以耦合壓密有限元素分析探討基樁之負摩擦力行為

本研究採用有限元素軟體ABAQUS探討基樁受負摩擦力之行為，以耦合排水分析模擬超額孔隙水壓隨時間消散，土壤壓密沉陷對樁引致之負摩擦力。首先探討兩種土壤組成模式(Porous elastic+Modified Drucker-Prager/Cap與Porous elastic+Critical state plasticity)模擬黏性土壤行為之適用性。繼則利用前人在曼谷黏土中所施作之負摩擦力現地試驗，採用Porous elastic +Critical state plasticity模式模擬樁基礎周圍土壤以建立負摩擦力分析模型。最後以所建立之分析模型進行一系列參數研究，探討在土壤滲透係數、樁土間界面摩擦係數及門檻剪位移量、地表載重、樁頭載重、群樁效應等不同樁土系統條件下樁受負摩擦力之反應。樁頭荷載對於不同承載型式樁的負摩擦力行為有不同的影響。對於摩擦樁，樁頭荷載會降低樁所受的負摩擦力，其降低幅度隨著樁頭力愈大而愈顯著，中立點深度約在0.3-0.7倍樁長處。對於座落於堅實砂土層之點承樁，樁頭受載亦會使樁所受的負摩擦力降低，其降低幅度小於摩擦樁，中立點位置約在0.6-0.8倍樁長處。對於座落於極堅硬層上方之理想點承樁，由於中立點位置位於樁底，樁頭載重對其負摩擦力行為的影響不顯著。考量樁之使用性，樁頭變位也是重要的設計因子。雖然對於摩擦樁，樁頭荷載能大幅減少負摩擦力大小，但樁頭變位亦是前述三種承載型式的樁中最大者，且在較大的樁頭力作用下，亦可能發生承載破壞。考量樁之使用性及所受之軸力大小，對於座落於堅實砂土層上方之點承樁，當受到樁頭力，一方面會有負摩擦力減小的效果，另一方面，由於底層土壤較黏土層具有較高之勁度與強度，因此較摩擦樁具有較小的樁頭變位，且在較大的樁頭力情況下樁底較不會發生承載破壞。

關鍵字

樁基礎；負摩擦力；耦合壓密分析；有限元素法；中立點

並列摘要

In this study, the finite element software ABAQUS is used to investigate the behavior of piles subjected to negative friction. Coupled consolidation analysis is used to simulate the dissipation of excess pore water pressure of soil with time and the soil consolidation-induced negative friction. Firstly, the applicability of two soil constitutive models (Porous elastic+Modified Drucker-Prager/Cap and Porous elastic+Critical state plasticity) to simulate clay behavior is investigated. Then, field negative friction tests on Bangkok clay are utilized to build the negative friction analysis model, in which the soil is modeled using Porous elastic+Critical state plasticity model. Eventually, the established analytical model is further applied to carry out a series of parametric analyses for exploring the negative friction behaviors under different pile-soil system conditions, including soil permeability coefficient, friction coefficient and threshold shear displacement of pile-soil interfaces, soil surcharge, pile-head loading and group pile effect. The application of pile-head loading has different effects on negative friction behavior for piles with different end-bearing types. For friction piles, the pile-head loading will reduce the negative friction force, and the drgree of reduction is significant with increasing pile-head loading. The depth of neutral point is located between 0.3-0.7 times the pile length. For piles whose tip is on dense sand, the negative friction reduction is less than for the friction piles, and the depth of neutral point is located between 0.6-0.8 times the pile legth. For piles whose tip is on a hard layer, the pile-head loading has no significant effect since the neutral point is located at the pile base. Considering the serviceability of pile, the pile-head displacement is also an important design factor. Although the pile-head loading can greatly reduce the negative friction of friction piles, their pile-head displacement is the largest among the above three types of end bearing piles and they even have bearing failure under large pile-head loads. Therefore, considering the pile displacement and axial force, for an end bearing pile on dense sand, it will sustain smaller negative friction when subjected pile-head loading; on the other hand, compared to a friction pile, it has a smaller pile-head displacement and less bearing failure will occur under large pile-head loading because of larger stiffness and strength of the bottom soil than those of clay.