近年全球暖化、氣候變遷及能源危機已成為世界各地共同面臨之問題，各國積極投入再生能源的發展，藉以達到節能減碳之目標。台灣海峽被譽為世界最適合建置離岸風場之潛力場址之一。因此，離岸風力發電成為台灣近年積極發展再生能源的重點項目。基於台灣水深條件、鋼結構製造及海事自主施工等考量下，潛力場址申請開發商多擬採用之支撐結構基礎型式為大口徑單樁基礎與套管基礎。離岸風機上部結構受到風、波、流等週期性環境載重，環境載重造成之側向作用力經由上部結構傳遞至套管基礎之下部結構時，在迎風面與背風面分別對樁基礎產生軸向拉力及壓力。樁基礎長期受到反覆軸向作用力，造成樁基礎承載力的衰減，將影響風機運轉之支撐結構穩定性。關於樁基礎受反覆軸向拉力下之行為反應之相關研究成果十分稀少，因此目前較常用於工程界評估樁基礎受反覆載重作用下承載力衰減之分析方法為互制關係圖法。現有的互制關係圖並未納入樁基礎尺寸條件與土壤條件的考量，於互制關係圖中僅能得知樁基礎於反覆受力條件下容許作用次數且無法得知樁基礎於反覆軸向拉力下之承載力。
有鑑於此，本研究以有限元素分析軟體ABAQUS建立樁-土互制系統數值模型進行樁基礎受軸向拉力下之穩定性分析，以Jardine and Standing (2000)現地樁載重試驗成果作為案例分析對象，將打樁效應納入數值模型考量並對基樁承載力之影響進行探討。針對樁基礎靜態穩定性分析，於數值模型中透過基樁管壁膨脹方法模擬打樁過程中基樁擠壓樁周土壤，增加樁土間正向力以提高基樁承載力，達到現地試驗成果；針對樁基礎反覆穩定性分析，數值模型以土壤體應變衰減公式代入土壤體應變衰減模型(VSDM)，模擬樁周土壤受到反覆剪力作用後，土壤體應變隨著反覆作用次數增加而減小，使樁周正向應力降低造成樁身摩擦力隨之衰減，達到反覆軸向拉力作用下基樁承載力隨著反覆作用次數增加而衰減之目的。
本研究利用9組樁基礎尺寸之數值模型進行參數分析，產出考量土壤條件與樁基礎尺寸之互制關係圖，並搭配德國樁基礎設計規範[EAP(2012)]建議方法計算不同反覆拉力條件作用N次後之反覆拉拔力，分別探討樁徑、樁長、土壤緊密程度(固定樁基礎尺寸條件)、平均反覆拉力比及反覆拉力振幅比對基樁承載力之影響。由分析結果得知，樁徑、樁長與土壤緊密程度的增加使樁基礎承載力提高；反覆拉力振幅比相較於平均反覆拉力比，樁基礎反覆拉拔力比隨反覆作用次數衰減幅度顯著。

In recent years, Taiwan has been actively developing renewable energy and offshore wind power is one of the key projects in Taiwan's Renewable energy development. Jacket foundation which has high stiffness and is well suited for the deeper water depth in Taiwan. The piles of jacket foundation are subjected to highly cyclic tension and compressive loading and the design of jacket type foundation for offshore wind turbine is mainly controlled by axial tension loading. Cyclic axial tension loads may cause degradation of pullout capacity due to the reduction of pile shaft resistance. This study presents a numerical model to simulate the behavior of pile under cyclic axial tension loads. The numerical model calibration was done by using the data from the field test results of Jardine and Standing(2000). The results from the field test and predictions made by the calibrated model based on the effect of pile driving and the calculation method of soil volume strain degradation were generally in agreement and simulated axial capacity degradation of piles under cyclic tension loads. This thesis used the methods of EAP (2012) along with the interaction diagram produced from this numerical method to evaluate the axial capacity of pile under cyclic axial tension loads and the result of this analysis showed that the increase of pile diameter, pile length and relative density of sand increased the pile capacity. The ratio of axial cyclic load amplitude had a more significant effect on the pile capacity which decreased with the number of loading cycles compared to the ratio of mean axial cyclic load.

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