基樁廣見於各項工程中，藉由樁身之摩擦力與樁尖的支承力，將上部結構物之荷重傳遞到深層較堅硬之土層或岩盤上，而樁身摩擦力之多寡與其所承受之側向大小有關。本研究主要利用改良式直接剪力試驗儀，進行樁－土界面之定體積摩擦試驗，且在試驗中改變預壓密壓力與過壓密比等因素，以探討基樁承受側向荷載後之表面摩擦特性。
試驗結果顯示剪動過程中，樁－土界面之有效正應力、摩擦比、有效應力路徑均受過壓密比影響。當土壤之過壓密比愈大，樁－土界面之有效正應力隨著界面滑動位移之增加而增加的量愈多。在不相同預壓密壓力但相同過壓密比之情況下，所得之摩擦比曲線趨近於ㄧ條曲線。然而樁－土界面之極限摩擦比則隨過壓密比之增加而增加。經由反覆加壓與解壓應力變化之試驗結果，可知反覆加壓與解壓應力變化之次數對樁－土界面之摩擦性質影響不明顯。根據定體積摩擦試驗與分析結果，本研究以過壓密比為參數，建立破壞前之摩擦比與界面滑動位移之關係曲線。

Pile foundation is very popular seen in various constructions. It transmits load from the superstructure by means of friction force around pile and bearing capacity of the pile tip to stiffer soil or rock bed in deeper stratum. However, the magnitude of friction force around pile foundation is related to the lateral load which was exerted by pile foundation. This study carried out a series of constant volume friction tests of pile-soil interface by means of a modified direct shear apparatus. By changing preconsolidation pressures and overconsolidation ratios in this study, it was possible to investigate the frictional characteristics of surface after the lateral load applied on the pile.
The results exhibited that the overconsolidation ratio affected effective normal stress, friction ratio, and effective stress path when shearing. If the overconsolidation ratio of soil was higher, the increasing of effective normal stress due to the increasing of sliding displacement of interface got higher. The friction ratio curves of pile-soil interface under different preconsolidation pressures but the same overconsolidation ratio exerted on the soil approximate to a similar curve. The ultimate friction ratio of pile-soil interface increased with the overconsolidation ratio. From the results of cyclic loading and unloading tests, they showed that the frequency of cyclic loading and unloading did not affect the frictional characteristics of pile-soil interface obviously. According to the analysis results of constant volume friction tests, the relationship between the friction ratio curve before failure and overconsolidation ratio is established for the behavior of surface friction.

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