進行離岸風機水下基礎設計時，為進行不同設計載重組合下之整體動態反應分析，需考量不同載重條件下之基礎受力-變形反應，以建立合適之基礎勁度矩陣。對於埋置於無凝聚土壤中之樁基礎，當受到ULS條件之設計載重作用時，樁基礎之側向行為可以ULS設計載重等級之受力-變形曲線割線斜率作為基礎勁度；此時，樁週土壤之受力變形反應可透過靜態三軸剪力試驗之應力-應變曲線求得。當樁基礎受SLS條件之作用力時，同時需考量樁基礎受反覆作用力下之永久變形量；此時，樁週土壤受反覆應力作用下之永久塑性應變將反應於土壤應力-應變曲線之割線斜率。對於受到FLS條件下作用力之樁基礎，其受力變形反應近乎彈性。此時，樁週土壤於反覆作用力下之彈性應變將反應於應力-應變曲線之回彈模數。隨著樁基礎所受之反覆作用力持續增加，樁週土壤之回彈模數趨於定值。為了方便計算樁基礎受反覆側向作用力下之永久變形量及回彈變形量，本研究以動態三軸剪力試驗建立反覆作用力條件下之塑性應變與彈性應變關係，結合Kuo (2008)建立之勁度衰減模型，以及陳威廷(2021)之回彈勁度硬化模型，提出無凝聚性土壤於不同設計載重條件下之應力-應變反應變形模數計算建議，提供有限元素樁-土互制有限元素數值模型進行離岸風機基礎勁度計算應用。

In the process of designing underwater foundations for offshore wind turbines, in order to establish an appropriate foundation stiffness matrix, analyzing the overall dynamic response under various design load combinations requires considering the foundation's force-deformation response under different loading conditions. For piled foundations embedded in cohesionless soil, when subjected to Ultimate Limit State (ULS) design loads, the lateral behavior of the pile foundation's force-deformation curve can be used to determine the stiffness, employing the secant slope of the force-deformation curve at the ULS design load level. The soil-pile interaction response can be obtained through static triaxial shear tests to determine the stress-strain curve. Under Serviceability Limit State (SLS) conditions, in addition to the applied loads, the permanent deformation due to repeated loading must be considered for pile foundations. In this case, the permanent plastic strain of the soil surrounding the pile under cyclic loading will be reflected in the slope of the stress-strain curve. For pile foundations subjected to Frequent Limit State (FLS) loads, the force-deformation response is nearly elastic. The elastic strain of the soil surrounding the pile under cyclic loading will be reflected in the modulus of resilience of the stress-strain curve. As the cyclic loading on the pile foundation increases, the modulus of resilience of the soil surrounding the pile stabilizes.To facilitate the calculation of permanent and resilient deformations of pile foundations under cyclic lateral loading, this study establishes the relationship between plastic and elastic strains under cyclic loading conditions using dynamic triaxial shear tests. This is combined with the stiffness degradation model proposed by Kuo (2008) and the resilient stiffness hardening model by Chen Wei-Ting (2021). The study proposes recommendations for calculating stress-strain response and deformation modulus of cohesionless soil under different design load conditions, providing guidance for the calculation of foundation stiffness in offshore wind turbine applications using finite element pile-soil interaction numerical models.

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