點承與摩擦基樁負摩擦力之模型試驗

本研究利用中央大學自行設計之基樁負摩擦力量測系統，探討樁體在工作載重條件下，因鄰近地表覆土壓力增加而形成之土層壓密，所造成樁身承受負摩擦力之現象。並變化點承土層之性質，以模擬點承樁與摩擦樁之負摩擦力發展情形。試驗過程以循環加載方式逐步增加及減少覆土壓力，並利用樁體各深度黏貼之應變計電阻值的變化，可得知樁身軸力分佈情形進而推算樁表面單位面積摩擦力隨深度的變化、下拉荷載隨壓密時間的發展情形，以及負摩擦力因土體回漲而減少的情形；並以樁頂、土表沉陷量配合壓密理論來推估樁、土相對位移與摩擦力之間的關係。由試驗結果得知，點承樁所受下拉荷載遠大於摩擦樁，承載砂層在樁體產生有限沉陷時即可發揮承載效果以承受下拉力與工作載重；而摩擦樁試驗中，由於樁體產生較大沉陷量，而拉高中立點位置，使負摩擦力影響範圍減少。由於摩擦樁受下拉力造成的軸力增量遠小於點承樁之情形，故建議在需求承載力不高時，可藉由摩擦樁設計方式，減少因下拉力而提昇的樁身強度要求，所造成的工程成本的增加。

關鍵字

模型試驗；中立點；樁基礎；負摩擦力

並列摘要

Negative skin friction is one of the most common problems in the design of pile foundations in soft soils, which is governed by soil slip behavior at the pile-soil interface, and highly depends on friction coefficient of interface, surface loading, and axial load. The research designed two types of long-term model pile tests under working load situation, point bearing pile and friction pile, to discuss the mechanism of negative skin friction in different kinds of end bearing layers when its surrounding ground consolidated in multi-stage, which is cyclically added on the ground surface. From the research results, the neutral plane of point bearing pile appears at about 85-90% of the compressible layer, and the incremental axial force of pile rises to 50kg when the surcharge increases to 1kg/cm2. In case of friction pile, the settlement of pile is larger than the one of point bearing pile. The neutral plane obviously moves downward as the surcharge and compact time increases, the location of which ranges between 30 to 50% of the pile length. The incremental axial force of pile rises to 14kg when the surcharge increases to 1kg/cm2. Due to negative skin friction highly developing in point bearing pile, it is suggested to adopt fiction pile for lowering construction cost when the construction only needs low demand of bearing capacity.

並列關鍵字

negative skin friction ； piles ； downdrag ； neutral plane ； model test

參考文獻

5.謝依航，「基樁負摩擦力之模型試驗」，碩士論文，國立中央大學土木工程學系，中壢(2006)。

17.Alonso, E.E., Josa, A., and Ledesma, A., “Negative Skin Friction on Pile：A simplified Analysis and Prediction Procedure.” Geptechnique, Vol.34, No.3, pp.341~357 (1984).

20.Bjerrum, L., Johannesson, I.J., and Eido, O., “Reduction of Skin Friction on Steel Piles to Rock” Proceedings of the 10th International Conference on Soil Mechanics and Foundation Engineering, Vol.2, pp.27~34 (1969).

22.Comodromos, E.M., and Bareka, S.V., “Evaluation of Negative Skin Friction Effects in Pile Foundations Using 3D Nonlinear Analysis” Computers and Geotechnics, Vol.32, No.3, pp.210~221 (2005).

26.Fellenius, B.H., “Results from long-term measurement in piles of drag load and downdrag” Canadian Geotechnical Journal, Vol.43, pp.409~430 (2006).

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點承與摩擦基樁負摩擦力之模型試驗

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