本研究利用南投縣眉溪中上游之河砂作為試體使用，進行尺寸為長300mm、寬300mm、高250mm之兩種反覆直接剪力試驗，不同單位重γ= 14kN/m^3 、15kN/m^3 、16kN/m^3、不同圍壓與不同水平剪動位移之反覆直接剪力試驗、與不同單位重不同圍壓但固定剪動位移反覆剪動40次之反覆直接剪力試驗，從而得到各個實驗之剪應力與水平位移之關係等數據，再根據實驗數據分別求取兩模型(修正R-O模型、修正H-D模型)所需之參數回歸公式，並依此回歸公式分別建立兩模型，再與先前所作之實驗數據繪圖作比較，用以預測土壤之反覆剪動行為。
本研究根據實驗結果與模型預測結果得知：一、乾砂直接剪力試驗之尖峰剪應力會隨著圍壓的增加而增加，也會隨著單位重之增加而增加，其尖峰剪應力發生時之位移量也隨圍壓增加而增加。二、變位移之反覆直接剪力試驗之剪應力會隨著圍壓上升而增加，也會隨著位移增大而增加，也會隨著單位重之增加而增加，呈現正相關，而體積變化的部分可以觀察到隨著圍壓增加與單位重之增加，試體體積的總壓縮量也會隨之增加。三、定位移之反覆直接剪力試驗顯示土壤剪動會有硬化之情形但此硬化現象有一定的限制，其會與圍壓呈現正相關之關係。四、修正H-D模型在剪動變位較小試驗之模型預測很好，但剪動變位較大試驗之模型預測有明顯的差距，但若將預測剪動位移加大至過尖峰值後的情況下，修正H-D模型卻有很好的預測量。五、修正R-O預測模型在尖峰值前剪動變位較小試驗有很好的預測結果，但若將預測剪動位移加大至過尖峰值後的情況下，修正R-O預測模型反而會有誤差量的產生。

Cyclic direct shear tests on a river sand were carried out to study the relationship between shear stress and the shear displacement by using a medium-scale direct shear test apparatus. Two models, namely, the modified Ramberg-Osgood (R-O) model and the modified Hardin-Drnevich (H-D) model, were used to simulate the behavior of sands subject to direct shear. Model parameters were derived based on curve fitting techniques, and the model-generated stress-displacement relationships were compared with the experiment ones. Results of cyclic direct shear test showed that strain hardening is valid up to a shear displacement of about 10mm, regardless of the number of cycles and the density of sands.
The modified H-D model satisfactorily simulates the behavior of sand subjected cyclic shearing under relatively small displacement states. This is not true when using modified R-O method. Under large displacement condition, (shear displacements larger than 10mm), the modified R-O method outperformed the modified H-D method in the sense that the modified R-O method rendered hysteresis loops similar to those obtained in the tests.

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