Effect of Dry and Wet Cycles on the Strength Characteristics of Biochar–Clay Mixture

Abstract

Biochar is an economical and environmentally friendly “green material” with potential benefits in greenhouse gas emission reduction, soil performance improvement, and environmental restoration. Mixing biochar with clay and using it as a landfill cover can effectively reduce the escape of greenhouse gases into the air, which is important for environmental protection. It is suggested that biochar should be mixed with clay and used as a landfill covering layer. In this study, the shear strength was obtained by direct shear test, and the shear strength characteristics of biochar–clay mixture under the influence of different dry and wet cycles, biochar contents, and moisture conditions were studied. The results show that the shear strength of the biochar–clay mixture in the saturated state decreases with increasing wet and dry cycles, with shear strength decreases ranging from 6% to 19%. The cohesion and internal friction angles of the clays and mixtures show a decreasing trend under wet and dry cycles, with the cohesion and internal friction angles decreasing in the range of 2% to 16%. The shear stress–shear displacement curve for the biochar–clay mixture in the saturated state shows strain hardening after wet and dry cycles; the curve in the dry state shows strain softening with a distinct peak and a platform at the front end of the curve. The shear strength of clay in a dry state is larger than that of biochar–clay mixture and always larger than that of clay in a saturated state. The shear strength difference of the mixture between dry and saturated states is obviously smaller than that of pure clay. This paper, therefore, provides theoretical guidance for the application of biochar–clay mixtures to landfill covers.