植生護坡利用植物的根系根力以達提供邊坡土壤補強效應作用，然而植物根系是有生命的，根系網絡的發展隨種植成長期齡而漸長。本研究以本土性生態植物－五節芒為例，於現地採樣不同高度的五節芒根系，進行根系幾何資料統計，並利用碎形觀念計量根系分佈複雜度之發展，以瞭解根系隨時間之成長演變行為。另一方面，現地採取根繫土壤試體，進行根繫土壤之室內直剪試驗，以求得根繫土壤之剪力強度參數；並於實驗室模擬根繫土壤，製作不同根量之試體進行直剪試驗，可控制土壤根量及根分佈之位置、角度，探討根量與剪力強度及其參數之關係。本研究目的主要藉由五節芒生長期長短求出地下根系複雜度，並探討根量與土壤強度增加關係。 研究結果得致以下結論：(1)五節芒幾何參數與碎形參數之關係可以雙曲線模式預測之，碎形參數會隨草高、莖徑、根量增加而漸大，但均有一極限值，顯示根系補強土壤在生長到某一時期後將趨於固定不再增加，顯見地下根系網絡分佈特徵確可透過五節芒幾何參數予以評估。(2)根據地下根系幾何參數根寬幅、根深度與碎形參數之關係，可由碎形參數推估根寬幅、根深度，應用於工程上可約估得知植生間距及加勁深度。(3)由實驗室模擬含根土壤直剪試驗中發現，根於土壤中之生長方向與剪力方向，可分為有效根及無效根。根與剪力方向呈順向放置時，根明顯有效提供土壤抗剪強度，此方向之根則為有效根，無效根則反之。(4)直剪過程中根與土壤顆粒間之行為，上下固定之有效根受剪時，根直接受拉力，此拉力顯現在剪力強度上，試體剛開始受剪力時，剪力強度參數凝聚力、摩擦角皆會增加，而後摩擦角趨於固定不再增加，只增加凝聚力。 The stability of soil slope is reinforced by the vegetation roots. However, the root length and root system are gradually increased and complicated during the vegetation growth. In this research, Miscanthus floridulus samples were collected in various heights. The geometric of root system are measured first. The fractal dimension to demonstrate complexity of root system to counted for Miscanthus floridulus samples in different height. On the other hand, the direct shear tests of rooted-soil are performed to obtain the shear strength parameter. In addition, the samples of compacted sand with roots in different arrangements are also sheared to study the shear behavior of rooted soil. The main purpose of this research is to study the shear behavior of Miscanthus floridulus rotted-soil in different growth period. Some conclusions are drawn as following: (1) The hyperbolic model can capture the relationship between Miscanthus floridulus geometric parameter in height and fractal parameter of root system complexity. The fractal parameter of root system increases gradually with plant height and tend to a limit value. This implies that the complexity of roots and the soil reinforcement will keep constant finally. (2) According to the relationship of root system in height and its fractal parameters, we can estimate rooted effect in different time period. (3) The root reinforcement effect in soil is positive when the roots are forced in tension during shearing movement. (4) The internal friction angle and cohesion of rooted soil increase gradually in the beginning of shearing movement of root and soil particle. Then, the internal friction angle will keep constant and the cohesion of rooted soils increases due to the root deformation by tensile stress.
