利用直接剪力試驗來獲得土壤的工程特性是相當常見的方法，但一般直剪試驗盒不透明，不易觀察直剪盒內部在受剪過程的微觀破壞機制，且前人研究發現直剪試驗在剪動面應力分佈不均勻，因此本研究利用離散元素法，使用PFC2D軟體模擬直剪試驗。由於前人研究已探討無鍵結強度顆粒在不同微觀參數下對直剪試驗結果以及微觀力學機制的影響，因此本研究模擬具鍵結強度顆粒材料之直剪試驗，探討顆粒微觀參數對巨觀凝聚力與摩擦角的關係，並從中監測直剪試驗過程試體之應力分佈、孔隙率變化和鍵結斷鍵情形，分析剪動帶上的平均應力變異程度並觀察應力路徑與主應力平面的旋轉角度。
由直剪模擬結果可得知具鍵結顆粒材料之巨觀與微觀力學行為如下: (1) 顆粒粒徑越小則凝聚力越大、摩擦角越小，當鍵結強度越大時凝聚力也越大。(2)直剪模擬剪脹多來自於直剪盒右上及左下區域。(3)破壞時鍵結斷裂與剪應力極值多分佈於剪動帶上，且由整個試體觀察到張力破壞數目大於剪力破壞數目。(4)破壞時最大主應力面旋轉角度會介於50度到60度。(5)剪動帶上平均水平應力與垂直應力之變異係數越接近破壞則越大;剪應力之變異係數則越接近破壞越小越穩定。

Direct shear testing is among the most common laboratory tests for obtaining the engineering properties of soils. However, the microscopic behavior during direct shear test of this type of material is not always easy to monitor. Previous studies have found that the non-uniformity of the stress can be developed along the failure plane have been brought up by previous study. Therefore, a discrete element method is employed and the PFC2D is used to simulate the direct shear test in this study. Since prior researchers have been observed the influenced of the microscopic parameters and the microscopic behavior of the dry granular material during the direct shear test, I simulate the direct shear test by bonded-particle and investigate the related of the particles microscopic parameters and the macroscopic cohesion and friction angle in this study. I also monitor stress and porosity distribution, and bond break condition during direct shear test as well as analyze the variations of average stress, stress path and major principal plane along the pre-determined failure plane were also observed.
The following summaries are addressed: (1) smaller particles size has stronger cohesion and smaller friction angle, and greater bond forces have stronger cohesion; (2) the dilation of the overall particle assembly comes from the right of the upper shear box and left of the lower shear box; (3) most breaks of the bonds locate on the shear zone, and the number of normal bonds breaks exceeds that of the shear bonds breaks; (4) the directions of major principal plane ranges from 50 and to 60 degrees in the counterclockwise direction until the test specimen reaches failure; (5) the coefficients of variation of the horizontal and vertical stress become larger when near approaching the failure, and the coefficient of variation of the shear stress becomes smaller and more stable.

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