Abstract：In order to improve the accuracy of stability analysis of slopes under the condition of rain softening，the modes and criterion of failure of soil slopes of road subgrade were analyzed. For the different rain softened depths，three types of failure modes occurred including soil slice type of unit width，composite type，and integral type. The ratio ? of the length of slip body to the softened depth was taken as the characterization parameter of deep and shallow slope failure category，and the thresholds λcr and λf of three modes were presented. According to the limit equilibrium condition of rigid body，a unified formula of stability safety factor based on the composite type failure mode was established. The results show that threshold ?cr of unit width soil slice type and composite type and threshold ?f of composite type and integral type are not significantly affected by the strength and geometric parameters of the slope，and are between 15.2–20.0 and 3.0–6.3 respectively. If ? was less than ?cr，applying the parallel slope-plane mode produced the conservative results due to the ignoring of the mechanical effect of the upper and lower edge. If ? was more than ?f，applying the integral curve represented by circular slip surface yielded larger safety factor. But if ? was less than ?f，applying the logarithmic spiral or circular arc sliding mode yielded no significant difference.
2016, Vol. 35 
