Satoshi Goto

Landslides represent a significant geological hazard in Vietnam, affecting various regions and posing threats to human lives, property, and the environment. On October 12, 2017, a massive landslide struck Khanh Village, Hoa Binh Province, Vietnam, resulting in 18 fatalities and the destruction of five houses. The event highlighted the vulnerability of limestone regions in Vietnam to landslides, particularly during periods of heavy rainfall. This paper first provides preliminary survey results to examine the cause and mechanism of the landslide. The results show that heavy rain was triggering factor in the 2017 Khanh waterfall landslide. Using the LS-RAPID computational model, we reproduce the 2017 event and assess the risk of future landslides from the remaining steep slope of the Khanh waterfall area. The findings provide valuable data to assist authorities in enhancing landslide hazard assessment, as the remaining steep slope poses a significant risk of future landslide occurrence.

Introductions: A large-scale flume experiment was performed to evaluate the mechanism of landslide occurrence due to rainfall using weathered granite sand. The dimensions of the flume were 9 m (length), 1 m (width), and 1 m (depth). The weathered granite sand from the actual landslide site at Da Nang City, Vietnam was used. The pore water pressure was measured by a pore-water pressure transducer at two depths (middle and bottom) to determine the process of rainwater infiltration into the soil. The surface deformation was measured with extensometers at three positions of the slope. The deformation of the entire slope was determined by the 160 cylindrical-shaped makers evenly spaced in the slope and three cameras. Results: The results showed that the rainfall infiltrated into the slope process, increasing from negative pore water pressure to approximately 0. The maximum shear strain contour has been plotted in total and in time increments. The shear band was detected from the time increments maximum shear strain contour. The localization in the shear band formed just before failure. Conclusions: To the best of our knowledge, this is the largest scale laboratory test ever conducted to calculate the shear band. Moreover, it was found that the failure occurred when the sand was in an unsaturated phase. Failure does not seem to depend on the increase in pore water pressure but on the maximum shear strain. This feature can be used to explain the phenomenon of landslides that occur even when the groundwater level does not increase but large deformation occurs.

There are three approaches to defining the potential instability zonation of natural slopes. The first approach is to understand the failure mechanism through soil properties in a slope, and the mechanisms of failure and movement. The second approach aims to understand the tendency of slopes to fail, with terrain factors that form the essential characteristics of slopes. Understanding the spatial distribution of slope failures and their patterns on a particular slope is the third approach and the focus of this research. All approaches require field verification with expert knowledge.Slope failure zones contain unique topographic patterns that can be used to identify the failure shape and its dimensions. Slope failures are one of many natural denudation processes. Most slope failure landform units, considered as past slope failures tend to expand naturally. If there is no human involvement, zones of past natural slope failure can be also categorized as potential zones of future slope failure. The large-scale geomorphological analysis is the best approach for clearly identifying landform units associated with potential zones of slope failures. The best scale is 1:10,000. Two-dimensional or three-dimensional raster interpretation of slopes can be used to visualize more clearly the actual shape of slope failures. For the study, raster geomorphological mapping uses LiDAR survey data to characterize the landform units of slope failures and to prepare a landslide susceptibility evaluation.

The freezing of scoria and snowing on the slopes are common in winter at Mt.Fuji, Japan． From early winter to late spring， slush avalanches often occur due to the rise of atmospheric temperature and the event of heavy rainfall． Shallow surface erosion and transportation of scoria, trees and so on with ice and water sometimes trigger a series of slush avalanche‐slush flow‐slush lahar， which has been called “Yukishiro” in Japanese. Yukishiro often devastated villages on the flank of Mt.Fuji. We will show the methods and the results for the simulation of these slush avalanches and sediment movement phenomena using Cellular Automaton (CA) and Multi-Agent System (MA) methods. For slush avalanches simulation, a lightweight and high-speed simulation tool based on this CA/MA simulation method were used. The CA/MA simulation tool used here is suitable for evaluating the risk of slush avalanches and debris flow arrival areas in a large-scale region. The CA/MA method does not require matrix calculations because it is an explicit method. Hence, parallel processing is easy, which will significantly reduce the calculation time. The probabilistic evaluation of sediment disasters is a possibility by studying many parameters. In future, we are going to carry out improvement of the local rule by carrying out a lot of reproduction analysis of real disasters.

