台灣雨量充沛且多集中於夏季，因此，降雨所造成的沖蝕行為對土壤之影響不可忽視。根據目前求得現地土壤強度之試驗，以直剪試驗為較簡單且快速的方法，而為探討沖蝕對土壤強度之影響，開發一「沖蝕直剪試驗儀」，此試驗儀不僅能進行入滲沖蝕試驗，亦能進行剪動試驗，以求得土壤之剪力強度。
本研究藉由沖蝕直剪試驗儀，模擬現地受入滲沖蝕後，造成土壤細粒料流失，藉此探討沖蝕作用對土壤力學行為之影響。
根據試驗分析結果，不同正向應力與細粒料含量影響沖蝕行為及剪力強度。整體而言，土壤孔隙比愈大，滲流係數愈大，且流失的細粒料含量高，達滲流沖蝕破壞所需時間較短。
此外，土壤剪力強度受粗顆粒與細顆粒控制，由試驗結果可知，細粒料含量10%與20%屬於粗顆粒控制，受沖蝕擾動後細粒料流失，使土壤之強度增大；反之，細粒料含量30%正落於粗細顆粒轉換區間，受沖蝕擾動後土壤之剪力強度較低。綜觀上述之結果，細粒料含量與正向應力，對土壤強度及受沖蝕行為之影響甚大。

Due to the abundant rainfall in Taiwan, the erosion behavior caused by rainfall cannot be ignored. The easiest and most popular way to obtain soil shear strength is direct shear test measurement. This research designed and developed a Seepage Flow Direct Shear apparatus (SFDS) to simulate the seepage flow behavior caused by fines erosion in the field to discuss how fines erosion impacts the soil behavior.
According to the test results different normal stress and fines content affects the seepage flow behavior and shear strength. Overall, while the soil void ratio increases the coefficient of permeability and erosion of fines content increases, causing erosion time at failure to be short.
Coarse and fine grains control the soil shear strength. The test results shows that when the fines content FC = 10% and 20% controlled by coarse grain, the soil strength is stronger after fines seepage flow erosion. On the other hand, when the fines content FC = 30% located in the transition zone, the soil strength will be weaker after seepage flow erosion. In summary, the fines content and normal stress have a huge impact on soil strength and erosion behavior.

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