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  • 學位論文

矩形壁樁垂直承載行為之研究

The Bearing Behavior of Barrette Piles under Vertical Loading

指導教授 : 邱俊翔 陳正興

摘要


本研究藉由位於台北盆地及高雄市區中達極限破壞之壁樁載重試驗案例,完整分析矩形壁樁之垂直承載行為。所選壁樁試驗案例均加載至樁頭位移大於10%壁樁寬度(B),並根據埋設於樁體內不同深度之鋼筋計應變,計算出各土層之完整t-z曲線及q-w曲線,用來探討矩形壁樁之樁周摩擦阻抗及樁底阻抗特性。 根據矩形壁樁極限載重試驗所歸納得到之t-z曲線,除了含有礫石之地層外,無論是黏性土壤或無凝聚性土壤主要均呈位移軟化現象,其樁周摩擦阻抗隨有效覆土壓力增大而增加,尖峰摩擦應力大約發生於位移20mm處;於較大位移時則呈現小幅度軟化現象,在位移達0.1B時之殘餘摩擦應力與尖峰摩擦應力之比值約介於0.81~0.92。此外,整理樁周摩擦阻抗與標準貫入試驗N值及不排水剪力強度su之關係,並與工程實務中常用之經驗公式進行比較,結果顯示,使用國內建築物基礎構造設計規範中之經驗公式估算壁樁之樁周摩擦阻抗時,其結果可能均偏向保守。 比較台北華中橋案例中有、無施作樁底灌漿壁樁之試驗結果,可量化樁底灌漿之成效,結果顯示,樁底灌漿確能改善樁底土壤之承載性能,有效提高樁底之極限承載能力,其對本案例中長樁之樁頭荷重-位移(Q~S)曲線的初始勁度幾乎没有影響,然而於樁頭承受大荷重及位移情況下,樁底灌漿明顯提高樁頭反應曲線之塑性勁度及整體極限承載力。 為簡化模型方便工程應用,可將t-z曲線簡化為完全彈塑性模式,q-w曲線則簡化為彈塑性雙直線模式;利用此簡化雙線性模式進行試樁曲線之反算分析,其模擬結果與現場試驗數據相當一致。此外,本研究建議亦可將樁頭Q~S曲線簡化為雙線性模式,此雙線性基樁垂直承載模式簡單且具代表性,方便應用於群樁分析或樁筏複合基礎之土壤-結構互制分析使用。

並列摘要


A comprehensive analysis for the bearing behavior of barrette piles is carried out based on ultimate load tests on piles located in the Taipei Basin and Kaohsiung City. All piles are loaded to their ultimate conditions with pile head displacements larger than 10% of the barrette’s thickness (B). Based on the strains measured at several depths along the pile shaft, complete side resistance t–z curves for various soil strata and end bearing q-w curves are obtained for detailed investigations. The characteristics of side resistances for cohesive and cohesionless soils are then quantitatively determined. Except for soil strata with gravel, most of the t–z curves exhibit a deflection-softening behavior. The peak resistances occur at a local pile displacement of approximately 20 mm, and the residual strength at a local pile displacement of 10% of the thickness of the barrette pile will be slightly reduced to a ratio of 0.79–0.92 of the peak strength. The side resistance generally increases with the effective overburden pressure. In addition, the relationship between the side resistance and the soil’s SPT-N values, as well as the undrained shear strength, are calculated and compared with the empirical formula commonly used in engineering practice. Results show that the empirical formula defined in the Design Specifications of Structural Foundations in Taiwan are conservative for estimating the side resistance of barrette piles. Comparing the test results of piles with and without base grouting in the Taipei case, the effects of base grouting can be quantitatively evaluated. Results show that the base grouting has almost no influence on the initial stiffness of the pile head response curve (Q-S curve) for such a long piles, but significantly increases the ultimate bearing capacity and plastic stiffness of the pile under large pile head displacement. Based on the t-z and q-w curves obtained in this study, it is suggested to simplify the non-linear curves by corresonding bilinear models for the convenience of engineering applications. Results of back analyses using the simplified model agree well with the load tests. In addition, the bilinear models for load-deflection curves at pile head as well as the corresponding equivalent stiffness are suggested for the analysis of group piles or soil-structure interaction of pile-raft foundation.

參考文獻


蔡煜青 (2006),「壁樁垂直承載力試驗之案例分析」,台灣大學碩士論文,台北。
何樹根 (2000),“場鑄基樁施工之考量”,樁基工程,地工技術研究發展基金會,台北。
Decourt, L. (1995), "Prediction of load-settlement relationships for foundations on the basis of the SPT-N". Ciclode Conferencias Intern. "Leonardo Zeevaert", UNAM, Mexico, 85-104.
Lin, S. S., Wang, K. J., Hsieh, H. S., Chang, Y. H., and Huang, C. S. (2007), "Field testing of axially loaded drilled shafts in clay-gravel layer." Journal of GeoEngineering, 2(3), 123-128.
Lin, S. S., Liao J. C., Wang K. J., and Chang, Y. H. (2008), "Effect of tip grouting on capacity of shafts socket in gravel layer." Journal of the Southeast Asian Geotechnical Society, 39(1), 13-17.

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