Synergic effects of biochar and polyacrylamide amendments on the mechanical properties of silt loam soil under coastal reclamation in China

Recent surveys have reported severe erosion and slope collapse caused by a combination of the poor structure of silt loam and heavy rainfall in reclaimed areas of China. Biochar and polyacrylamide (PAM) have been widely used as amendments to protect soil from degradation and enhance plant growth. However, few studies have focused on their synergic effects on soil mechanical properties. In this study, biochar and PAM were incorporated into silt loam at rates of 0%, 2%, and 5% and 0‰, 0.4‰, and 1‰, respectively, to investigate the extent to which biochar and PAM additions affect soil consistency limits and shear strength and to evaluate the slope reliability under natural conditions. The results show that the plastic limit (PL) and liquid limit (LL) were significantly enhanced by amendments of 5% biochar and 1‰ PAM. Soil consistency limits were significantly correlated with the soil 3D fractal dimension (FD), μ-CT porosity and electrical conductivity (EC). Amendment of coastal silt loam soil with biochar resulted in decreased cohesion. The application of PAM reduced the negative effect of biochar on cohesion and considerably increased the cohesion value of the silt loam. The results of the slope stability analysis were closely related to the shear strength parameters. Biochar-amended silt loam soil in the coastal area exhibited a much lower factor of safety (FS) compared with the control soil, irrespective of the level of the water and the slope angle within the slope system. A PAM amendment in coastal silt loam soil increased the FS significantly (P < 0.05) and minimized the negative effects on the FS that was caused by the biochar amendment. Therefore, biochar and PAM could be applied to silt loam soil together to improve soil quality to enhance the slope stability under coastal reclamation in China.