Optimal Utilization of Biochar, Polyacrylamide, and Straw Fiber for Subgrade Stabilization of Forest Roads

Subgrade stabilization is crucial for forest road construction, especially in Northeast China and the Russian Far East, with great economic growth potential. This study explored a novel and green solution of integrating biochar (BC), polyacrylamide (PAM), and straw fiber (SF) in the form of a ternary composite for stabilizing forest subgrade soil in cold regions. Using central composite design-based response surface methodology, the optimal mix ratio design was obtained, and the composite stabilizer was designated as BPS. Afterward, the stabilizing performance of BPS was studied by conducting an unconfined compression strength (UCS) test. The results showed that the optimum composition of BC:PAM:SF stood at 81:9:10. The UCS and deformation modulus with 3% BPS at 28 days reached 565.42 kPa and 17.24 MPa, respectively, which were 3.36 and 6.05 times higher than those of the untreated samples. The BPS-treated soil also possessed better resistance to freeze–thaw cycles. The freezing–thawing-induced loss ratio of strength was 49.3% lower than that of natural soil. Moreover, empirical models for the UCS of BPS-stabilized soil, as well as its relationships with the modulus, were established and validated by data in the literature. Finally, the “filling, cementing, and reinforcing” stabilization mechanism of BPS was elucidated by scanning electron microscopy analysis.