實際工程中，為了施工方便及成本考量，以及滿足挖填方平衡之需求，常需使用現地細顆粒含量較高的土壤來回填，這類的土壤內含有較高的細顆粒，因此滲透性較差，遇到豪雨來襲，會導致排水不良及孔隙水壓快速累積的問題，最後造成邊坡滑動或是擋土牆體損壞。為了改善低滲透性回填土構造物受到豪雨滲流之排水效果，本研究設計一座橫向滲流儀，以粉土為低滲透性土壤、地工不織布為排水材料、搭配不同厚度之薄砂層(過濾排水層)進行各種複合排水系統之長短期滲流試驗。並於試體上方施加垂直載重模擬覆土壓力，試驗結果發現，無載重時地工不織布對系統滲透性有顯著的提升，但隨著載重作用與長時間滲流使排水層厚度減少以及地工不織布孔隙被土壤顆粒阻塞而降低其排水效率。本研究發現，在地工不織布周圍設置薄砂層後不論有無載重其排水效果皆能有效發揮，且系統滲透係數隨著薄砂層厚度的增加而增加。本研究成果有助於了解地工不織布以及薄砂層對低滲透性土壤的長短期排水改善效果，模擬現地長期暴雨下土壤與地工不織布間之互制行為，作為排水系統最佳化設計依據，於工程實務上減低構造物在豪雨下之受損情形，達到建立永續安全家園之最終目標。

This study carried out a series of horizontal permeability tests to evaluate the permeability and drainage efficiency of low permeable soil with geosynthetic drainage system. The experimental variables include: number of drainage layers, with and without sand layer, and sand layer thickness. The objective of this study is to improve the drainage capacity of low permeable soil for the application of geosynthetic-reinforced soil (GRS) structures backfilled with in-situ soil, typically with low permeability, under typhoon and heavy rainfall conditions. Test results indicate that system short-term permeability is improved as number of drainage layer, and number and thickness of sand layer increase. However, the system permeability decreases as the applied normal pressure, simulating the overburden pressure within retaining structures, increases. The reason of decreasing trend is because the normal pressure would compress the thickness of soil and drainage layer, and meanwhile fine soil particles are likely forced to penetrate into drainage layer which results in the clogging of drainage layer. Regarding the long-term permeability, test results demonstrate that system long-term permeability decreases as time elapse increases and the sand layer thickness decreases. By calculating the degree of retention (DOR), the ratio of mass of soil retained in a geotextile to the mass of the pure geotextile, this study finds DOR decreases as sand layer thickness increases, which provides a strong experimental evidence that the sand layer can effectively improve system permeability by preventing the drainage layer from clogging in long-term. Last, the measured results were compared with the predicted results using theoretical equation for equivalent horizontal hydraulic conductivity of multi-layered soil. The comparison results indicate with absence of normal stress, the measured and predicted system permeability are in a good agreement. However, the theoretical equation was found to overestimate system permeability as normal stress increases.

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