Byung-Gon Chae

Suggestion of a method for landslide early warning using the change in the volumetric water content gradient due to rainfall infiltration

An early warning system can be an effective measure to reduce the damage caused by landslides by facilitating the timely evacuation of residents from a landslide-prone area. Early detection of landslide triggering across a broad range of natural terrain types can be accomplished by monitoring rainfall and the physical property changes of soils in real time or near-real time. This study involved the installation of a real-time monitoring system to observe physical property changes in soils in a valley during rainfall events. This monitoring included the measurement of volumetric water content, which was compared with the results of laboratory flume tests to identify landslide indicators in the soils. The response of volumetric water content to rainfall events is more immediate than that of pore-water pressure, and volumetric water content retains its maximum value for some time before slope failure. Therefore, an alternative method for landslide monitoring can be based on the observation of volumetric water content and its changes over time at shallow soil depths. Although no landslide occurred, the field monitoring results showed a directly proportional relationship between the effective cumulative rainfall and the gradient of volumetric water content per unit time (t/tmax). This preliminary study thus related slope failure to the volumetric water content gradient as a function of rainfall. Laboratory results showed that a high amount of rainfall and a high gradient of volumetric water content could induce slope failure. Based on these results, it is possible to suggest a threshold value of the volumetric water content gradient demarcating the conditions for slope stability and slope failure. This threshold can thus serve as the basis of an early warning system for landslides considering both rainfall and soil properties.

Landslide prediction, monitoring and early warning: a concise review of state-of-the-art

Landslide is one of the repeated geological hazards during rainy season, which causes fatalities, damage to property and economic losses in Korea. Landslides are responsible for at least 17% of all fatalities from natural hazards worldwide, and nearly 25% of annual casualties caused by natural hazards in Korea. Due to global climate change, the frequency of landslide occurrence has been increased and subsequently, the losses and damages associated with landslides also have been increased. Therefore, accurate prediction of landslide occurrence, and monitoring and early warning for ground movements are very important tasks to reduce the damages and losses caused by landslides. Various studies on landslide prediction and reduction in landslide damage have been performed and consequently, much of the recent progress has been in these areas. In particular, the application of information and geospatial technologies such as remote sensing and geographic information systems (GIS) has greatly contributed to landslide hazard assessment studies over recent years. In this paper, the recent advances and the state-of-the-art in the essential components of the landslide hazard assessment, such as landslide susceptibility analysis, runout modeling, landslide monitoring and early warning, were reviewed. Especially, this paper focused on the evaluation of the landslide susceptibility using probabilistic approach and physically based method, runout evaluation using volume based model and dynamic model, in situ ground based monitoring techniques, remote sensing techniques for landslide monitoring, and landslide early warning using rainfall and physical thresholds.

Suggestion of a Landslide Early Warning Method Using a Gradient of Volumetric Water Content

This study involved the installation of a real-time monitoring system to observe physical property changes in soils in a slope during rainfall events. This monitoring included the measurement of volumetric water content, which was compared with the results of laboratory flume tests to identify landslide indicators in the soils. The response of volumetric water content to rainfall events is more immediate than that of pore-water pressure, and volumetric water content retains its maximum value for some time before a slope failure. Therefore, an alternative method for landslide monitoring can be based on the observation of volumetric water content and its changes over a time at shallow soil depths. Although no landslide occurred, the field monitoring results showed a directly proportional relationship between the effective cumulative rainfall and the gradient of volumetric water content per unit time (t/t max ). This preliminary study thus related slope failure to the volumetric water content gradient as a function of rainfall. Laboratory results showed that a high amount of rainfall and a high gradient of volumetric water content can induce slope failure. Based on these results, it is possible to suggest a threshold value of the volumetric water content gradient demarcating the conditions for slope stability and slope failure. This threshold can thus serve as the basis of an early warning system for landslides considering both rainfall and soil properties.