A 6.6-M_w earthquake struck the Iburi region of Hokkaido, Japan, in 2018, triggering massive landslides. Most of these landslides were shallow and occurred mostly in the Atsuma and Abira towns. Ta-c and Ta-d tephra layers have been found in the Towa landslide at Atsuma from the Tarumai volcano, while Ta-d, En-a, and Spfa-1 tephra layers have been found in the Mizuho landslide at Abira from the Tarumai and Eniwa volcanos, as well as the Shikotsu caldera. Field observations from previous studies revealed that the sliding layers were located in the Ta-d and En-a layers at the Towa and Mizuho landslides, respectively. Unlike previous research on earthquake-induced landslides, which were investigated using mechanical properties, this study investigates the characteristics of physical properties, saturated permeability properties, and content of clay minerals on sliding and surrounding tephra layers. Results from this study reveal that the physical properties of sliding layers from two landslides demonstrated the same characteristics: non-plastic soil with a low density of soil particles, void ratio, and dry density; these characteristics could influence earthquake-induced landslides. It also reveals a relationship between the plasticity chart and the age of tephra materials, including the relationship between the weathering process and density of soil particles and the dissimilarity in characteristics of saturated permeability properties in tephra materials.

In recent years, many landslides have occurred in Vietnam, particularly in the Northern mountainous region during the rainy season from May to October. On the morning of October 12, 2017, the Khanh waterfall landslide in Khanh Village, Hoa Binh Province, Northern Vietnam occurred. The landslide killed eighteen people and destroyed five houses. Topographical and geological surveys were conducted around the area to determine its causes. The rainfall data and flow discharge were also analyzed. The results showed that this collapse was different from some previous ones collapsed due to the erosion at the toe of the slope. Khanh waterfall landslide occurred due to the increasing amount of water in cracks and caves in the limestone layer in the slope. The collapse process was simulated based on Coulomb mixture theory. The numerical simulation results show similarities with the actual collapse process. The results provide indicators for assessing the risk of such limestone waterfall landslides in the future.

Heavy rainfall frequently occurred in Kyushu and triggers the landslides every year. This study observes the landslides which occurred in Oita Prefecture, Kyushu, Japan. The landslides in this study, consisting of the same soil materials, tuff breccia and andesite materials; however, the landslide mechanisms were different. Two landslides occurred caused by heavy rainfall in the different timing of the landslide occurrence, and another landslide occurred without the heavy rainfall or an earthquake occurs. Therefore, this study aims to analyse the physical and mechanical properties of tuff breccia and andesite materials with diverse landslide mechanisms. This study performed soil stratigraphic analysis and soil hardness measurements in the field, and performed physical properties, saturated permeability, mechanical properties, and XRD tests in the laboratory. This study found that characteristics of tuff breccia and andesite in diverse mechanisms of landslides were not very different, especially on the landslides caused by heavy rainfall. Furthermore, the landslide in the andesite and tuff breccia areas could be divided into three types based on the timing of the landslide occurrence, scale of the landslide, and landslide mechanisms.

Shallow landslides occur frequently on the Aso volcanic mountains. The soil materials on the Aso volcanic mountains consist of tephra layers formed by volcanic activities. This study is aimed to specify the physical properties of soil that correlate with the sliding layer of a shallow landslide on the volcanic mountain area. Tephra layers consist of kuroboku and scoria layers and the differences between these layers were specified using the physical properties of soil methods. Results showed that the plasticity index and the fine fraction content can be used for estimating the sliding layer in the Aso volcanic area.

Shallow landslides occur frequently on the Aso volcanic mountains. The soil material on the Aso volcanic mountains consists of tephra layers formed by volcanic activities. This study is aimed at determining the physical properties of soil that correlate with the slip surface of a shallow landslide on the volcanic mountain area. Tephra layers consist of kuroboku and scoria layers and the differences between these layers have been determined using the physical properties of soil methods. This study showed that the plasticity index and the fine fraction content can be used for estimating the slip layer in the Aso volcanic area.