Dynamic response and damage evolution of polypropylene fiber-reinforced loess subjected to freeze–thaw cycles

Freeze–thaw heave and thaw settlement are inevitable phenomena that affect soils in seasonally frozen regions. Under repeated traffic loading, these processes significantly reduce the strength and stability of subgrades. Therefore, various stabilizing materials have been incorporated into loess to improve its mechanical properties. A GDS dynamic triaxial apparatus under controlled temperature was employed to investigate the effects of polypropylene (PP) fiber content and the number of freeze–thaw cycles on the dynamic behavior of loess. Specimens containing 0%, 0.25%, 0.5%, 0.75%, and 1.0% PP fibers were subjected to 0, 1, 3, 5, and 7 freeze–thaw cycles. Variations in dynamic stress, dynamic elastic modulus, dynamic modulus ratio, and damping ratio along the backbone curve were analyzed. In addition, the dynamic coupling behavior and freeze–thaw degradation characteristics of the loess were investigated. Experimental results indicate that PP fibers enhance the dynamic properties of loess. The optimum PP content was found to be 0.75%, at which the dynamic modulus ratio increased by 13%, 17%, 21%, 30%, and 51% relative to untreated loess after 0, 1, 3, 5, and 7 freeze–thaw cycles, respectively. However, further increasing the PP content beyond 0.75% resulted in a decline in the dynamic improvement. For a given PP content, dynamic stress, dynamic modulus ratio, dynamic elastic modulus, and damping ratio all decreased with increasing freeze–thaw cycles and tended to stabilize after five cycles. Based on the experimental results, models describing PP fiber reinforcement and freeze–thaw degradation were established to elucidate the reinforcement mechanism of the modified loess. The findings provide a theoretical basis for the design and improvement of loess subgrades in seasonally frozen regions.