The long-term stability of compacted soil liners in landfill barriers depends on maintaining extremely low water permeability and resisting cracking induced by wet-dry cycles. This study investigated the potential of biochar as an amendment to improve the characteristics of granite residual soil, a commonly used material in barrier construction. Laboratory experiments were conducted on soil-biochar blends at different compaction levels (60% and 80%) and biochar concentrations (0%, 5%, 10%, and 20% by mass). The results showed that biochar addition gradually reduced saturated soil water permeability by up to one order of magnitude. Alterations in pore size distributions indicated a shift towards smaller diameters, suggesting the role of biochar in blocking macropores. The crack experiments demonstrated that biochar lowered surface crack ratios by 75% compared with untreated soil. Moreover, biochar affected the drying behaviour of residual granite soils, prolonging the evaporation period from 10 to 12 days and increasing the residual moisture content from 5% to 8%. In conclusion, biochar exhibited the potential to diminish soil permeability coefficients and alleviate soil cracking, providing valuable insights for enhancing the long-term performance of landfill containment barriers.

Urban loess subgrades are affected by considerable vibrations from traffic, especially when the underground pipelines leak, and seepage under vibrations often causes road damage. However, the influence of vibrations on the water permeability of loess subgrades remains elusive. Here we address this issue by performing vibrationassisted permeability tests and scanning electron microscopy, mercury intrusion porosimetry, and suction-nuclear magnetic resonance measurements. This allowed the evolution of the saturated hydraulic conductivity (Ks), water-air migration, soil microstructure, and pore water forms to be evaluated. The water permeability of the loess subgrade is promoted by vibrations due to the increase in Ks, the acceleration of wet front migration, and the escape of entrapped air. Moreover, the value of Ks under vibration is 3-14 times greater than that without vibration, and the maximum increase occurs at a vibration frequency near the natural frequency of the loess. Furthermore, a theoretical framework of loess vibration permeability is proposed, and the mechanisms by which vibration accelerates the permeability behavior of the loess subgrade are revealed. Vibration promotes the expansion of soil pores, a decrease in the binding capacity of pore water, the mobilization of fine particles, and the formation of local low-permeability layer. Moreover, it accelerates the opening of entrapped air bubbles and the displacement of water-air, the reduction in seepage resistance. Thus, seepage water flows rapidly along infiltration channels. These findings are highly important for the road safety performance and the sustainable development of the traffic environment in loess regions.