Abstract: According to imbalance thrust model, calculation format of stability coefficient was studied and the corresponding procedure of nubbly rock mass slope can be derived. It is very difficult to establish limit state equation under the condition of the format. An approximate method can resolve the problem. It is suggested that the sample data can be provided according to imbalance thrust analysis model which is used as a means of calculation and realized the smoothing function which takes rock mass mechanical parameters as independent variable as an approximate analytic expression. The limit state equation can be given according to the function and with the design point method slope stability reliability can be given easily. Finally, the stability probability of a project was analyzed by the above-mentioned method and the traditional mean deviations method respectively. Compared with the traditional mean deviations method, the approximate method can meet engineering requirement on accuracy and its calculation workload was decreased greatly. It is indicated that the approximate way is available.

Key words: Imbalance thrust, slope, multi-sliding surface, reliability