本研究係採三維有限元素程式Midas/GTS模擬樁筏基礎在靜態荷載(垂直、水平和彎矩作用)下所產生的力和變形特性。研究以座落在均質黏土層和均質砂土層的簡易樁筏基礎(27m×23m×1.5m)為數值模型，改變樁距樁徑比值(S/D)、樁長筏寬比值(L/B)等結構設計參數、地層深度以及土壤材料參數，討論該基礎承受靜態荷載時的力學行為。黏土土壤模式使用修正劍橋黏土模式進行模擬，砂土則以莫爾庫倫破壞模式進行模擬。除基本參數影響分析外，並以前人所發表的群樁基礎力-沉陷關係驗證Midas/GTS分析的可信度，同時進一步針對本研究分析之結果與國際相關設計準則進行比較。 研究結果顯示：1. 黏土模式之參數影響較砂土層明顯，土壤參數選取將影響分析結果。修正劍橋模式以壓縮指數與預壓密壓力的影響較大，莫爾庫倫模式中土壤抗剪強度參數影響甚微，楊氏係數與柏松比影響相對較為明顯。2. 沉陷量的變化會隨樁距和樁長的不同而改變，並與一般工程的認知相似，間距愈密、樁數愈多、樁長愈長能降低沉陷量。沉陷量以基版中心為最大，邊緣次之，角隅最小；然應力分布則相反。3. 摩擦樁沉陷量較點承樁為大，點承樁為摩擦樁的百分之十。4. 群樁荷載和基礎荷載的比值隨S/D減少、樁長變長而放大。本研究所觀察的垂直向荷載比值約介於06~0.8，而水平向荷載比值約介於0.8~1。5. 彎矩作用影響亦是黏土層較為明顯，受壓側的位移量將較受拉側位移量大。6. 直接基礎差異沉陷量約為1cm，樁筏基礎則可減少約百分之五十的差異沉陷。7. 本研究數值模型之樁數和樁長對沉陷量影響較Katzenbach et al. (1998)所建議者為高，其原因係為本數值模型土壤參數設計值偏低所致。由於本研究所考慮者係以已完成壓密下的地盤結構瞬時行為為主要考量，為使軟弱地盤筏樁基礎能顯現其在長期荷載下的壓密沉陷行為，建議未來能以依時性壓密土壤模式模擬該基礎受長時力的行為。同時，建議使用土樁介面元素以使內力分布掌握更為確實。 This study intends to discuss the load and deformational behaviors of pile-raft foundations using the three-dimensional finite element analyses based on Midas-GTS program. A simple pile-raft foundation with a 27m×23m×1.5m (L×B×H) raft underlain by a number of piles is considered. The influences of the pile-to-pile distance (S) and the pile length (l) are monitored. Homogeneous soil layer consisting of either sands or clays is assumed on top of the rigid rock. Friction piles and end bearing piles can be thus studied. With the Mohr Coulomb model for sand and the Modified Cam Clay model for clay, rational model behaviors can be found. In general for the vertical loadings, it is found that the largest displacements on the order of 2~6 cm will occur at the center of the foundation, while the foundation edge will exert smallest displacements. In contrast, the loads transmitted to the corner piles are found larger than the side piles and much larger than the center pile(s). Vertical loads carried by the numerical piles are found approximately 60%~80% of the total load varying the ratios of S/D and l/B (where D is the pile diameter) whereas horizontal loads were found carried more by the piles. Frictions distributed at the upper parts of the piles will be much larger at the friction piles rather than those of the end bearing piles. Moments applied to the pile-raft foundation will cause tensile forces to some piles. The value of initial modulus of the soil is very sensitive to the results. In this study, the soft clayey site was assumed fully consolidated and a homogeneous ground was considered for foundation settlement due instantaneous load. For foundation behaviours including the consolidation effects along with the time, more rigorous simulations needs to be taken. Furthermore, the interface element (contact element) between soil and pile should be used for better solutions.
