The Mohr-Coulomb and Drucker-Prager

yield criteria commonly used in slope stability analysis

which have not considered the intermediate principal stress effect or the differential effect of tension and compression

are difficultly to reflect the three dimensional stress state of rock slope. Unified strength theory contains a series of failure criteria; and the parameter b of this theory reflects the intermediate principal stress or the intermediate principal shear stress effect. This paper developed a user material subroutine (UMAT) based on unified strength theory using ABAQUS; taking a three-dimensional slope including discontinuities as a numerical example

and using strength reduction approach to calculate the dynamic safety factor curves for three-dimensional rock slope under five conditions (b = 0、0.25

0.5

0.75 and 1)with different intermediate principal stress extents. The results show that the safety factor varies with the b; but the curve shapes are similar; the smallest value occurs at same time. To understand the influence of intermediate principal stress on the safety factor clearly

a two-dimensional cross-section is extracted from the three-dimensional slope to carry out the calculation under same conditions. The results show that safety factor of three-dimensional model is larger than two-dimension’s under the same parameter b. The safety factor is increasing with b increases. The two-dimensional results underestimate the safety factor magnitude; and the reality stress state of three-dimensional rock slope should be of higher stability.