Granular geomaterials under different loading conditions manifest various behaviors, such as hysteresis. Understanding their hysteretic behavior and deform

Progressive failure of spoil slopes under extreme rainfall involves multiscale processes, including rainfall infiltration, fine-particle migration, and weakening of the particle skeleton, while their cross-scale evolution remains insufficiently understood. To investigate this mechanism, laboratory model tests were conducted under a rainfall intensity of 108 mm/h with degrees of compaction of 75%, 80%, and 90%. Scanning electron microscopy (SEM) and PFC2D simulations were further employed to examine failure characteristics at the macroscopic, microstructural, and particle scales. The results show that, at 80% compaction, the slope underwent a progressive sequence of fine-particle migration and toe deposition, erosion-gully development, toe undercutting, and intensified shoulder erosion. Pore-water pressure responses also showed clear spatial differences between the crest, middle slope, and toe. As the degree of compaction increased from 75% to 90%, the maximum deposition length, maximum deposition depth, and dry mass of deposited material decreased by 29.8%, 31.0%, and 38.6%, respectively. SEM observations indicate that increasing compaction generally enhances particle contact and structural integrity, thereby reducing local structural disturbance caused by prolonged rainfall. PFC simulations show that particle displacement is mainly concentrated in the shallow slope, particularly near the shoulder and toe, while the number of bond failures increased from 276 at Step 130 to 3488 at the final failure stage. The experiments and SEM observations provide the basis for comparing compaction levels, whereas the PFC results offer a particle-scale interpretation of progressive failure at 80% compaction. The numerical findings do not establish how particle-scale damage varies with compaction.