In the traditional strength reduction method

only the minimum safety factor and the critical slide surface can be acquired as its reduction region covers the whole slope model. However

in the slope engineering practice

we should treat the slope area not only surrounded by the global critical slide surface

but also located in the vicinity of the secondary slide surfaces. Thus

a modified strength reduction method is proposed to overcome the shortcomings of the traditional strength reduction method in searching the secondary slide surfaces. In general

the yield regions of slope are always distributed on both sides of a slide surface

which presents as a shear zone. Thus

a slide surface and its safety factor can be obtained by reducing the strength parameters of local slope area located within the shear zone. Based on the above understanding

it is supposed that the shear zones are distributed along the logarithmic spiral. Shear zones at different locations and ranges can be gained by changing the form of the logarithmic spirals

and then different secondary slide surfaces with their safety factors can be achieved through the strength reduction method. At last

the proposed method is verified through two case studies

and the effect of the shear zone breadth on calculated results is discussed. The results show that the proposed method can get not only the critical slip surface

but also different secondary slide surfaces corresponding to any safety factor.