自1960年以來，各地的重大工程因負摩擦力之問題，導致工程危害與經濟損失，諸多學者以現地試驗法、室內試驗法和數值分析探討負摩擦力之行為。數值分析擁有低成本、重複利用性多等優點，運用合理參數與力學模式，模擬實際的施工過程，得到可靠且完整之分析結果。
本研究以FLAC3D有限差分程式進行分析，模擬三維群樁受負摩擦力之行為，並選用MIDAS/GTS程式建立群樁模型。研究首先以摩擦與點承兩種形式基樁，進行單樁受負摩擦力分析，探討樁之下拉力(Dragload)與額外的樁沉陷量(Downdrag)，並與FLAC2D之二維數值結果對照，驗證三維數值之正確性;群樁分析考量樁位置、加載荷重與樁帽之影響因子，探討群樁效應對下拉力與樁土沉陷量之影響;同時彙整高鐵資料，進行深層降水引致橋墩基礎受負摩擦力影響。研究結果整理如下:(1)加載初期，群樁效應的束制導致樁土位移量減少，越接近群樁中心，負摩擦力折減程度越高，特別以中心樁受群樁效應影響最大。(2)隨著地表荷重增加，群樁下拉力分佈逐漸接近，並與單樁之下拉力相近，顯示群樁效應逐漸消失。(3)樁帽束制對摩擦群樁有重要影響，對角落樁造成額外下拉力，使樁基礎破壞之可能性提高。(4)高鐵案例分析，當地下水位降低至52公尺，群樁之負摩擦力達到極限狀態，足以危害群樁之承載力。

Since 1960, negative skin friction (NSF) is one of the common problems in the design of pile foundations. In the past, several literatures reports can be found regarding the NSF on single pile and group piles in a compressible clay. NSF induces additional vertical load on the pile and pile settlement. it must to reduce the safety of the structure.

The objective of research is to investigate the dragload and downdrag on friction and end-bearing single pile with the finite different package FLAC3D. The simulation results of FLAC3D were compared with the results of FLAC2D to examine their difference. Group piles analysis considers the influence of various parameters including pile group configuration, position of piles in the group, surface loading and pile cap. The group effect are significantly influenced axial force and soil settlement. Finally, using FLAC3D to simulate the NSF of bridge foundations at High Speed Railway when the ground water was dropdown from the ground table to the lower elevation.

Based on the results and the comparisons from the analysis for group piles the following conclusions can be drawn: (1)Group effect causes reduction of soil settlement and dragload significantly on pile groups for the relatively small surcharge. The maximum reduction of dragload is obtained in central pile. (2)The magnitude of surface loading increases the group effect decreases. (3) Additional vertical load on friction corner piles was caused by fix-head cap.(4) In the THSR analysis case, when the ground water table was dropdown from the ground table to the depth of 52 meters, NSF would be limit stated to cause the maximum dragload on group piles. It might reduce the bearing capacity of the pile foundations seriously.

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