Analysis of Rainfall Infiltration Velocity for Unsaturated Soils by an Unsaturated Soil Column Test : Comparison of Weathered Gneiss Soil and Weathered Granite Soil

The unsaturated soil column tests were carried out for weathered gneiss soil and weathered granite soil in order to obtain the relationship between rainfall intensity and infiltration velocity of rainfall on the basis of different unit weight conditions of soil. In this study, volumetric water content and pore water pressure were measured using TDR sensors and tensiometers at constant time interval. For the column test, three different unit weights were used as in-situ condition, loose condition and dense condition, and rainfall intensities were selected as 20 mm/h and 50 mm/h. In 20 mm/h rainfall intensity condition, average rainfall infiltration velocities for both gneiss and weathered granite soils were obtained as cm/s ~ cm/s for different unit weight values and cm/s ~ cm/s, respectively. In 50 mm/h rainfall intensity condition, rainfall infiltration velocities were obtained as cm/s ~ cm/s and cm/s ~ cm/s respectively. The test results showed that the higher rainfall intensity and the lower unit weight of soil, the faster average infiltration velocity. In addition, the weathered granite soils had faster rainfall infiltration velocities than those of the weathered gneiss soils except for the looser unit weight conditions. This is due to the fact that the weathered granite soil had more homogeneous particle size, smaller unit weight condition and larger porosity.

Assessment of the alteration and acid-generating potential of deep boreholes for geological disposal

An integrated study, including mineralogy, geochemistry, lead isotopic composition, acid–base accounting, and factor analysis, was conducted on the rock core samples from two deep boreholes (AH1 and BH3) to assess the alteration and acid-generating potential of two representative lithologies of Korea (granite and geniss) for geological disposal. The alteration condition represents water circulation features in crystalline basement rocks, which is one of the key points to be assessed in repository sites. In addition, this study proposes that the acid-producing potential and acid-consuming minerals in the host rock are important for the long-term safety of a radioactive waste repository, given that sulfide oxidation may occur in artificial fractures created during construction. Fe3+ ions can then react with other sulfide minerals in reducing conditions and release H+ and SO42−, which can deteriorate cement-based grouting materials used for carven walls as well as the host rock. The results of the study show that the composition of Pb isotopes is a good indicator of the alteration of rock core samples compared to the chemical index of alteration and loss on ignition. The acid potential ratio and net acid production potential indicate that most of the core samples in both boreholes have a high alkali-generating potential. Factor analysis shows that the neutralizing capacity in AH1 is highly correlated with the alteration, while the neutralizing capacity in BH3 is correlated with secondary minerals. The alteration appears to be related to secondary minerals and the acid-producing capacity, while the changes in major elements are not related to the Pb isotopes in BH3 due to leaching along fractures.

A STUDY OF THE PRESSURE SOLUTION AND DEFORMATION OF QUARTZ CRYSTALS AT HIGH pH AND UNDER HIGH STRESS

Bentonite is generally used as a buffer material in high-level radioactive waste disposal facilities and consists of 50% quartz by weight. Quartz strongly affects the behavior of bentonite over very long periods. For this reason, quartz dissolution experiment was performed under high-pressure and high-alkalinity conditions based on the conditions found in a high-level radioactive waste disposal facility located deep underground. In this study, two quartz dissolution experiments were conducted on 1) quartz beads under low-pressure and high-alkalinity conditions and 2) a single quartz crystal under high-pressure and high-alkalinity conditions. Following the experiments, a confocal laser scanning microscope (CLSM) was used to observe the surfaces of experimental samples. Numerical analyses using the finite element method (FEM) were also performed to quantify the deformation of contact area. Quartz dissolution was observed in both experiments. This deformation was due to a concentrated compressive stress field, as indicated by the quartz deformation of the contact area through the FEM analysis. According to the numerical results, a high compressive stress field acted upon the neighboring contact area, which showed a rapid dissolution rate compared to other areas of the sample.

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.