Polypropylene (PP) fiber-reinforced soil slopes undergo progressive resistance degradation under wet–dry cycling (WDC), while stochastic seismic loading introduces additional uncertainty, challenging deterministic seismic assessment. This study develops a time-dependent seismic reliability framework integrating the probability density evolution method and the equivalent extreme value event method. The cohesion and internal friction angle of unreinforced soil measured at different WDC states are represented as cross-correlated lognormal random fields and combined with random fiber configurations and weighted nonstationary stochastic ground motions in a nonlinear dynamic model. The main contribution is a unified uncertainty-propagation scheme that incorporates experimentally characterized WDC degradation and multiple uncertainty sources into the evolution of response probability and multilevel first-passage reliability. With increasing WDC number and PGA, the extreme displacement distributions shift toward larger values, accompanied by increased response dispersion, tail risk, and reliability loss. The reliability evolution exhibits three stages, namely initial stability, rapid degradation, and residual convergence, during the 70 s excitation. PP fiber reinforcement improves reliability, although the marginal gain becomes limited when the fiber content exceeds 0.15% under the present numerical conditions. The proposed framework provides a probabilistic basis for the seismic assessment and deformation control of PP fiber-reinforced soil slopes at different WDC degradation states.

1. Reinforced soil slopes are widely used in critical infrastructure, such as highway embankments and containment structures, because of their favourable seismic performance and geo-environmental a...