Strength and microstructure of a lignin fiber-reinforced expansive soil in cold regions

An experimental program was undertaken to investigate the effects of lignin fibers and freeze-thaw (FT) actions on the shear strength behavior of an expansive soil. Soil specimens were prepared at five lignin fiber contents. Consolidated undrained triaxial tests were carried out on as-compacted specimens and specimens subjected to 10 FT cycles. Scanning electron microscopy and nuclear magnetic resonance tests were performed to reveal the microstructural characteristics. Test results indicate that the inclusion of lignin fibers leads to a significant increase in the cohesion due to the fact that lignin fibers form spatial networks that increase the bonding among soil aggregates. The cohesion-fiber content relationships are linear and their slope is constant regardless of FT actions, which indicates that the contribution of lignin fibers to the cohesion is not impaired by FT cycles. Lignin fibers alleviate the development of FT-induced cracks and the associated reduction in cohesion. Meanwhile, lignin fibers have little impact on the arrangement and contact stress and roughness among soil particles. Therefore, they have little influence on the friction angle of the soil with or without FT histories.