以雙向水流探討五種地工織物之過濾行為

Abstract

河川護岸的治理可採用兼具安全性及對生態友善的地工合成材(geosynthetics)，如將地工織物鋪設於護岸表面，使其與土壤互制，以形成土壤-地工織物過濾系統。在土壤-地工織物過濾系統的設計上，系統除了要具備良好的透水性，亦需要有適當的阻留能力，以避免土壤流失造成護岸的破壞。當此系統穩定形成時，便可達到抗沖蝕及過濾的功能，進而有效地穩定河岸的邊坡。 目前相關的研究大多採用單向水流的試驗，然而河岸的水利條件較接近雙向循環水流的作用，故本研究採用雙向循環水流試驗儀，進行一系列的室內試驗，探討五種地工織物對現地土壤的過濾行為。試驗材料選用細粒料含量大於20%的大漢溪沿岸土壤，及不同特徵開孔徑及厚度的不織布。試驗中，將含地工織物之土壤試體於覆土壓力70 kPa作用下，分別受到不同週期水流(600、300、150、75 sec/cycle)的反覆作用，同時量測試體內之孔隙水壓力變化及試體頂部的歷時沉陷；試驗後並收集被濾出的土壤，進行粒徑分佈試驗，以探討系統之過濾機制是否有效。 試驗結果顯示: (1)從濾出土壤的重量及粒徑分析得知，本試驗採用的不織布皆有一定的阻留能力，並不致造成試體明顯的沉陷及土壤的濾出；(2)細粒料含量高的土壤，其顆粒易受水流作用而移動，使得水壓力變化較不穩定，尤其是水流週期越短，變化趨勢愈明顯；(3)於流速較快的水流作用下，細顆粒的移動較明顯，使得顆粒重組及土體結構改變，導致發生較大的沉陷；(4)厚度較厚的不織布，對於削減試體受水流擾動的影響有較大的作用。

The use of geosynthetics for revetment protection is an environmentally friendly method. For example, the geotextiles lying on the revetment will interact with the soil to form a soil-geotextile filtration system. In a proper designed system, it must have the capabilities of good drainage and preventing the on-site soil from being washed away. Most of the researches so far have conducted tests under uni-directional flow condition. In real condition, a flow goes into-and-out of a revetment due to water table change is more like bi-directional and cyclic than uni-directional. Hence, this research utilized a bi-directional flow apparatus and performed a series of tests on five non-woven geotextiles to test their filtration function. The soil, containing more than 20% of fines, was obtained from the bank of Tan-Han creek. The geotextiles for testing have different characteristic opening sizes and thicknesses. When tested, specimens of soil with geotextiles were under an overburden pressure of 70 kPa and various flow periods, ranging from 600 to 75 sec/cycle. During each test, the pore water pressure in the specimen and the settlement at the top of the specimen were measured. After each test, the soil washed away from the specimen was collected for analyzing its grain size distribution in order to understand if the filtration system was effective. The test results are listed as follows: (1) from the analysis of grain size distribution and the weight of collected soil, it is known that all the geotextiles tested functioned well; (2) in the soil with high fines content, the variation of pore water pressure was not very stable due to the movement of fines. This phenomenon was obvious when the flow period was short; (3) under fast flows, the movement of fines induced rearrangement of soil structure and lead to apparent settlement; (4) the thicker geotextiles could reduce the disturbance from the flow than the thinner.