風化花崗岩森林斜面におけるソイルクリープ挙動と土壌水分変化との関係

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Soil creep behavior and suction changes of soil water were observed with the automatic strain probes and tension meters on a forest slope of weathered granite in Nara-prefecture for about one year. The correlations among soil creep behavior and soil moisture change and their variation with the location on the slope are analyzed. Two displacement types are detected in the topsoil on the slope. They are displacement of whole topsoil and that of upper part of topsoil. The former is caused by the relatively large shear strain at the bottom of topsoil. It makes whole topsoil to incline toward upslope or down slope. The latter is a shallow movement of flow type. The correlation between soil displacement and soil moisture change varies with the locations on the slope as well as the time scales of soil moisture change. At the lower part site in the middle slope where the observation was carried out, upslope and downslope displacement appear in dry and wet soil moisture conditions, respectively. During a single rainfall event, upslope displacement appears at the beginning, while the displacement turns downslope after the rain and then the displacement is decreased. At the upper slope site, upslope displacement occurs on the way of soil wetting. Downslope displacement is observed both in wet and dry soil moisture conditions, during the periods of plentiful and scarce rainfall. Moreover, large soil displacement happens sometime at the upper part of topsoil in this site. It is assumed that soil-structure destruction occurs at the boundary soil horizons according to drying and wetting. The hypothesis that soil creep due to drying and wetting is brought through the process of soil expansion and contraction, applies both at the upper slope and the lower part of middle slope in this study. However, some soil creep mechanism that cannot be explained only by soil expansion and contraction is likely at the upper slope. It can be said that sandy soil is sensitive to suction changes of soil water. On the contrary, clayey soil does not immediately responded to suction change, allowing for a slow and secular change. As to distribution pattern of soil creep velocity in soil profile, known soil creep pattern that the velocity decreases with soil-depth were observed at four sites out of five instrumented sites. At the site closely below the break of slope angle between upper and middle slopes, the different soil creep distribution pattern was observed, suggesting a small-scale sliding. The displacement as derived from the strain measurement involves ambiguity, because the data processing is based on several unverifiable assumptions. It is, however, proved that the strain-probe method brings invaluable information about the quantity and mechanism of soil creep.
日本地形学連合の論文
2005-04-25

著者

奥西一夫
国土問題研究会

風化花崗岩森林斜面におけるソイルクリープ挙動と土壌水分変化との関係

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