台灣地理環境特殊，易形成顆粒細小且不具塑性之土壤，西南部地區此類型之低塑性粉土分佈廣，由於低塑性粉土易受沖蝕作用影響而土壤流失之特性，近年來造成許多地下開挖工程發生管湧破壞之重大工程災害，故對於低塑性粉土之內沖蝕行為與土壤穩態有進行更深入探討之必要。
前人以Flexible Wall Pin Hole (FWPH)試驗儀進行驟變壓力差試驗，已探討不同狀態之低塑性粉土試體的內沖蝕性質，故本研究將利用FWPH試驗儀及三軸不排水試驗，控制孔隙比、細粒料含量與有效圍壓，更進一步探討土壤細粒料含量變化對抗沖蝕能力及土壤穩態行為之影響。
根據試驗結果顯示，低塑性粉土的抗沖蝕能力隨著有效圍壓或孔隙比的增加而降低，當細粒料含量增加時，整體而言，抗沖蝕能力會有降低趨勢，但於土壤排列最緊密之30%細粒料含量時，會有提升之現象，由此可知，土壤顆粒排列緊密程度為主要影響土壤抗沖蝕能力之條件。
另外，根據試驗結果所得之土壤穩態線，當土壤應力狀態位於穩態線下方，其將不受內沖蝕作用產生破壞，反之，則會產生破壞。綜觀以上之結果，就低塑性粉土而論，可提高土層緊密程度或降低地下水位以增加其抗沖蝕能力，且利用土壤穩態線可判斷土壤內沖蝕破壞的發生與否，以供工程上之設計參考用。

Due to the geographically special environment, large amount of low plasticity slity sand with fines widely spread in Southwestern Taiwan. The low plasticity silt sand is easy to lose due to the internal erosion effect and result to piping failure in many underground excavation cases. Therefore it is necessary to investigate the properties of low plasticity slity sand under condition of internal erosion.
In previous study, the internal erosion properties of low plasticity silty sand under sudden pressure difference has been investigated by using Flexible Wall Pin Hole apparatus (FWPH). In this study both FWPH test and undrained triaxial test are used to study the effect of fines content on the resistance of internal erosion and the steady state behavior of the low-plasticity silty sand.
The test results indicated that the resistance of internal erosion of low plasticity sand would decrease as the effective confining pressure or void ratio increases; while as fines content increases, there is a decreasing trend of resistance of internal erosion, but it world increases at 30% fines content, which world be packed most closely. The degree of soil particles packing density is the principal condition affected the resistance of internal erosion of soil.
The steady state line obtained from test results showed that as the effective stress and void ratio of the silty sand below the steady state line, it will not cause piping failure by internal erosion; on the other hand, it will cause failure. In summary, low plasticity silty sands may increase the resistance of internal erosion by improving the density of the soil or reduce the groundwater level, and the steady state line of soil may be used as the reference for engineering design to determine if failure condition of erosion may occur.

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