地工合成物包裹砂柱試體之應力-應變行為探討

Stress-Strain Behavior of Geosynthetic-Encapsulated Sand Column Specimens

蔡君平(Chun-Ping Tsai)

指導教授 ： 吳朝賢

淡江大學/工學院/土木工程學系碩士班/碩士(2006年)

https://doi.org/10.6846/TKU.2006.00794

摘要

在眾多之土壤改良工法中，砂石樁改良法施工迅速，砂石樁除了可傳遞荷重外，並能加速地盤排水及減少土壤沈陷，因此對砂石樁力學性質之研究為一重要且富價值之課題。打設於軟弱土層中之砂石樁，於樁體上部結構易因側向束縛力不足而產生破壞，對砂石樁加勁可增加側向束縛應力並減少其破壞機率。本研究將地工合成物縫製成圓管狀並包覆於砂柱試體外圍，進行實驗室靜態三軸試驗，以模擬加勁砂石樁於土層中受軸向力時之應力－應變行為，探討將地工合成材料以外包加勁模式加勁砂石樁之效益。

在試驗方面建立一改良式體積量測系統，藉由量測所得之體積應變，計算各軸向應變下加勁材之張應變，並以雙曲線模式模擬加勁材張力－應變關係，依此求取加勁材之張力，計算加勁材所提供之束縛應力，以分析不同勁度之加勁材對試體所提供之加勁效果。

研究結果顯示：在不同拉伸速率之寬幅拉伸試驗上，以慢速拉伸之不織布其勁度較低，由於ASTM規範之試驗速率高於加勁材之受張速率，故在計算上若使用ASTM規範之試驗結果，則會高估加勁材之束縛應力。三軸試驗進行至軸向應變32%時，試體徑向應變尚未達加勁材最大張力所對應之應變，此致加勁試體軸差應力持續上昇並無尖峰值。此外加勁試體之視凝聚力與 角均隨著所施加之軸差應力而上升，加勁材勁度越高，加勁後試體所發揮之視凝聚力亦越高，加勁材勁度對試體 角並無明顯影響。在加勁試體之數值模擬上，試體軸差應力理論值與試驗值之平均誤差均在15.99%以下。

藉由本文中之分析，可瞭解外包加勁模式之效益，及對砂土材料強度參數之影響，提供設計加勁砂石樁時選材及加勁方式上之參考，並對後續試驗與理論研究上提供一概念性之建議。

關鍵字

砂樁 ； 加勁 ； 地工合成材 ； 砂土 ； 三軸試驗

並列摘要

Granular columns were introduced into engineering practice to improve bearing capacity and reduce settlement in a weak or soft soil. This method is considered one of the most versatile and cost effective techniques for improving in-situ ground conditions. Skirting the granular column by wrapping a geosynthetic liner around the top section of the column and enveloping the granular column in a membrane were proposed to strengthen the column. When vertical and coincident lateral deformations of the granular column occur under a vertical load, the encapsulating geosynthetic generates additional confining stress to the column. Because the mechanical behavior of the granular material is usually controlled by the lateral confining pressure, the improvement in bearing capability from the geosynthetic-encapsulated granular column depends upon the mobilization of lateral or confining stresses exerted by the geosynthetic. The reinforcing effect of the granular column thus depends on the characteristics of the geosynthetic and the filled granular material. A numerical procedure to investigate the stress-strain relation of a reinforced column subjected to monotonically increasing confining pressure is therefore developed in this investigation. The theoretical analysis was validated through experimental triaxial compression tests on cylindrical specimen encapsulating with geotextile.
Hyperbolic model was used to simulate tensile stress-strain relation of the geosynthetics. Results from three geotextiles encapsulating sand specimen of two relative densities show: (1) Geotextile encapsulating sand column enhances the axial strength; the reinforced specimen exhibited significant apparent cohesive strength. A remarkable increase in apparent cohesive is noted for geosynthetic with high stiffness. (2) The volumetric strain is not sensitive to the confining pressure; the magnitude of confining stress provided by the encapsulating geotextile varies slightly for different confining pressures. (3) Results obtained form the proposed theoretical analysis has good agreement with the experimental results.

並列關鍵字

Granular column ； Reinforced ； Geosynthetics ； Triaxial Test ； sand

參考文獻

Chandrasekaran,B.,Broms,B.,＆Wong,K.S., (1989),“Strength of Fabric Reinforced Sand under Axissymmetric Loading, ” Geotextiles and Geomembranes, Vol. 8,pp.293-310.

Desai, C.S., (1972), “Nonlinear Analysis Using Spline Functions.” ASCE, Vol.98, No.SM9 ,pp.967-971.

Gray, D. H., & Al-Refeai,T, (1986), “Behavior of Fabric vs. Fiber-reinforced Sand,” Journal of Geotechnical Engineering Division, Vol.112, No.8,pp. 804-820.

Gray, D.H.,& Ohashi, H., (1983), “Mechanics of Fiber Reinforcement in Sand,” Journal of Geotechnical Engineering. ASCE,Vol.109, No.3, pp.335-353.

Haeri, S.M., Noorzad, R., & Osakoorouchi, A. M., (2000), “Effect of Geotextile Reinforcement on the Mechanical Behavior of Sand, ”Geotextiles and Geomembranes, Vol.18, pp.385-402.