Based on the fact that the force is a vector, a new method for slope stability analysis is proposed, called Vector Sum Analysis Method (VSAM). In this method, the stability analysis is done on the basis of the current stress state of slope and its whole potential sliding direction. Without excessive assumptions, the stability safety factor of slope can be obtained with this method and is called safety factor of VSAM. It is defined as the ratio of projection value of vector sum of ultimate anti-sliding capability to that of current mobilized forces to the whole potential sliding direction, and the key problem is the determination of whole potential sliding direction, which depends on the distribution of maximum anti-sliding shear forces on the potential slip surface. At the end of this paper, two classical examples of 3D slope are used to verify this method. The calculation results show that the safety factors of VSAM are well comparable to that of Limit Equilibrium Method (LEM). There has been a successful attempt about the application of VSAM in 2D slope stability analysis. So it is natural to extend it to three-dimensional analysis. Compared with the traditional methods of 3D slope stability analysis, the new method has two advantages:One is that there is a definite physical meaning and simple calculation, which is especially simple for 3D slope analysis. And the other one is that the safety factor is obtained through the current stress state without excessive assumptions unlike that of LEM and Strength Reduction Method (SRM). So, the application of this new method has a good future.
Since the first literature had been published by Fellenius, which was about slope stability analysis. There have been lots of research articles related to slope stability. Nowadays, a number of methods are available for assessing the stability of slopes, and these methods are the limit equilibrium methods(LEM) of slices, finite element method, limit analysis[3-4], The calculus of variation approach, boundary element method, and so on. Frankly speaking, the conventional limit equilibrium methods and the finite element-strength reduction technique are commonly utilized to obtain the safety factor of slope, especially in three-dimensional slope stability analysis.
The limit equilibrium method (LEM) is widely use by engineers and researchers and this is a traditional and well established method. Although the LEM does not consider the compatibility condition, and the constitutive relation of the material, it can approximately provide the safety factor of a slope with many unknown variables. Due to simplicity and ease of use, LEM are the most commonly used methods among others. Duncan  gave a comprehensive review on the limit equilibrium methods and the status of the finite element slope stability analysis in the period of 25 years up to 1996. As is well known, the stress distribution of inter-slice force is a statically indeterminate problem and it is quite difficult to obtain the statically determinate solution under no assumptions.