台灣西南部土層多含有顆粒細小且低塑性之低塑性粉土，且近年在施作地下開挖工程或是進行潛盾隧道工程時，常因發生管湧破壞而產生諸多重大工程災害，因此有必要藉由相關室內試驗來探討低塑性粉土之內沖蝕性質。
先前研究以Flexible Wall Pin Hole(簡稱FWPH)試驗儀進行穩定壓力差試驗，其試驗結果証明，FWPH試驗能有效探討低塑性粉土受長期穩定滲流之土壤內沖蝕性質，本研究為模擬深開挖工程因地下水滲入開挖面而引發土壤流失現象，故以FWPH試驗儀與不同狀態之試體進行驟變壓力差試驗來探討土壤內沖蝕性質，其中試體控制因素為孔隙比、有效圍壓、細粒料含量與細粒料塑性指數。
由試驗結果顯示，低塑性粉土之抗沖蝕能力隨著有效圍壓或細粒料含量增加而降低，隨著試體孔隙比減少而提高。另外，土壤抗沖蝕能力隨細粒料塑性指數增加而有明顯提高之現象。綜觀上述結果可知，FWPH試驗可有效探討土層之內沖蝕性質，且就低塑性粉土層而言，可藉由降低地下水位差、提高土層緊密程度與降低開挖深度等方式來避免管湧破壞現象之產生。

The low-plastic silt with the characteristic of fine grains is considerably popular appeared in the strata those spreading area of Southwestern Taiwan. Recently many severe disasters, due to the piping, occurred in those low-plastic silt strata, especially related to the deep excavation or tunneling by Tunnel Boring Machine. Therefore, it is necessary to investigate the engineering properties, which related to the deep excavation of the low-plastic silt in field.
The Flexible Wall Pin Hole apparatus (FWPH) was used to investigate the behavior of low-plastic silt under stead pressure condition. The results showed that FWPH was able to study the long-term properties of internal erosion in low-plastic silt. In this study the FWPH apparatus is also adopted to simulate the piping effect in a deep excavation due to the seepage of groundwater. The specimens of low-plastic silt with different void ratios, various fine contents and plastic indices under the conditions of sudden pore pressure changes are performed in the tests.
The test results showed that the resistance of internal erosion of low-plastic silt would decrease as increase in confining pressure and fine content, however the resistance of internal erosion would increase as void ratio decrease. In addition, the resistance of internal erosion would increase as the plasticity index of fines increase. In summary, FWPH test can be used to investigate the property of internal erosion of stratum under conditions which sudden pore water pressure change. As deep excavation in low-plastic strata, the possible piping failure can be mitigated as both reducing the groundwater and the depth of excavation.

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