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  • 1
    Publication Date: 2020-10-01
    Description: The infiltration of rainfall into a slope surface may affect slope stability; thus, it is important to understand the amount of rainfall infiltration (hereafter referred to as the “infiltration capacity”) for a slope surface layer when evaluating slope stability. This research focuses on slope gradient, a factor affecting the infiltration capacity, and performs two types of water-spraying experiments using pit sand under the same conditions but with different slope gradients. In the first experiment, the surface flow rate and soil loss were measured using an earth-tank model with a horizontal distance of 0.5 m, depth of 0.1 m, and width of 0.2 m to form slope gradients of 2°, 20°, and 40° to clarify the effect of slope gradient on the infiltration capacity. In the second experiment, a water-spraying experiment that closely simulated natural rainfall was performed at a large-scale rainfall facility owned by the National Research Institute for Earth Science and Disaster Resilience (NIED), Japan. This experiment used an earth-tank model with a horizontal distance of 1.21 m, depth of 0.5 m, and width of 0.5 m to form slope gradients of 2°, 10°, 20°, 30°, and 40° with the aim of proposing a quantitative evaluation method for the relationship between the slope gradient and infiltration capacity. The results showed that the soil loss and infiltration capacity increased as the slope gradient increased in the case of the pit sand used in the experiments. This was confirmed to be due to the fact that an increased gradient allowed grains with diameters of
    Print ISSN: 1881-2473
    Electronic ISSN: 1883-8030
    Topics: Technology
    Published by Fuji Technology Press
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  • 2
    Publication Date: 2018-10-01
    Description: The heavy rain in Northern Kyushu District on July 5, 2017 caused a sediment disaster, resulting in the loss of many lives and damage to buildings. In this study, the primary causes (topography and geology) and trigger factors (rainfall) for the sediment disaster were spatially analyzed to examine factors contributing to slope failure. As a result, it was found that the number of slope failures was highest in metamorphic rock areas and the occurrence density of the landslides was highest in plutonic rock areas. In addition, the slope angle of the slope-failure source point was sizable in volcanic rock areas and many landslides occurred in the valley-formed areas. A rainfall analysis showed that the Akatani, Shirakitani, Sozu, Kita, Naragaya, Myoken, Katsura river basins and Ono, Ohi, Sata, Inaibaru river basins are different rainfall distributions, which significantly affected the slope-failure occurrence density.
    Print ISSN: 1881-2473
    Electronic ISSN: 1883-8030
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  • 3
    Publication Date: 2019-12-01
    Description: The July 2018 heavy rain, which was actually a series of intermittent downpours instead of a short-term continuous heavy rainfall, triggered a large number of sediment disasters. This study was conducted to evaluate sediment disaster triggers. In the study, an interview-based survey was conducted on the occurrence times of the sediment disasters caused by the heavy rain and a rainfall analysis was completed using analyzed rainfall data from the Japan Meteorological Agency. These were followed by an analysis of estimated occurrence times of the sediment disasters and the temporal changes in rainfall indices determined through the rainfall analysis. An analysis of disasters at 36 sites examined for the purposes of this study showed that many occurred when the soil water index (SWI) during the study period (June 28, 2018, to the estimated occurrence time of a sediment disaster) was maximized. The analysis also indicated that slope failures tended to occur when hourly rainfall was relatively low and the SWI was high and debris flows occurred when the SWI was high and hourly rainfall was relatively high. Examination of the data, considering the alert level of the SWI, showed that in cases where the SWI continued to increase after exceeding the alert level, 75% of the sediment disasters analyzed occurred within approximately 19 h.
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  • 4
    Publication Date: 2017-09-27
    Description: The failure time of a slope is predicted by a method based on creep failure theory for slope displacement on natural slopes, embankments, and cutting slopes. These prediction methods employ several equations based on the relationship between the displacement rate (displacement velocity) and time. However, such methods harbor problems because the shape of the tertiary creep curve is affected by many conditions, and it is difficult to identify the phase of tertiary creep. This study examines the time change in the displacement rate of the slope and derives an index for identifying the phase of tertiary creep. Two models of large-scale composite granite slopes were tested by using a large-scale rainfall simulator. In the experiments, the slope displacements were monitored in real time. From these results, inflection points were found in the velocity of the slope displacement. It was found that the corresponding inflection points at different locations in the sliding soil mass occurred with the same timing. This paper discusses the effectiveness of the prediction method for slope failure time by using the inflection points of displacement rate in real-time monitoring records.
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    Electronic ISSN: 1883-8030
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  • 5
    Publication Date: 2017-09-27
    Description: Every year in Japan, slope failures often occur due to heavy rainfall during the wet season and typhoon season. The main reasons for soil failure are thought to be the increase of soil weight from infiltrated precipitation, the decrease in shear strength, and effects of the increase groundwater elevation. It is therefore important to consider to characteristics of groundwater behavior to improve slope disaster prevention. Kiyomizu-dera experienced major slope failures in 1972, 1999, and 2013, and a large slope failure occurred nearby in 2015. The two most recent events occurred since observation of precipitation and groundwater conditions began at the site in 2004. In this research, we determine the relationship between rainfall and groundwater level using both a full-scale model experiment and field measurements. Results indicate strong connection between rainfall intensity and the velocity of increase in groundwater level, indicating that it is possible to predict changes in the groundwater level due to heavy rainfall.
    Print ISSN: 1881-2473
    Electronic ISSN: 1883-8030
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