite under compression

Abstract To study the damage process of microscale to macroscale in coarse-grained granite specimen under uniaxial compressive stress, we have observed micro-damage localization and propagation by using a newly developed experimental system that allows us to observe the damaging process continuously. The results showed that pre-existing microcracks lead to macroscopic shear fracture through the damage development process. The mechanism of micro-damage initiation in a granite specimen under uniaxial compressive stress may be considered for two cases. One is that two grains such as quartz and feldspar contact each other in the same direction as the axial stress, and the other is that a biotite grain inclined to the axial stress direction is surrounded by feldspar grains. The homogenization theory was applied to verify numerically the micromechanics of stress-induced damage in the mineral contacts. Local stress distribution in the periodic-micro structure was also calculated by the homogenization theory. It is shown that this analysis, which takes into account the initial state of the specimen, is well adapted to the behavior of two grains for which microcracking is the fundamental mechanism of damage.

Numerical analysis on micro-damage in bisphere model of granitic rock

Microcracking in contact with constituent minerals plays an important role in the nonlinear deformation process and micro-damage, leading to shear fracture of brittle materials such as granite. For damage propagation, the surface energy stored in the grain as a result of applied stresses thus tends to be relieved on the plane firstly, and pre-existing intracrystalline microcracks perpendicular to axial stress direction are closed. On the other hand, those parallel or subparallel to the axial stress direction are open. These stress-induced microcracks are predominantly tensile in nature, and the directions are parallel or subparallel to the axial stress direction. Numerical analysis provides some basis for a micromechanics of stress-induced micro-damage occurring at the major constituent mineral contact portions in feldspar. A homogenization theory was applied to analyze the damage development for the vicinity of pre-existing microcrack and stress-induced microcrack at grain contact. Numerical analysis with the bisphere model for damage development at micro-damage portions was discussed, assuming that feldspar grains surround quartz grain.

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.

Effects of Rainfall Intensity, Soil Slope, and Geology on Soil Erosion in Korea

Extended Abstract In recent decades, there has been a growing concern over the soil erosion in Korea which is mostly induced by extreme rainfall during a rainy summer season. Soil erosion is partly associated with landslide or failures of slope where thick soil profile develops. The supply of fine particles to dams and rivers induced by soil erosion causes a critical turbidity composed of suspended matters over a long period. It was shown that landslides induced by debris flow in Korea are commonly related to the geotechnical properties and clay mineralogy of precursor soils developed on bedrock [1]. This study aims to elucidate the importance of geological characteristics, soil slope, and rainfall intensity in relation to soil erosion. The analytical results by XRD show that primary clay minerals contained in the slopes where intensive soil erosion takes place are illite, chlorite, kaolinite, and montmorillonite. It is evident that erosion rates were proportional to rainfall and clay contents. Because illite is the most abundant clay in soil, it serves as one of the most effective factors that induce intensive soil erosion. According to monitoring on field sites, the rates of soil erosion increased with increasing slope angles and precipitation rates. Soil erosion was initially controlled by rill development and original topography. The runoff amounts discharged from soil increased turbidity and clay contents removed, especially high in metamorphic rocks. This study demonstrated that it is possible to predict a critical depth and the change in the safety factors of a slope subject to rain infiltration and that a rainfall-induced landslide largely depends on clay mineralogy in the soil and slope parameters affecting soil erosion. Environmental management for soil erosion, especially landslide related to significant soil loss, needs systematic observations and monitoring on different lithologies at varying rates of outflow, which can be supported by understanding mechanical properties and clay mineralogy of soils at slopes.

A Study on Improving Installation Guideline of Facilities to Protect Groundwater Contamination: Applications of Packer Grouting to Contaminated Wells

Because the present groundwater law broadly regulates a simple and impractical guideline ignoring aquifer characters and geology, general purpose facilities for protection of groundwater contamination is still considered unsatisfactory to ensure groundwater resources. In recent, there have been growing attempts in the packer development as crucial techniques and devices for groundwater protection. This study investigated the application of packer grouting techniques to contaminated groundwaters of two well sites in the Andong and Yeongi areas, both of which revealed a satisfactory effect with improved water quality: 94% decrease in turbidity at the Andong area and 60% decrease in -N, respectively. Based on aquifer characters including geology, weathering depth, fracture pattern, hydraulic gradient, and the flow path of contaminants, the integrated properties of groundwater contamination should be evaluated and treated with the help of accurate analyses such as bore hole imaging and monitoring data. Packer grouting and casing on well to ensure the useful aquifer free of contaminant are expected to play important role in inhibiting the inflow of contaminants when adequately applied. Therefore it is concluded that these can serve as reliable tools in remediation and protection of contaminated groundwater as well as efficient utilization of groundwater.

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.