Abstract: Slope stability analysis is a classical subject of soil mechanics, through which the safety factor of a slope can be obtained. However, the critical slip surface corresponding to the minimum safety factor is not necessarily the first failure surface to slide, while the one relating to instability acceleration possibly slides first. The theory of instability acceleration is applicable to all structures with free faces such as slopes and tunnels; for the calculations, we can use the conventional assumption of the limit equilibrium method, or a stress analysis via a numerical method such as the finite element method. Besides instability acceleration, traditional safety factors can also be calculated through minimizing the modulus of instability acceleration, a case similar to the special pattern recognition method. This paper sums up the historic evolution and main principles of the instability acceleration method. And we extend the theory to the calculation of relative instability acceleration and relative safety factors, relax its related constraints, and allow it to have a wider application prospect.

Key words: slope, stability analysis, relative instability acceleration, relative safety factor, tunnel, pattern recognition