Biochar, as an environment-friendly soil amendment, has been extensively proposed in landfill cover, primarily for promoting soil hydraulic properties, such as hydraulic conductivity and soil water retention. However, the impact of biochar derived from various feedstocks on soil-biochar mix properties, particularly gas permeability under unsaturated conditions, remains under-explored. This study evaluates how different types of biochar influence gas permeability and soil water retention. Five biochars pyrolyzed using different biomass waste, such as apple wood, reed straw, walnut, corn cob and corn straw, were each mixed with sandy soil in a 5% mass ratio. Gas permeability and hydrological response (water content, matric suction) were measured during wet-dry cycles. Results indicated that biochar amendments generally enhanced water retention compared to bare soil. Apple wood biochar, in particular, significantly improved both water content (reaching 90% of the control's maximum moisture content) and suction (peaking at 2755 kPa), outperforming reed straw, walnut, corn cob and corn straw biochars. This enhancement stems from apple wood biochar's hydrophilic functional groups (e.g., -OH), which improve soil hydrophilicity and water-biochar interactions. Its large specific surface area and tightly arranged micropores further enhance suction. Gas permeability rose with increasing suction, with reed straw and apple wood biochars increasing gas permeability by 196% due to their larger average pore sizes and the formation of more meso-macro pore structures in the sandy soil. Conversely, walnut and corn cob biochars reduced soil permeability, suggesting their suitability for high-pressure applications. These findings guide the use of biochar-amended soil in landfill covers to mitigate gas emissions.

Featured Application The findings of this study establish the behavior of sanitary landfill cover materials, such as compacted clay and compacted polyurethane-clay, in unsaturated conditions under several wet-dry cycles, which would aid in predicting the performance of the material under varying environmental conditions. By predicting the unsaturated hydraulic conductivity and understanding the effects of environmental stresses, the findings can aid in the design and implementation of more durable and efficient landfill liners and covers.Abstract Sanitary landfill covers are exposed to varying environmental conditions; hence, the state of the clay layer also changes from saturated to unsaturated. The study aimed to predict the unsaturated hydraulic conductivity of the locally available compacted clay and clay with polyurethane to determine their behavior as they change from wet to dry using matric suction and empirical models proposed through other studies. The specimens underwent three wet-dry cycles wherein the matric suction was determined for several moisture content levels as the specimen dried using the filter paper method or ASTM D5298. The results showed that the factors affecting the soil structure, such as grain size difference between clay and polyurethane-clay, varying initial void ratios, and degradation of the soil structure due to the wet-dry cycles, did not affect the matric suction at the higher suction range; however, these factors had an effect at the lower suction range. The matric suction obtained was then used to establish the best fit water retention curve (WRC) or the relationship between the matric suction and moisture content. The WRC was used to predict the unsaturated hydraulic conductivity and observe the soil-water interaction. The study also observed that the predicted unsaturated hydraulic conductivity decreases as the compacted specimen moves to a drier state.

This study was conducted to explore the use of non-expansive soil as protective cover for expansive soil slopes. Laboratory model experiments were carried out on expansive soil systems with varying thickness of non-expansive soil cover. The models were subjected to three wet-dry cycles. Variation in soil moisture content was monitored using moisture probes. Surface and internal cracking of soil was observed using cameras. Variation of infiltration rate of the cover with wet-dry cycles was measured in-situ. Results of the study show correlation between cover thickness and evaporation rate and crack formation in the expansive soil. Crack size, quantity, depth, and interconnectivity in the expansive soil increased with decreasing cover thickness. Even the thinnest cover significantly reduced the the number and depth of cracks. The infiltration rate of the cover remained unchanged after three cycles wet-dry cycles. The final water content, after the third drying, in the expansive soil increased with increasing cover thickness.