本研究以飽和砂土試體進行模型樁受長週期反覆軸向載重試驗，以觀察樁土反覆軸向承載之互制行為，目的在模擬離岸風機套筒(Jacket)群樁基礎長期承受上部結構重量及週期性波浪作用力，對樁基礎產生週期性的垂直軸向加載力及卸載之行為，此項承載行為將造成樁基礎產生差異沉陷，使風機逐漸傾斜，影響風機系統長期運轉之穩定性。
施加反覆載重之前，需先進行靜態極限抗壓試驗，得到靜態極限抗壓力Pu。反覆樁載重試驗之初始條件為模擬風機基礎於自重作用下安全係數為3的情況，然後再進行單向反覆軸向載重試驗，總共進行6組，3組承受反覆軸向先加載後卸載型式，3組承受反覆軸向先卸載後加載型，3組反覆荷載振幅分別為1/3Pu、1/6Pu及1/12Pu，反覆作用週數依序為10000次、10000次及20000次。
綜合前人乾砂反覆載重試驗之研究結果，均顯示基樁受較大的反覆軸向振幅下，樁頭等效勁度隨著反覆作用次數衰減明顯，樁頭位移增加速率亦越大。相同振幅之先加載後卸載、先卸載後加載試驗，乾砂試驗基樁先加載後卸載型造成之殘餘位移較大，而濕砂試驗則是先卸載後加載型影響大。

In this study, a model pile is used to perform the long-term cyclic axial load tests in saturated sands, and the behavior of the pile-soil interaction is observed. In order to simulate jacket foundation of offshore wind turbine is long-term subjected to the upper structure weight and periodic wave force. The long-term cyclic axial pressure and tension forces are behaved on the pile, causing differential settlement for pile foundation and slowly tilting of the bottom of wind turbine, affecting the stability of the fan system long-term operation.
Before the cyclic load test is performed, static ultimate compressive load test is carried out, and then gets static ultimate load P_u. Loading static load 1/3 Pu initially owing to considering the static loads safety factor of the offshore wind turbine equal to 3, before the one-way cyclic axial load test carried out. A total of 6 sets cyclic axial load are carried out, 3 sets are performed cyclic compression type, and other 3 sets performed cyclic tension type. Three sets cyclic axial load amplitudes are1/3Pu, 1/6Pu and 1/12Pu respectively, and the number of cyclic cycles are 10000, 10000 and 20,000, respectively.
When comparing with previous studies done by dry sands, these are performed cyclic axial load tests show some of same results. Long-term cyclic axial load tests show that the stiffness of pile head decreases obviously owing to large of axial amplitude. And the incremental rate of pile head displacement velocity faster when axial amplitude large.
Under the tension and compression load tests of the same amplitude, the residual displacement caused by the long-term cyclic axial load tests. When cyclic axial load tests in dry sands, the residual displacement are larger in condition that first pression then tension. In contrast, where tests in saturated sands, the residual displacement are larger under the first tension then pression conditions.

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