Improvement of mechanical properties of clay in landfill lines with biochar additive

Compressibility and shear strength of clay barriers are some of the most important geotechnical properties in designing an effective and stable landfill containment system. Biochar has been widely used in soil improvement, soil remediation, and carbon sequestration. In this research study, clay that is used in landfill liners has been pre-mixed with biochar as an additive. Biochar was added to the soil with 0, 2.5, 5, 10, and 20% of biochar by weight. The biochar clay is tested for its effectiveness in improving the consolidation and shear strength properties of the material. Numerical modeling is used to examine characteristics of the capillary forces between biochar particles and soil particles. The effects of the amount of biochar on the shear strength of the clay under different consolidation pressures are analyzed. The results indicate that a biochar content of 5% provides the optimum improvement on the mechanical behavior of biochar-added soils. At a biochar content of 5%, under normal stresses of 50, 100, 200, and 400 kPa are equal respectively to 1.81, 1.39, 1.20, and 1.08 times the shear strength of the soil under the same normal stress with no biochar; the shear strength reaches the maximum value mainly due to a maximum increase in cohesion of the material while the angle of internal friction is decreasing. Although the shear strength of the biochar-added clay is increased with biochar content, the increase in strength relative to the soil without biochar is gradually decreasing with increased consolidation pressure. With increase in the distance between the biochar and clay particles, the capillary force first increases and then decreases. For the same separation distance between the clay and biochar particles, the capillary force between clay and clay particles is the smallest, while the capillary force between biochar and biochar particles is the largest, and the capillary force between the biochar and biochar particles is 1.78 times of that between the clay and clay particles under the same largest values. In general, biochar improves the shear strength of biochar-added soils.