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  • 學位論文

軟弱黏土中外包加勁砂柱承載行為之數值模擬

Numerical Modeling of Geosynthetic-encapsulated Sand Columns on Loading-settlement Behavior in Soft Clay

指導教授 : 洪勇善

摘要


軟弱黏土因承載力不足,無法承受上部構造的載重,常需進行土壤改良以增加土壤的工程性質。在眾多改良方法中,砂石樁工法具備加速土層排水、減少液化潛能,以及增加承載力等優點。然,砂石樁於軟弱黏土中常因上部側向應力不足而產生腫脹破壞。為提高砂石樁之承載能力,以具抗張的加勁材包裹於砂石樁的外圍,不但可避免腫脹破壞,亦可因加勁材提供圍束應力而提升砂石樁的承載能力;在實務應用上已有許多成功案例。 本研究利用有限差分軟體FLAC撰寫數值分析程式,藉由分析結果探討軟弱黏土中外包加勁砂柱的力學行為。首先,透過砂土三軸試驗結果,以塑性理論非諧合流法則,建立砂石材料的剪脹力學行為模式;同時,由黏土壓密試驗結果,依據臨界狀態理論獲得黏土Modified Cam - clay model中之參數。另,由加勁材拉伸試驗與砂土-鋼板間之界面直剪試驗結果,分別描述張力-應變行為與界面剪向力學特性。依據實驗室模型試驗的邊界與條件,建立軟弱黏土中外包加勁砂柱承載行為的數值模型。經模擬各種外包加勁砂柱的模型試驗,數值分析結果皆與試驗值接近,亦可知軟弱土壤經置入外包加勁砂柱後,承載能力明顯提升;且當砂樁外包加勁材料勁度愈大時,柱體可承受之垂直壓力也愈高。 參數研究以現場實體尺寸為分析標的,分別探討邊界效應、砂石樁徑、加勁材勁度以及加載形式等,對外包加勁砂石樁承載行為的影響。數值分析結果剖析柱體變形、黏土側向壓力、加勁材徑向應變以及圍束應力等於受載過程中的變化。可知,砂石樁承載行為受邊界效應影響甚大,當邊界愈小,承載能力愈高,達20倍樁徑時已不受邊界的影響。另,樁徑愈小與加勁材勁度愈大,承載能力愈高。最後,承載能力也受加載形式不同而有所差異,當加載範圍愈大時,砂石樁受到垂直壓力也愈小。

並列摘要


Due to inadequate bearing capacity on soft clay often need to increase soil engineering properties through soil improvement. Granular columns are a common improvement technique for foundation embankments or structures on soft clay. This technique has been introduced into engineering practice to improve the bearing capacity, reduces the liquefaction potential of granular column foundations resting on the soft clay. However, granular columns may not be appropriate in very soft soils that do not provide enough lateral confinement to the columns. To increase the lateral confinement of the columns, and consequently their vertical capacity, encasing the columns with geosynthetics has been a successful solution in recent years. In this study, the finite difference software FLAC is used to complete the numerical analysis. This numerical study investigates the behavior of geosynthetic-encased sand columns installed in soft ground. Base on an elastic-plastic constitutive model used with the non-associated flow rule, the shearing-dilatancy behavior of granular materials is characterised through laboratory triaxial tests. The parameters of Modified Cam-clay model are extracted from the consolidation tests. By using the tensile force-strain relation obtained from wide-width test, the reinforcement properties are obtained. The shear stiffness of interface between sand and steel plate is obtained via direct shear tests. According to the boundary conditions of the laboratory model tests, establish a numerical model for encased granular columns in soft clay. The proposed numerical method is validated through laboratory model tests on un-reinforced and encased sand columns in soft clay. The clay boundary, column diameter, reinforcement stiffness and loading patterns influence encased column in soft clay response is also studied. Numerical results indicate the larger clay boundary the lower the bearing capacity of column. Up to 20 times the column diameter not effect the column bearing capacity. In addition, the larger column diameter or smaller the reinforcement stiffness result the lower the bearing capacity. Finally, the vertical pressure of column is smaller when the greater load range.

參考文獻


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被引用紀錄


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張智堯(2017)。水平層狀加勁砂樁於長期荷重下之行為〔碩士論文,淡江大學〕。華藝線上圖書館。https://doi.org/10.6846%2fTKU.2017.00600
林明瑋(2017)。軟弱黏土中層狀加勁砂柱承載行為之數值模擬〔碩士論文,淡江大學〕。華藝線上圖書館。https://doi.org/10.6846%2fTKU.2017.00234
李晏如(2017)。黏土中外包加勁懸浮砂柱承載行為之數值模擬〔碩士論文,淡江大學〕。華藝線上圖書館。https://doi.org/10.6846%2fTKU.2017.00173
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