| Summary: | 碩士 === 國立臺灣大學 === 生物環境系統工程學系暨研究所 === 90 === The main purpose of this study is to investigate the relationship among seepage, electrical conductivity, and the occurrence of debris flow caused by piping. However, if the soil which was deposited in the stream with a high potential of debris flow or located at the sides of the stream, was eroded due to seepage for a period of time, the fine particles of the soil might be carried away and subsequently, the soil deposit or the soil slope might tend to be unstable. When it reached to a critical state, piping might occur. Because of the increasing effects by the piping, the whole soil deposit or the slope might collapse. Therefore, to study the critical hydraulic gradient at which piping occurred, field dimension analyses were conducted and accordingly, seepage tanks were designed and fabricated. On the other hand, the variety for electrical conductivity of the water was also evaluated with the seepage experiments at the same time, and tried to estimate the relationship between the electrical conductivity and the debris flow caused by piping.
In this study, Feng-Chiu of Nan-Tou prefecture was selected as the site. The soil samples were collected from the upstream site of Feng-Chiu. After tests, the grain size distribution curves and basic soil index properties were obtained. Then, by using the obtained data and properties, the seepage experiments were designed and carried out. Therefore, the critical hydraulic gradient and the equations for calculating critical piping might be evaluated. It was found that the critical hydraulic gradients for the soil samples with the slope steepnesses of 45, 60, and 75 degrees were 0.296, 0.294, and 0.290 respectively. It also found that the critical seepage angle for the soil sample collected from Feng-Chiu was 26.4 degrees.
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