Mechanical properties and freeze–thaw durability of expansive soil reinforced with polypropylene and lignin fibers

To address the deterioration of expansive soil in seasonally frozen regions subjected to freeze-thaw cycles, this study investigated the effects of fiber modification by incorporating polypropylene fiber (PP) and lignin fiber (LF) on the swelling behavior, mechanical properties, and freeze-thaw durability of expansive soil. Using expansive soil as the base material, the modification effects of individual fiber incorporation were evaluated through free swell ratio, no-load swell ratio, and direct shear tests. Freeze-thaw cycle tests were then conducted on the composite fiber-modified soils, and the microscopic mechanism was analyzed in combination with SEM observations. The main conclusions are as follows: (1) For single-fiber modification, lignin fiber (LF) at a dosage of 1.2% exhibited the best overall performance, producing the greatest reduction in free swell ratio and the largest increase in shear strength. The incorporation of polypropylene fiber (PP) also significantly improved these parameters, although its effect was weaker than that of lignin fiber (LF). (2) Based on the results of single-fiber modification, the addition of 1.2% polypropylene fiber (PP) on the basis of 1.2% lignin fiber (LF) further improved the mechanical properties of expansive soil, under which the shear strength and unconfined compressive strength of the modified expansive soil were both enhanced. (3) In terms of freeze-thaw durability, the composite fiber-modified soil subjected to freeze-thaw cycles showed significantly lower attenuation rates of shear strength and unconfined compressive strength than the untreated specimens. SEM results showed that local overlapping and bridging structures formed between the fibers and soil particles, which helped delay crack propagation and weaken freeze-thaw damage. Through physical restraint and reinforcement effects, lignin fiber (LF) and polypropylene fiber (PP) jointly improved the mechanical performance and frost resistance of expansive soil, thereby providing an experimental reference for research on fiber modification of expansive soils in seasonally frozen regions.