隨著工程技術與施工品質的進步，樁基礎近來被廣泛應用於結構物基礎，其中多以群樁形式存在於建築物的主要基礎或輔助結構。決定樁基礎極限承載力的方法有很多種，樁載重試驗之結果最為可靠，但受場地與費用限制。而數值模擬分析正好能彌補其缺點，只要輸入合理的參數與條件便能有效求得近似解，了解複雜的樁土互制行為、樁與樁之間的相互影響、樁間土壤變化與群樁效應等。因此，本研究採用Abaqus有限元素軟體進行分析。由於二維軸對稱分析無法完全模擬真實情況，僅能反應簡化條件下的平面應變行為。三維數值分析可考慮土壤及基樁在三度方向上的位移，與不同位置之樁身受群樁效應的影響，符合實際之狀況。首先模擬場鑄樁的單樁行為，以此單樁分析的網格組成群樁分析模型，透過荷重位移曲線決定群樁承載力，計算群樁沉陷量，並研究群樁間距與群樁效應之關係。本研究之分析結論整理如下: (1) 在砂土與黏土中，群樁效應的產生主要是應力重疊造成中心樁的承載力下降所致。群樁效率隨樁距增加而上升，隨樁數增加而下降。然而砂土中的群樁效率會隨沉陷量增加而明顯變化；但黏土中的群樁效率與沉陷量關係較小。(2) 群樁在2倍樁徑間距的破壞模式會越接近整體基礎塊破壞模式，而非單樁承載力的總和。 (3) 砂土中，加載初期為群樁效應最顯著的階段。樁間土壤受到群樁擠壓造成的勁度上升，使中心樁的承載力在加載後期提高。 (4) 黏土在沉陷量達10cm之前，群樁效率維持不變，但黏土的群樁效應影響範圍較大，即使10D樁距仍有群樁效應。 In recent years, pile foundation develops very quickly, especially in high buildings and skyscraper. There are kinds of methods to determine the ultimate bearing capacity. The pile loading test is the most useful, but it is not economical and takes long time. Therefore, numerical analysis is suitable for simulation. The two dimensional axisymmetric analysis can’t fully simulate the real condition, it can only reflect the simplified condition of plain strain. Therefore, this study uses FEM software ABAQUS to analyzes the three-dimensional simulations. First, simulate the single bored pile, then apply the setting and mesh into pile groups. Second, set the three major factors (a) spacing (b) pile number (c) settlement to understand the relations with pile group effect. The analytic results evaluated from the FEM software ABAQUS show that (1) in sand and clay layer, the cause of group effect comes from the stress overlapped by piles. Group efficiency increases with the spacing increases, it decreases with the pile number increases. Group efficiency in sand has obvious behavior with pile settlement increases. But in clay, it only occurs when the load approaches to ultimate capacity. (2) When the spacing is 2D, pile group failure behavior will become to the whole block failure. (3) In sand, group effect is very obvious in the earlier stage of loading. The center pile’s bearing capacity will increase by the pile-soil pressing. (4) In clay, the group efficiency is small and almost the same before the pile settlement reaches 10cm. And the effect of range is larger than in sand.
