Effects of Biochar Type and Addition Rate on Pore Seepage Characteristics of Saline Soil and Associated Mechanisms

Water seepage in saline soils can compromise the stability of landfill cover layers and cause ecological pollution. Biochar, recognized as a novel sustainable material for soil improvement, has garnered significant attention due to its large specific surface area and complex pore structure. Its addition can alter the physico-chemical properties of soil, including pore structure characteristics, compaction behavior, and liquid seepage paths. This study therefore focuses on saline soil, investigating the effects of biochar type and addition rate on the soil’s hydraulic conductivity through variable-head permeability tests. The correlation between changes in microscopic structural characteristics and hydraulic conductivity was analyzed using SEM and MIP techniques. The results indicate the following: (1) Both corn straw and wood biochar affect the hydraulic conductivity of the modified soil, with the degree of influence varying based on the presence and dosage of biochar, generally leading to a reduction. (2) The corn straw biochar-modified soil exhibited a more significant decrease in hydraulic conductivity—by approximately one order of magnitude—compared to wood biochar, reaching a minimum value of 1.04 × 10−6 cm/s at a 10% addition rate. (3) Analysis from SEM and MIP tests reveals that incorporating biochar alters the original pore structure of the soil, causing macropores to transition towards mesopores and micropores, and resulting in the narrowing or clogging of pore channels, which ultimately reduces the hydraulic conductivity. The finding that biochar amendment decreases hydraulic conductivity provides valuable data to support the design of landfill cover layer schemes.