Synergistic effects of biochar and plant fibers on the hydro-mechanical behavior of landfill cover soil

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Biochar has been widely incorporated into landfill cover soils to enhance moisture regulation due to its porous structure and strong water retention capacity. Plant fibers, being abundant, inexpensive, and renewable, also show potential as soil amendments. However, little research has addressed the coupled effects of biochar-fiber composites on soil hydraulic and mechanical behavior under unsaturated conditions. This study investigates their influence on CO 2 permeability ( k a ), hydraulic conductivity ( k w ), and shear strength in sandy loam. Laboratory experiments examined the effects of biochar (5% and 10% by mass) and plant fibers (0.1% and 1% by mass) on k a and shear strength, while monitoring suction and volumetric water content. Results showed that k a increased by 117.46% with 5% biochar (SRS-5%) due to macropore formation, but only 2% with 10% biochar (SRS-10%) because of pore blockage. Fiber addition further enhanced ka by 22.25-64.79%. Saturated hydraulic conductivity ( k s ) increased by 186% with SRS-5% and 32.67% with SRS-10%, with fibers contributing additional gains of 78.09% and 15.58%, respectively. Shear strength improved with confining pressure (50-200 kPa), rising 27.7-124.22% with biochar alone, and by a further 25.42-52.31% when fibers were included. The combined treatment of biochar and plant fiber increased the shear strength by 94.49%-198.81%. Improvements are attributed to pore-filling and compaction by biochar and fiber reinforcement via three-dimensional bridging. These findings underscore that biochar and plant fiber can effectively improve the gas permeability, hydraulic conductivity and mechanical stability of landfill cover soil, offering a promising strategy for cover system optimization.