水下樁基礎受震反應分析方法之改良－考量動態水壓力與地盤運動效應

Abstract

樁是離岸結構常見的基礎形式，受震時不僅受到結構慣性力之作用，亦受到動態水壓力及地盤運動之影響，其行為複雜，且直接以水－樁－土互制模型分析其反應相當耗時。針對此課題，本研究透過理論與數值分析探討樁受動水壓作用之反應，並發展了簡化動態分析模型，可考量動水壓與地盤運動效應，以完整模擬水－樁－土互制行為。本研究進一步探討以擬靜力方法分析樁在地盤運動作用下反應的適用性，並發展改良方法。 首先，本研究修正前人提出之理論解，用以分析水下樁基礎水中段之受震反應特性，並與數值模擬結果相互驗證，能同時適用於剛性和柔性基盤條件。利用此修正理論解進行參數研究，發現動態水壓力會降低共振頻率並放大樁頂位移的共振反應，而影響的程度隨著土壤勁度的增加而減少。在受震反應分析中，動態水壓力會顯著改變反應大小，但其改變的方向與地震反應譜有關，並不固定。 接著，本研究驗證附加質量法於水下樁基礎的適用性。附加質量法係將額外的質量附加於水下結構以模擬動態水壓力影響。本研究分別介紹了精確和簡化的附加質量分布之表達式，儘管兩種分佈並不相同，但在數值分析中皆能夠得到足夠精確的樁基礎受震反應。此外，本研究以修正理論解作為驗證目標，評估簡化附加質量分布在不同條件下的適用性。將經過驗證之簡化附加質量分布應用於模擬完整的水下樁基礎之受震反應，發現地盤運動會降低水對樁反應的影響。 最後，本研究改良分析樁受地盤運動作用下之反應的擬靜力方法。針對不同地震類型、樁頭束制與土壤條件下，評估幾種常用決定特徵地盤位移剖面方法的適用性，然而沒有能適用於所有條件的方法。因此，本研究提出基於地盤在不同深度處之尖峰相對變形或旋轉之多特徵地盤位移剖面方法。考慮加載順序後，此改良方法在單層黏土層、強度遞增或夾弱層的三層黏土層中皆有良好的表現。然而，在砂土層中可能因阻尼力過大而低估樁在自由樁頭條件下的剪力反應。

Piles serve as a common foundation for offshore structures, enduring inertial forces, hydrodynamic pressure, and ground movement during seismic events. This study employs theoretical and numerical analyses to investigate the response of piles under hydrodynamic pressures and simplifies the dynamic analysis model considering both hydrodynamic pressures and ground movement effects. Furthermore, the study examines the applicability of a pseudo-static method for analyzing pile response under ground movement and proposes an improved method. First, this study modified theoretical solutions to examine seismic responses of underwater piles, validated by numerical analysis under both rigid and flexible base conditions. Parametric studies reveal that hydrodynamic pressure decreases resonance frequency and increases pile-head displacement, with reduced impact as soil stiffness increases. The magnitude of seismic response is influenced by hydrodynamic pressure. This study also verified the added mass method for underwater piles, in which additional mass is applied to simulate hydrodynamic pressure. Both precise and simplified added mass distributions yielded good seismic response prediction in numerical analyses. Application of the simplified distribution to complete underwater piles indicated that ground movement diminishes the water’s influence on pile responses. Lastly, this study improved the pseudostatic method for assessing ground movement effects on pile responses by evaluating numerous characteristic ground displacement profiles under various conditions. Since no single method suits all conditions, this study developed a multi-characteristic profile method. This improved method forecasts dynamic responses in various soil conditions more accurately than previous methods. Keywords: Pile foundations, earthquake excitation, hydrodynamic force, ground movement, pseudostatic analysis.