植生護坡利用植物的根的錨定力達提供提高邊坡淺程滑動之穩定性，係透過根的拉伸作用以達到提伸根系土壤的抗剪強度。故本研究針對現地五節芒進行採樣，在了解根主要的抗拉特性，進而得到探討不同根徑對於抗拉強度之碎形關係；而也進行室內直剪試驗，以求土壤內根含量與土壤剪力強度之關係，並探討根繫土壤於破壞過程中，根與土壤顆粒之間及其剪力強度參數凝聚力(c)、摩擦角之變化；另一方面同步進行現地十字片剪試驗，並於試驗後取回完整現地草根，透過影像處理量化五節芒根量，對應十字片剪試驗結果加以討論。 研究結果得致以下幾點結論：(1)氣乾草根的吸水程度之吸水率(Q)於第八天達到飽和狀態，而草根強度在提升吸水率(Q)對強度沒有明顯影響。(2)根徑的拉伸行為皆為延性行為，隨根徑越粗其斷裂變形較大且勁度衰減率越高。抗拉強度隨著根徑增大而遞減，以雙對數表示其五節芒碎形關係有碎形維度關係，其表示五節芒抗拉強度隨根徑變化之影響。(3)不同根折角對土體加勁影響，當根折角大者較早產生抗拉作用，根折角小則較晚，現地草根於邊坡滑動為漸進式破壞早期根折角大者作用，後期根折角小者產生抗剪強度。而根數較多者(植生較久)可提高抗剪強度且破壞時機越晚。(4)根於土體中可分為拉出摩擦與拉斷破壞，根於土體中拉動初期受摩擦阻抗影響，隨根量增多在高應力下剪力強度提高，但只貢獻於強度參數c之上對強度參數φ沒有明顯貢獻，為拉伸拉斷機制；在極低正應力下反映在摩擦角φ上為拉出機制，故產生雙直線模式破壞包絡線。(5)探理論與模擬試驗之結果時，理論值與較高應力之試驗值接近，概因在Gray and Megaham(1981)剪力強度模式推導中，係假設根拉力已完全發揮拉力強度而斷裂。(6)隨草高的提升根量越大(Ar)，隨深度越深根量面積(Ar)亦變小的趨勢，相對片剪強度(Cu)隨隨根量增減所影響。 The stability of soil slope is reinforced by the vegetation roots. Therefore, this research starts with taking samples of miscanthus floridulus on the spot and then studies main tensile characteristics of roots aiming at obtaining the fractal relationship between various root diameters and the tensile strength. And also direct shear test should be done to achieve the relationship between roots content in the soil and soil shear strength. In addition, during the process of rooted soil being destroyed, the change between roots and soil particles, shear strength parameter cohesion(c) as well as friction angles is to be studied. On the other hand, spot vane shear test will be taken at the same time and when the test is finished the complete spot root should be taken out and by image processing the amount of miscanthus floridulus roots will be determined so as to discuss the test results. Some conclusions are deawn as following: (1) a saturation condition of air drying grass roots for the water absorption (Q) is reached on the eighth day and after that the improvement of absorption content exerts no obvious influence on roots strength. (2) All the root tensile behaviors are considered to be ductile which means the bigger the size is, the larger the fracture deformation is and the higher the stiffness attenuation is. The tensile strength decreases as the root diameter increases. And double logarithm shows a hyperbolic model in the miscanthus floridulus fractal relation which indicates changes of root diameters have an effect on tensile strength. (3) Different root fracture angles strengthen influence on the soil so that it is earlier to produce tensile stress when the angle is larger than that of smaller root fracture angle. The progressive failure acts on the early larger root fracture angle when spot grass roots slide along the slope, but for the smaller root fracture angle produces shear strength. So the vegetation with lots of roots (elder vegetation) can increase its shear strength and its failure moment comes later. (4) Two mechanisms on roots in the soil may be classified as one is called pull-out friction and the other is rapture. During the early pull-out period, frictional resistance affects roots in the soil and with the increasing amount of roots the shear strength improves. But the tension fracture mechanism only contributes to the strength above cohesion C and there is no obvious contribution to the friction angle; Whereas the pull-out mechanism can be reflected on the friction angle φ under very low normal stress, which produces bilinear model to destroy the envelope curve. (5) In the Gray and Megaham(1981) shear strength model deduction, the stress is supposed to be developed completely so as to fracture the roots. (6) The higher the grass height is the larger amount of roots (Ar) becomes while the deeper the root is the smaller the area of roots (Ar) covers. And the amount of roots has an influence on relative vane shear strength.
