Abstract:In view of the characteristic of complex slope,an improved method of selecting search variable is presented. The positions of two exiting points and arc height of the slip surface are taken as search variables,and assumptions aren¢t required. Divided-interval search has been introduced so as to locate the most dangerous slip surfaces and the next critical slip surfaces,of which the obvious advantage is employed to divide the search region,increasing the possibility of finding the global optimum. Chaos optimization method and improved complex method are combined to search the minimum safety factor and critical slip surface. Mixed search algorithm combines the merits of two algorithms,which can effectively avoid local minimum,and overcome time-consuming disadvantage of chaos optimization method. Mixed search algorithm is applied to searching the minimum safety factor of a complex slope;and the result shows that the rate of successful computation of basic complex method with improved search variable is 40%–60% higher than that with the current search variable. The minimum safety factor obtained by mixed search algorithm is ranging from 1.70 to 1.72. So mixed search algorithm is much more likely to find the global optimum of complex slope. And analyses of multi-step slope engineering example show that there are four next critical slip surfaces with small differences in safety factor;the relative difference is 7.6% in maximum,and 3.6% in minimum,but large differences in the location of critical slip surface.