考慮剪脹行為下加勁砂柱之力學機制與應用

機構典藏 > College of Engineering > Graduate Institute & Department of Civil Engineering > Research Paper > Item 987654321/54439

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Title:	考慮剪脹行為下加勁砂柱之力學機制與應用
Other Titles:	Mechanical Responses and Applications of Reinforced Sand Columns- Pondering Soil Dilative Behavior
Authors:	洪勇善;吳朝賢
Contributors:	淡江大學土木工程學系
Keywords:	砂石樁;數值分析;剪脹機制;外包;層狀;地工合成材;sand column;numerical analysis;shearing dilation mechanism;encased;laminated;geosynthetics
Date:	2010
Issue Date:	2011-07-06 11:30:51 (UTC+8)
Abstract:	砂石樁應用於軟弱地層中，除了增加地層承載力、減低沈陷與加速壓密外，對於地震來襲時也可大幅降低液化的可能。然而，應用於極軟弱的粘土中，卻因近地表處的側向圍束應力不足，而導致腫脹破壞或超額沈陷；因此，近幾年掀起如何在砂石樁內或外圍加勁，使其應用更為廣泛的研究趨勢。在國外，砂石樁外包具抗張特性的地工合成材於現場應用逐漸增加，此方式的加勁提升砂石樁受載時的圍束應力，避免砂石樁於地表附近的腫脹破壞。此外，抗張材分層置入樁體也是另一種目前實務上較為少見，但具研究價值的加勁方式。不論是外包或層狀加勁，當砂石樁軸向受載壓縮而產生側(徑)向變形，加勁材即逐漸藉由側向擴張(外包加勁)或摩擦阻抗(層狀加勁)而發揮張力，進而增大對砂石樁的圍束應力，提升砂石樁的承載能力。因此，若砂石樁承受軸向加載會產生剪脹行為，將使得砂石樁的側向擴張更加明顯，加勁材發揮更大的張力和提高對砂石樁的束制應力。因此，正確模擬砂石材料受力時剪脹行為，及圍束應力遞增條件下砂土材料力學行為的分析模式，是成功探討加勁砂石樁行為的基石。線彈性-完全塑性或雙曲線Mohr-Coulomb 模式，常用於文獻中的砂石行為模擬，此模式隱含砂石在降伏前體積壓縮的假設。由於對砂石未達降伏前即發生剪脹的實際行為無法正確分析，以致於嚴重錯估加勁材的加勁能力，對於整體加勁砂石樁的反應行為也無法正確掌握，造成工程上認知的錯誤。有鑑於此，本研究擬分三階段進行加勁砂柱力學行為與加勁機制的探討。第一階段透過三軸試驗的方式，觀察純砂與外包加勁砂柱力學行為的差異，分別由塑性理論建立純砂力學模式與外包加勁數值模型，並撰寫分析計算程式，以三軸試驗之應力-應變-體積變化結果為佐證，作為數值分析的驗證與敏感參數的校訂。第二階段進行層狀加勁砂柱三軸試驗，並依據第一階段所完成的純砂模式，建構層狀加勁的數值模型與驗證，並剖析加勁材與砂土互制行為機制。第三階段以各種不同加勁模式下(外包、層狀及混合加勁)所建立的數值模型，進行廣泛的參數研究，評估不同加勁模式的加勁效能，並供現場實務上施工與設計的參考。此研究的重點即在：正確的模擬砂石材料在整個受載期間的剪脹行為，及圍束應力遞增條件下砂石材料力學行為的分析模式，並藉由三軸試驗結果驗證分析模式的正確性。 Although construction on soft soils is most commonly undertaken using basal reinforcement, with or without prefabricated vertical drains, there is growing interest in the use of granular columns constructed by filling a cylindrical column with granular material. This technique has been introduced into engineering practice to improve the bearing capacity and reduce settlement of sand column foundations resting on the weak soil. The improvements on bearing capacity via granular columns are achieved through the inclusion of a stronger granular material. In response to a vertical load, an expanded granular column will squeeze the native soil, and result in an additional confining pressure onto the column. That leads to an increase in the stiffness and strength of granular column. However, insufficient lateral support at shallow column depth (top portion) frequently causes bulging failure at the top portion of the column. Therefore, reinforcement on granular columns, especially over the top few meters, is needed to provide lateral support to enhance the lateral confinement of column. The reinforcement can be achieved by enveloping a granular column with a flexible fabric or by placing horizontally laminated reinforcing sheets on the granular column either in full or partial height. Geosynthetic-encased sand columns were successfully used to found a dike in very soft soil for land reclamation. In this study, the load-responses of embedded reinforced-granular columns are evaluated using a numerical method based on the elastic-plastic theory. The material is assumed to obey the Mohr-Coulomb yield criteria. Because the granular material is subjected to increasing confining pressure compounded by the circumferential tensile stress developed in the extended encapsulating material and lateral restraint due to the counteraction of the surrounding soil to column expansion, difficulties arise in evaluating the variation in the Poisson’s ratio and hence, the lateral expansion of the column when the granular material is subjected to increasing confining pressure. A relationship between the volumetric change and axial strain of the soil is established enabling the expansion behavior of granular material prior to plastic state to be accounted in the analysis. In addition, a series of triaxial compression tests will be carried out in laboratory to investigate the response of reinforced sand columns. The reinforcement is achieved by enveloping a granular column with a geotextile or by placing horizontally laminated geotextile sheets on the granular column. The increase in deviatoric stress, the reductions in volumetric and radial strains, and the increase in confining pressure generated by the reinforcements are measured and analyzed. The experimental results are compared with data obtained from the present numerical method and analytical method reported in the literature.
Appears in Collections:	[Graduate Institute & Department of Civil Engineering] Research Paper

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