傾斜地盤中基樁側向荷載之非線性反應分析

Abstract

基樁承受側向載重時，因土壤及樁材之非線性效應使得其反應呈現高度非線性，以往多以水平地盤進行研究與探討，而對於傾斜地盤的影響方面則相對很少，仍有許多課題值得研究。本研究之主要目的為分析與探討傾斜地盤中基樁受側向荷載作用之非線性反應，同時考慮土壤非線性效應與樁土互制關係，包括樁身與土壤之相互滑動與分離，以探討傾斜地盤對基樁與土壤反應的影響。 為了解傾斜地盤中單樁受側向荷載時土壤可提供的極限側向阻抗，本研究首先利用樁周土壤破壞土楔模式推導一般c-ψ土壤之極限側向阻抗評估公式，經與前人針對純凝聚性土壤、純非凝聚性土壤之研究結果比較驗證，顯示所得評估公式對於水平地盤、下邊坡及上邊坡情況均可適用。並定義坡度影響係數為傾斜地盤土壤側向極限阻抗對應水平地盤土壤側向極限阻抗之比值，用以評估凝聚性土壤、非凝聚性土壤及一般c-ψ土壤中，地盤坡度對土壤極限側向阻抗之影響。 為探討傾斜地盤中基樁受側向荷載作用之反應行為，本研究利用數值分析法進行模擬分析，所用程式為有限元素法ABAQUS程式，採半對稱之三維實體元素模型，土壤破壞準則採Mohr-Coulomb模式，分別針對地盤坡度為0⁰、10⁰、20⁰與30⁰的情況，進行自由樁頭條件之單樁非線性側推分析，除驗證本研究所推導土壤極限側向阻抗理論評估公式外，並根據所得樁頭容量曲線(P-Y曲線)，探討地盤坡度對樁頭P-Y曲線之影響，包括容許側向荷載、0.01D割線勁度、0.1D割線勁度、降伏點與極限點等，評估地盤坡度所產生的折減效應，將其歸納為簡單折減公式供工程設計參考使用。 對於樁身與土壤之反應，則由數值分析得到的樁身側向變位，運用差分法推求所對應之彎矩及土壤反力分佈，建立樁頭附近土壤的p-y曲線，用以分析土壤彈簧勁度與土壤反力值隨所在深度之特性，並探討地盤坡度對土壤p-y曲線之0.01D割線勁度、降伏點與極限值的影響，也將0.01D割線勁度與極限值對地盤坡度的折減效應歸納為簡單公式。 在進行單樁之非線性側推分析時，本研究提出可同時考量基樁與土壤非線性之數值模式建立方法。考量溫克基礎模式，土壤採用非線性p-y曲線，對樁材非線性之模擬方面，除修正傳統ATC法中的塑性曲率定義，並推求估算塑性區最小範圍之理論公式，以及分佈塑鉸的佈設方式與數量，以正確模擬塑性區的發展情形。據此建立數值模式的側推分析結果與實體元素模式數值分析及現地試驗結果比較驗證，均獲得不錯的成果。 最後，以直樁式棧橋式碼頭為例，建立可同時考量地盤邊坡、土壤與基樁非線性以及樁土互制作用等影響的群樁溫克數值模式，依一般易損性分析程序進行側推分析、耐震性能評估、以及易損性曲線及其相關參數之建立，確認本研究所採用的分析方法與程序，可快速且方便的進行具群樁基礎結構物之耐震性能評估與易損性分析。

Pile foundations are used to support a wide variety of structures. In many cases, lateral loads are often the critical factors considered in the design of piles. The response of piles subjected to lateral loading is highly nonlinear due to the inherent nonlinear behavior of soils and piles. In many practical situations, structures subjected to lateral loading are located near or in the slopes or embankments. But, the research to examine the effects of soil slope on lateral pile capacity is limited. Therefore, this study aims at the nonlinear analysis for lateral response of a pile on sloping ground. To achieve the purpose, this study developed the analytical solution to estimate the ultimate soil resistance in the lateral loaded pile located in slope. The analytical solution is assumed passive failure wedge modal, and can be applied to normal type of soils. Furthermore, this study developed a simple, but rational, analytical model to simulate the behavior of single piles subjected to lateral loading. The analytical model adopted will be based on the Winkler hypothesis and p-y approach, which is commonly used in engineering applications. In this study, the non- linearity of the soil is simulated by applying nonlinear soil springs, while the nonlinearity of a pile is modeled by placing distributed plastic hinges in beam elements. The distributed plastic hinge model can completely capture the development process of plasticity in piles. Finally, this study proposed the procedures of the fragility analysis for pile-supported wharf case. In modeling a wharf structure, the nonlinearity of the soil and the piles should be considered to account for their effects on the lateral response of the structure. And the proposed fragility analysis procedure can successfully establish the fragility curves of pile-supported wharf structures from illustrative examples.