地工織物常應用於週期性水流(bi-directional flow)作用的區域,然而傳統試驗方法雖能進行單向度的滲流試驗,但是對於雙向週期性水流則無法進行模擬;如水力坡降比試驗,對於有週期性水流作用的影響進行評估,於理論上並不適用。為了探討雙向週期水流作用下,覆土壓力及細料含量對土壤-地工織物過濾行為之影響,本研究研發一台可模擬雙向週期水流的試驗儀器-即雙向週期水流試驗儀。試樣由沈泥細料與砂土調配混合,分別調配之沈泥重量百分比為0% 、 10% 及 20% 三種,而軸向壓力分別為模擬覆土深度約為 3.5m (σ = 70 kPa)及7.0m (σ = 140 kPa)二種;週期水流速率則分別模擬一週期為600秒、300秒、150秒、75秒四種流速。 由試驗結果得到以下結論:(1)有摻入細料的試體發生管湧,原因是軸向載重 σ = 70 kPa 不足以束制土壤的移動,造成土壤的流失進而形成管湧;(2)織物之開孔徑尺寸雖符合地工織物於雙向水流狀況下之阻留準則,但由試驗結果發現,試體有發生管湧現象,原因與土壤條件、軸向壓力及水流速率有密切的關聯;(3)當軸向壓力為140 kPa 時,細料含量20% 者,試體中的砂土與細料緊密接觸,在軸向壓力140 kPa束制下不易被淘洗流出。
Geotextile is often used as a filter material in the area subject to cyclic flows such as seashore or river banks. Conventional permeability tests can generate uni-directional flow or bi-directional flow, but are unable to simulate cyclic flows, and therefore unsuitable for testing a soil-geotextile filter system under cyclic flow conditions. In this research, an apparatus which can simulate bi-directional cyclic flow conditions is developed for testing soil sample with geotextile. The soil samples are composed of sand mixing with various silt contents: 0, 10, and 20% by weight. Two axial pressures simulating the depths at 3.5m (70 kPa) and 7.0m (140 kPa) are applied on the samples. Four kinds of cyclic flow periods are 600, 300, 150 , and 75 seconds per cycle. The results from the tests are: (1) Piping occurs in the samples having fine content under axial pressure 70 kPa. The axial load may not be large enough to confine soils and limit their movement; (2) Though the geotexitle used satisfies the retention criteria under bi-directional flow, it still results in piping. The causes are related to soil condition, axial load, and the period of cyclic flow; (3) When the axial pressure is 140 kPa, the sample with 20% silt content does not have piping phenomenon.. It seems that sand and silt are packed well so that soil is difficult to be washed out