傳統邊坡穩定分析中常用的Mohr-Coulomb破壞準則，是以尖峰強度(或殘餘強度)進行分析，但該式子中係假設為C與ϕ同時到達尖峰值(或達殘餘值)，即假設C與ϕ剪動隨即發揮，但由許多前人所試驗的的結果，得知C與ϕ並非為一固定值。 在本研究中試驗部分則使用直接剪力試驗，採用不同正向應力下相同位移的剪應力驗證「C thenϕ」法，分別以砂/黏性土等進行試驗，由同一剪力曲線分析得到「定值」法與「C thenϕ」法之C、ϕ，從中得到兩者的關係加以應用至FLAC邊坡穩定分析。 本文在試驗確認「C thenϕ」法現象後，以FLAC探討土壤邊坡穩定性，先利用「定值」法對STABL與FLAC確認內部行為之適用性驗證，再進行「C thenϕ」法的應用。分別由凝聚力或摩擦角主控土壤剪力強度行為之1.比較「定值」法與「C thenϕ」法2.在「C thenϕ」法中探討C 殘餘衰減時機、及延遲到達ϕp之發揮時機，在邊破內部逐漸加載破壞的過程與傾斜管位置之水平變位比較。 結果得致下列主要結論：(1) 「C then ϕ」法分析：C隨變形增加而衰減至殘餘，ϕ則隨變形增加發揮致尖峰，並非同時發揮強度。(2) 「C then ϕ」法的Ci,max大於「定值」法的Cp，而ϕ p(或ϕ r) 兩分析法所得值相同。(3) 由C衰減與ϕ發揮時機：「定值」法的Cp是「C then ϕ」法的Cr， 而ϕ p(或ϕ r)值相同。(4) 「C then ϕ」法分析之延遲摩擦發揮時機：黏性土壤摩擦發揮時機較晚。(5) C主控剪力強度時，「C then ϕ」法與「定值」法塑性區變化過程、水平位移監測差異較小，反之，ϕ主控剪力強度時，差異較大。(6) 傾斜管在坡趾、坡面、坡頂與上坡頂監測時，所測得兩分析法差異較大；而下坡面差異較小。(7) 「C then ϕ」法分析中若C衰減較快或ϕ p發揮較慢時臨界破壞面較快形成則較危險。 In slope stability analysis, the coefficient of friction angle and cohesion adopted in Mohr-Coulomb failure criterion are kept constant. The friction angle and cohesion are assumed simultaneously to reach the peak value. However, the experimental result show that the cohesion (C) is decreasing rapidly after shearing from the maximum value to a constant residual value. The friction angle is increased to the peak gradually. In this thesis, FLAC is used to study the slope stability by considering the degradation in cohesion strength and the mobilization in friction strength according to the plastic shear strain of slope deformation. The following conclusions are drawn: (1) Using ‘C then ϕ’ concept to obtain C and ϕ value during shearing, it is found that C value is decreasing with shear displacement, but ϕ value is gradually increasing. The value of C and ϕ is not always kept as constant during shearing. The shear strength of soils by shearing is contributed first by the cohesive effect between soil grains and followed by the grain friction effect. (2) The value of friction angle is mobilized to the peak (ϕ p) after several displacement, but the initial value of cohesion is the maximum. At the peak shear strength state, the friction angle of soils reaches to the peak and the cohesion decayed to a residual value. The delayed friction effect in shear strength for cohesive soil is more obvious than that for cohesionless soil. (3) The maximum value of cohesion intercept using ‘C then ϕ’ concept is greater than the value of cohesion used in Mohr-Coulomb criterion. However, these two peak values of friction angle are very close. (4) The lateral displacement of slope in FLAC modeling using ‘C the ϕ’ concept is greater than that keeping C and ϕ as constant value. The fracture propagation of slope to form the failure surface is remarkable in FLAC modeling using ‘C the ϕ’ concept. The safety factor of slope stability obtained by using the ‘C the ϕ’ concept is smaller than that keeping C and ϕ as constant. This difference is more significant in sandy soils with high fraction angle and low cohesion. However, there is little difference in clayey soils with high cohesion and low frictional angle.