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    Title: 外包加勁砂柱之數值模擬
    Other Titles: Numerical modeling of geosynthetic-encapsulated sand columns
    Authors: 黃振業;Huang;Chen-yeh
    Contributors: 淡江大學土木工程學系碩士班
    洪勇善;Hong, Yung-shan
    Keywords: 地工合成材;外包加勁;數值模擬;柏松比
    Date: 2009
    Issue Date: 2010-03-16 10:33:24 (UTC+8)
    Abstract: 在基礎工程設計中,砂石樁改良法具有經濟、迅速及彈性大等優點。傳統砂石樁雖可承受軸力與剪力,但於樁體上部結構容易因束制力不足而產生破壞,一直以來工程師不斷的尋求對砂樁土體加勁的方式,以提高砂石樁之強度。其中,對砂石樁採用地工合成材外包加勁的方法,為目前可行的方法之一。

    本研究採用數值模擬方式,利用有限差分主程式FLAC針對外包砂柱的加勁行為進行模擬分析,藉由電腦快速疊代運算之功能,求得較過去實驗值或理論值分析下更為精確之結果。

    模擬結果顯示:(1)在過去前人的數值分析中,砂土力學行為大多使用雙曲線模式來描述。因此無法模擬砂土在尖峰應力前的體積膨脹現象,但本研究以塑性理論來模擬土壤之尖峰前的膨脹狀況。由砂土三軸模擬結果可知,其應力-應變曲線之模擬相當吻合試驗值,最大誤差百分率均低於2%。而體積應變之模擬與實驗值也在7%以內之誤差率;雖誤差較為明顯,但已可正確性模擬出砂土在尖峰強度前之膨脹現象,分析考量上較符合真實狀況。(2)在相同艙室圍壓下且軸向應變到達10%之後,加勁材勁度越高對於體積應變的抑制效果越好。而在軸向應變於10%之前體積抑制率並無一定的趨勢,此現象主要為數值分析上加勁材本身考慮柏松比之影響,當試體的體積開始膨脹時,由體積造成之側向擠壓會因為加勁材柏松比效應有所抵銷。(3)在低艙室圍壓下,加勁材勁度越高對於試體之徑向應變抑制能力越好,而在高艙室圍壓下亦有相同之趨勢,但徑向應變抑制率明顯下降。(4)觀察加勁材束縛應力增量可發現,試體經過外包加勁材後,以低艙室圍壓而言加勁之圍壓增量之效果低於理論分析下圍壓增量約35%~50%。此現象主要為加勁材因柏松比影響,與土壤同時產生側向應變情況,抑制土壤擴張之能力將會降低。

    由上述數值分析結果可知,外包加勁之效益若考量土壤膨脹行為以及加勁材柏松比影響下之加勁能力,其結果將更為精確保守。
    The stone column technique is a low cost, time saving, and flexible construction method for ground improvement. When the stone columns are installed in soft clay soils, the lateral confinement offered by the surrounding soil may not adequate to form the stone column due to not be able to develop the required load-bearing capacity. In such soil, the required lateral confinement can be induced by encapsulating the column with tensile geotextile material.

    The purpose of this study is to investigate the mechanical behavior of geotextile encasement on the sand columns using the finite difference analysis. The results from the numerical analysis studies are presented to quantify the effect of confinement and mechanism for improvement in load capacity due to encasement. Based on the results obtained from this study, the following conclusions are made:

    1. The sand that would be used in the model was tested by triaxial tests to develop its stress-strain volume relationship, which provide the basis for numerical analysis. The strain hardening - softening volume dilation model of the sand in present study proved that it could suitable simulates the dilative behavior on pre-peak deviatoric stress.

    2. Volumetric strain reduction percentage increase with the increase in the geotextile stiffness after 10% axial strain. Due to the Poisson effect, the volumetric strain reduction tendency is not obvious before 10% axial strain.

    3. At a specific axial strain, the radial strain reduction effect is higher for stiffer geotextiles to specimen subjected to low chamber pressure. However the radial strain reduction effect degrades with higher chamber pressure.

    4. The confinement stress increment in the reinforcement obviously underestimates 35% ~ 50% against the theoretical solutions.
    Appears in Collections:[Graduate Institute & Department of Civil Engineering] Thesis

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