Abstract: Relationships between the difference of 2D and 3D safety factor and parameters of landslide geometry (including ratio of slope, 3D sliding mass shape and length of landslide) and soil strength (including cohesion and internal friction angle) are studied. 2D and 3D safety factor of the different parameters homogeneous slopes are calculated based on the slope stability analysis program of the limit equilibrium method by modifying normal stress over slip surface. The ratio of 3D to 2D safety factor (denoted by F3/F2) is used to analyze the difference of 2D and 3D safety factor. Then, variation rule and mechanism of F3/F2 by changing parameters of landslide geometry and soil strength are also discussed. Finally, the standard of ignoring the difference of 2D and 3D safety factor are proposed from the view point of engineering practice. Results show that: F3/F2 decreases while ratio of length to height, internal friction angle and ratio of slope increases, and increases when cohesion increases; 3D safety factor is suggested to estimate the slope stability when (1) ratio of length to height is lower than or equivalent to 5; (2) ratio of length to height is more than 5 and lower than or equivalent to 10 while cohesion more than 25 kPa or internal friction angle lower than 15° or ratio of slope lower than 1. The conclusion can be referring to correct evaluation of slope stability.

Key words: slope, limit equilibrium, safety factor, 2D, 3D