Background When conducting ecological restoration of sandy and silty soil slopes in Dongcaoying village, Baodaoling town, Yanqing district, Beijing, issues such as substrate detachment, slope sliding and soil erosion occurred, resulting in poor stability of the restoration. Currently, the influence of different adhesive agent types and dosages on the properties of the substrate is not clear, and further research is needed to optimize the ecological restoration plan and enhance the stability of the slope.

Methods This study took sandy soil and silt soil as the research objects and conducted indoor experiments to explore the effects of polyacrylamide (PAM) and polyvinyl alcohol (PVA) at dosages of 0 (control group, CK), 0.2%, 0.4%, 0.6%, and 0.8% (w/w) on soil density, porosity, the mass fraction of water-stable aggregates, and shear strength. The soil solidification mechanism was also revealed through electron microscopy analysis.

Results 1) The adhesives significantly reduced soil bulk density (P < 0.05). The reduction in PVA (6.8%−10.3%) was higher than that of PAM (3.4%−4.8%) in sandy soil, and the reduction in PVA (11.7%−13.1%) was also higher than that of PAM (10.3%−11.7%) in silty soil. The effect of PVA was better. 2) The adhesives significantly increased soil porosity (P < 0.05), and the improvement effect of PVA was more significant than that of PAM, but there was no significant difference among the dosages (P > 0.05). 3) The adhesives significantly increased the mass fraction of large aggregates (P < 0.05), enhanced the stability of soil aggregates, and the effect of PAM dosage was more obvious. 4) The peak value of the principal stress difference of the soil modified by the adhesive increases in accordance with the Mohr-Coulomb law as the confining pressure increases. The strength improvement of sandy soil is better than that of silt soil, and the optimal effect is achieved at a range of 0.4% to 0.6%. Moreover, the cohesion of both types of soil increases after modification, but the internal friction angle shows different responses: the cohesion of sandy soil decreases with the increase of the admixture content, while there is no significant improvement in silt soil (P > 0.05). 5) The adhesives enhanced the stability of the soil body by aggregating small aggregates, forming thread connections and surface sheet-like structures.

Conclusions PAM and PVA have significant effects on improving the physical and mechanical properties of soil. The optimal dosage is clearly 0.4%−0.6%. The results can provide theoretical support for the differences in the application effects of adhesives in different soil types, and are of great guiding value for reducing soil erosion and promoting ecological restoration projects.