本研究是以分離元素法探討單剪試驗下其試體尺寸效應與邊界效應之影響，以釐清試體中最大粒徑與試體尺寸之關係，作為進行粗顆粒土壤單剪試驗之參考。研究先建立三軸試驗模型與鋼珠三軸試驗比對驗證，以驗證程式對顆粒性土壤剪動性質模擬之可信度。邊界效應探討是以不同試體高徑比和兩種單剪模型來進行數值模擬，觀察試體中對應力與應變分佈均勻性的影響，而尺寸效應之目的為找出最大粒徑與試體尺寸之關係，以決定固定試體尺寸下合宜的最大粒徑。模擬結果顯示：(1)降低單剪試體之高徑比能有效減低試體的邊界效應；(2)不同型式之單剪模型在邊界效應上差異不大；(3)尺寸效應模擬顯示當單剪試體高度與最大粒徑比值(H/dmax)大於7時，其剪力強度保持不變，但(H/dmax)小於7時，顆粒之最大粒徑對剪力強度有明顯的影響；(4)由DEM單剪模型中之主應力分佈，顯示DEM可模擬單剪在剪動過程中其主應力旋轉之現象。

　　This study aims to investigate the size and boundary effects in direct simple shear test with Distinct Element Method (DEM), and clarify the relation between the maximum particle size and the specimen size for element test on soils with large particles. The numerical procedure was verified with results from triaxial models and corresponding laboratory triaxial test using uniform steel balls. To evaluate the boundary effect, spatial variations of stress and strain within the specimen is studied for different height and diameter ratios (H/W) and two types of direct simple shear apparatus. The purpose of the size effect, is to determine the relation between the maximum particle size and specimen size for consistent shearing responses. The result shows that : (1)the boundary effect can be reduced effectively when H/W decreases, (2)the difference in boundary effect is insignificant between the Cambridge and NGI types of simple shear, (3)a relatively constant strength is available when the ratio of specimen height to the maximum particle size (H/dmax) is no less than 7 ; otherwise the shear strength is significantly influenced with the maximum particle size, and (4)DEM can simulate the rotation of principle stress during simple shear shearing process.

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