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    Title: 外包與層狀加勁砂柱力學行為之比較研究
    Other Titles: Comparative study on the mechanical behavior of encased and laminated sand columns
    Authors: 林致瑋;Lin, Chih-Wei
    Contributors: 淡江大學土木工程學系碩士班
    洪勇善;Hong, Yung-Shan
    Keywords: 砂柱;外包;層狀;加勁;地工合成材;三軸試驗;sand columns;Encased;Laminated;reinforced;geosynthetics;triaxial compression test
    Date: 2013
    Issue Date: 2014-01-23 14:22:19 (UTC+8)
    Abstract: 設置於軟弱土層中的砂石樁,樁體上部結構易因側向束縛力不足而發生破壞,對砂石樁加勁可增加側向束縛力並減少其破壞。而加勁方式又可以分為外包與層狀加勁,而本研究便是探討兩者加勁方式對承載能力及變形特性的影響。

    本研究透過砂顆粒基本性質試驗、單、雙向加勁材寬幅拉伸試驗、砂土-不織布界面直剪試驗及砂土三軸壓縮試驗,求取所需的力學參數。再依據加勁材不同的編織方向(機器與橫穿方向)進行外包加勁三軸試驗,以及不同的鋪設方式(同向與交叉鋪設)進行層狀加勁三軸試驗,依此瞭解加勁砂柱的應力-應變-體積變化關係。試驗參數包含四種圍壓(20 kPa、50 kPa、100 kPa 與200 kPa)、三種加勁材(GT1、GT2及GT3)、三種試體直徑(50 mm、70 mm及100 mm)以及低圍壓下不同加勁材層數(4層與8層)等參數,以闡述不同加勁方式力學行為的差異。

    由試驗結果獲得不論外包或層狀加勁,於低圍壓時加勁效果明顯大於高圍壓情形。由加勁材雙向拉伸試驗結果顯示,張力勁度遠高於單向拉伸,特別是較弱的橫穿方向(XD)可提升8 ~ 21倍,因而大幅提升加勁材周圍砂石材料的圍束應力,反應於砂柱的軸向承載能力;此清楚解釋為何層狀加勁砂柱於相同軸應變下遠高於外包砂柱的原因。使用等量加勁材時,交叉鋪設的層狀加勁效果明顯大於外包加勁,尤其於低圍壓且加勁材強度較大時,承載能力的差異更加明顯。以層狀GT3VD(交叉鋪設)為例,於圍壓20 kPa、軸應變20%時,承載力約為GT3MD外包砂柱的4.5倍。於較高圍壓時(100 kPa),則降為3.5倍。若採用較弱的加勁材(GT1),於低圍壓下(20 kPa),以層狀交叉鋪設方式較外包(GT1MD)承載力僅增加約63%。因此,選用高強度、高勁度的加勁材,運用於層狀交叉鋪設之砂柱中更能突顯加勁的效能。
    Granular columns have been used in engineering practice to improve the bearing capacity of soft clay, and reduced the settlement of foundations resting on weak soil. The lateral confining pressure of in situ generally increases with an increase in depth. Therefore, most granular columns that fail from bulging take place near the top due to insufficient lateral support. Reinforcement of the granular column, especially at the top section, was proposed to enhance lateral column confinement. The reinforcement is established through enveloping the granular column in a flexible fabric or applying horizontally laminated reinforcing sheets to the granular column. This research studies the relative efficiency of both forms of reinforcement in improving the bearing capacity.

    In this study, systematic series of triaxial compression tests are conducted on sand reinforced with geosynthetics in both forms, keeping the quantity of reinforcement the same. The results are analyzed to compare the forms and to study the effect of reinforcement form on the strength improvement in sands. Three types of geosynthetics are used to reinforce the sand specimens with 50, 70 and 100 mm diameter in these tests.

    Irrespective of encased or laminated reinforced form the test results show the reinforced effect was significantly better at low confining pressure. Biaxial stretching tensile stiffness is higher than the uniaxial condition. Especially cross machine direction (XD) can enhance the 8-21 times. Therefore, Geosynthetics surrounding gravel materials significantly improved confining stress and increased the bearing capacity of the granular column. The laminated cross-bedded column was significantly greater than the encapsulated granular column, especially for low confining pressure and higher strength geosynthetics. For the column GT3VD by cross-bedded laminated form in 20 kPa confining stress the bearing capacity at 20% axial strain was 4.5 times that of the encapsulated column GT3MD. However, using the weaker geosynthetics (GT1) the bearing capacity increased only 63% for cross-bedded laminated reinforced columns. Therefore, the selection of high strength and stiffness geosynthetics and using the cross-bedded laminated columns highlights the performance of reinforced granular columns.
    Appears in Collections:[土木工程學系暨研究所] 學位論文

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