Influences of enzyme-induced carbonate precipitation on the sheet erosion and sediment sorting of purple soil slopes

Enzyme induced carbonate precipitation (EICP) is one of the innovative geotechnical techniques to reinforce the soil. Specifically, urease can be directly utilized to catalyze the hydrolysis of urea, and then form the calcium carbonate crystals. EICP has promising potential to control rainfall-induced erosion. However, previous research has focused primarily on the overall effect of erosion reduction. It is still lacking in the influence of EICP on erosion processes under different erosion patterns. Among them, different erosion patterns are characterized by distinct processes and mechanisms, where sheet erosion is the initial phase of slope erosion to serve as the most important evolution of soil erosion. It is one of the most complex processes of slope water erosion, indicating significant sediment sorting. Therefore, this study aims to explore the influence of EICP spray treatment on sheet erosion and sediment sorting. The purple soil slope in the Three Gorges Reservoir Area (TGRA) was taken as the research object. Six spraying treatments (bare slope as control check (CK), only urease spray treatment, and EICP spray treatment with 4 cementing solution concentrations (0.5, 1.0, 1.5, and 2.0 mol/L)) were set. Simulated rainfall experiments with two rainfall intensities (60 and 120 mm/h) were carried out to analyze the sheet erosion (including runoff and erosion) and the sediment sorting of sub-rainfall events. Moreover, the microscopic influence mechanism of EICP on sheet erosion was revealed using scanning electron microscopy. The results indicated that the total runoff for only urease spray treatment increased on average by 10.68%, whereas, the total erosion was reduced on average by 43.74%, respectively, compared with the CK; The total runoff under EICP spray treatments increased on average by 29.51% to 50.76%, whereas, there was the decrease on average by 85.57% to 90.90%, respectively. The only urease spray treatment showed a certain effect on increasing runoff and reducing sediment. While the EICP spray treatment showed a significant effect on increasing runoff and reducing sediment (P<0.05). The increasing runoff and reducing sediment shared an increasing trend followed by the reduction with the increase of cementing solution concentrations. The cementing solution concentration of 1.5 mol/L performed the best. However, there was no significant difference in the total runoff and erosion among EICP spray treatments with different cementing solution concentrations (P>0.05), indicating the low concentration (0.5 mol/L) suitable for the control of sheet erosion. The erosion sediment exhibited a distinct sorting with the enrichment of clay, fine silt, and coarse silt particles and a depletion of sand particles. The degree of sediment sorting was reduced after EICP spray treatment, with a decrease in clay content and an increase in coarse sand particle content in the erosion sediment. Microscopic analysis showed that the spherical calcium carbonate precipitates and urease filaments often appeared in the gaps and/or on the surfaces of soil particles after EICP spray treatment. The soil was reinforced to enhance the erosion resistance, leading to reduced soil erosion. The infiltration was reduced to increase the runoff production. The finding can also provide theoretical support to the sheet erosion reduction of EICP for the soil and water conservation in the TGRA.