Abstract: The strength reduction method (SRM) has an advantage over other methods in evaluating slope stability for automatically
determining a critical slip surface. However, a key problem is that there is no corresponding definite standard for an instability criterion
in using SRM. Previous research has shown that slope stability can be determined based on the relative position between the limit-state
slope curve (referred to as the limiting slope curve) obtained from slip-line field theory calculations and the slope. Based on this
conclusion, a set of new SRM-based instability criteria is proposed. The limit slope curve calculated by the slip-line field theory will
change for different strength factors are calculated by different reduction factors. It can be concluded that the slope is in a stable state
when the limiting slope curve does not intersect the slope at the toe. The slope is in a limit-equilibrium state when they intersect at the
toe of the slope while the slope is in an unstable state when the limiting slope curve and the slope intersect. The calculation results of
standard slope test show that when the new instability criterion has an advantage in convergence, safety factor calculated by the new
criterion is close to the existing criterion and the standard answer. For the traditional criterion, there are no guidelines as to the choice
of a tolerance criterion and the optimal number of iterations, and it is not always easy to find the sharp break point immediately after a
slope failure, especially when the displacement curve is relatively smooth. Also, it is difficult to define a precise safety factor from this
criterion, moreover, the evaluation criterion with a presumed allowable iteration number is influenced by an artificial factor. The
objective quantification of the instability criterion can be realized by the new instability criterion. The critical slip surface shape and
position determined by the plastic strain in the slope are basically the same as the standard answer. The new instability criterion is
applicable for the strength reduction method.

Key words: slope stability, strength reduction method, slip line-field theory, instability criterion