Application of recycled lignin powder as a sustainable additive for soil improvement against water resistance

To evaluate the engineering performance against water resistance of lignin-modified soil, unconfined compressive strength (UCS) tests and direct shear tests were conducted on silty sand modified with varying lignin contents (2%, 5%, 8%, 12%, and 15%) following wet-dry cycles. The results demonstrate that 8% lignin-modified soil has the highest UCS and shear strength. The addition of lignin to soil enhances its wet-dry durability, with soil modified by 8% lignin exhibiting the highest durability. The structural properties of lignin-modified soil subjected to wet-dry cycles can be quantified using a combined structural stability index and structural variability index. A model relevant to the structural properties was proposed to predict the shear strength of lignin-modified soil following wet-dry cycles. Notably, the addition of lignin to silty sand does not result in the formation of new minerals, indicating that lignin addition is an environmentally-friendly soil treatment.