Enhancing the Compressibility of Seasonally Frozen Subgrade Clay Subjected to Freeze-Thaw Cycles Using Lignin Fibers

Repeated freeze-thaw cycles in seasonally frozen regions significantly degrade the mechanical properties of clay, posing serious challenges to geotechnical infrastructure stability. This study investigates the compressibility behavior of lignin fiber-reinforced clay under freeze-thaw conditions through one-dimensional consolidation tests and microstructural analysis. Clay specimens containing 0.0%, 0.5%, 1.0%, 1.5%, and 2.0% lignin fibers by mass were subjected to 0, 1, 4, and 10 freeze-thaw cycles to simulate typical seasonal variations. The results indicate that reinforcement with lignin fibers markedly enhances the soil’s resistance to freeze-thaw-induced degradation. Specifically, in unreinforced clay, 10 freeze-thaw cycles reduced the pre-consolidation pressure from 139 kPa to 97 kPa. With 2.0% lignin fiber, the pressure increased to 186 kPa under unfrozen conditions and remained at 120 kPa after 10 cycles. SEM and MIP analyses revealed that lignin fibers form interconnected networks that inhibit the formation and expansion of strip pores and constrained pore coarsening caused by freeze-thaw action, effectively stabilizing the soil structure. A model incorporating both fiber content and freeze-thaw cycle effects was proposed to predict compression behavior, and the model accurately captured the experimental compression curves across all test conditions. This study provides a theoretical and experimental basis for the application of natural fiber-reinforced clay in cold-region geotechnical engineering, offering a sustainable and effective alternative to traditional stabilization methods.