軟弱地盤中樁筏基礎構造之靜態力學行為

Static Behaviors of Piled Raft Foundations in Soft Ground Sites

林于茹(Yu-Ru Lin)

指導教授 ： 張德文

淡江大學/工學院/土木工程學系碩士班/碩士(2016年)

https://doi.org/10.6846/TKU.2016.00887

摘要

本研究係採三維有限元素程式MIDAS/GTS NX模擬版式與箱式筏樁基礎於垂直向和水平向靜態均佈荷載下之力學行為。樁筏基礎數值模型係位於砂土-黏土-砂土所組合之地層，土壤材料參數及結構參數變化對基礎靜態行為之影響為觀察重點。所選擇的力加載方式包括：1.依時性壓密分析、2.階段施力(排水參數)分析、3.階段施力(不排水參數)分析，其中依時性壓密分析與階段施力(排水參數)分析中，黏土層採用修正劍橋黏土模式進行模擬，砂土層則以莫爾庫倫破壞模式進行模擬；而階段施力(不排水參數)分析中，黏土層與砂土層皆以莫爾庫倫破壞模式進行模擬。透過不同分析，本研究可以了解相關基礎在短期和長期受力情況下的差異性。
研究結果顯示：1.依時性壓密分析與階段施力(排水參數)分析結果類似(可視為基礎長期行為)，基礎沉陷量隨超額孔隙水壓消散將逐漸趨於一致；在階段施力(不排水參數)分析中(基礎短期行為)，基礎變形、沉陷以及軸力等隨參數變化較不明顯。2.版式與箱式筏基沉陷量皆以筏基中心最大、邊緣次之，角隅最小；應力分佈則以角隅最大、邊緣次之，中心最小(基樁亦然)。3.箱式筏基雖降低沉陷量，但因樁長不變樁底接觸砂層，基樁軸力和摩擦力將高於版式筏基。4.土壤模式和土壤參數影響將隨模式而不同，莫爾庫倫模式之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明顯；修正劍橋模式中楊氏係數與柏松比影響相對不明顯，模式之壓縮參數對於版式基礎行為影響較為顯著。5.各影響參數對箱式筏基影響將低於版式筏基，其中，土層厚度、樁長和樁距樁徑比值(S/D)最明顯。6.軟弱黏土層係夾於砂土層間，其承載能力將優於單一均質黏土層。版式筏基在依時性壓密分析中垂直與水平荷載下之樁基礎佔總荷載比例(α)相似，約為55%與60%。箱式筏基在垂直與水平荷載下之樁基礎佔總荷載比例(α)較版式基礎低，約為34%與57%。7.當樁距樁徑比值增加時，雖然樁基礎佔總荷載比例(α)將隨之下降，但單樁承受之軸力增加，將不利於基礎內承載性能評估。8.垂直均佈荷載下，短期分析顯示箱式筏基之樁基礎佔總荷載比例(α)將較版式筏基礎為小；由於砂層和黏土層對樁長將有影響，上述現象是否與筏基周遭土層有關值得進一步研究。

關鍵字

樁筏基礎 ； 版式與箱式基礎 ； 靜態分析 ； 參數研究 ； 三維有限元素分析

並列摘要

This study intends to discuss the vertical and horizontal load on static behaviors of piled raft foundations using the three-dimensional finite element analyses based on the software MIDAS/GTS NX program. The numerical models respectively with plate raft and cellular raft underlain by a number of piles were considered. Sand-clay-sand sols are assumed for the ground site. Three types of the static analysis were considered which includes: a. Time-dependent consolidation, b. Stage load drained, and c. Stage load undrained condition. In time-dependent consolidation and stage load drained analyses, Mohr Coulomb model and Modified Cam Clay (MCC) model are respectively used for sand and clay. For staged loading undrained case, only Mohr Coulomb model is adopted. The different analyses were aimed to simulate the long-term and short-term foundation behaviors.
This study finds that: 1.The time-dependent consolidation analysis and the staged loading drained analysis will yield similar long-term results when excess pore pressure fully dissipates. The effects of the influence factors became relatively unimportant for short-term condition. 2. The foundation settlements are in the order where center>edge>corner; loads distributed at the foundation are the opposite. 3. The foundation settlement becomes smaller when cellular raft is encountered. However since pile length is remained, the stresses of piles were increased due bearing sandy layer at bottom of the piles. 4. Soil model and soil parameters in use will affect the results. Elastic constants are more important than strength parameters when Mohr Coulomb model is used. For MCC model, the compressibility parameters are typically important. 5. Thickness of the soil layers, pile length and pile-to-pile spacing ratio (S/D) are the most significant factors; and they are more important in the cases of plate raft. 6. The resistance of sand-clay-sand site is higher than that of a single layer of soft clays. For long-term analysis, vertical and horizontal loads carried by the piles are respectively 55% and 60% of the foundation loads for plate raft, whereas the loading ratios become 34% and 57% in the case of cellular raft. 7. As S/D increased, the loads carried by piles decreased, however the internal stress of piles will be enlarged in which the pile damages could occur 8. For short-term analysis, the loads carried by piles are also found smaller in the case of cellular raft. Since the results are indeed related to site condition, the interpretations should be careful.

並列關鍵字

piled raft foundation ； plate raft and cellular raft foundation ； static behavior ； parametric study ； three-dimensional finite element analysis

參考文獻

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