Evaluating the influence of enzyme induced carbonate precipitation on the sheet erosion processes and sediment sorting of purple soil slopes

Enzyme Induced Carbonate Precipitation (EICP) is an innovative geotechnical technique that directly utilizes urease to catalyze the hydrolysis of urea, and form calcium carbonate crystals, which serve to reinforce the soil. EICP demonstrates a promising and good potential for controlling rainfall-induced erosion. However, previous researches primarily focus on the overall effect of erosion reduction, and the influence of EICP on erosion processes for different erosion patterns is insufficient. Different erosion patterns characterized by distinct processes and mechanisms, among which sheet erosion is the initial phase of slope erosion and serves as most important process of soil erosion evolution. It is one of the most complex processes of slope water erosion and is characterized by significant sediment sorting phenomena. Therefore, to explore the influence of EICP spray treatment on sheet erosion processes and sediment sorting, this study took the purple soil slope in the Three Gorges Reservoir area (TGRA) 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 up, and simulated rainfall experiments with 2 rainfall intensities (60 and 120 mm/h) were carried out to analyze the sheet erosion process (including runoff and erosion processes) and the sediment sorting of sub-rainfall event, moreover, the microscopic influence mechanism of EICP on sheet erosion was revealed by used scanning electron microscopy. The results indicated that compared with the CK treatment, the total runoff and total erosion for only urease spray treatment increased on average by 10.68% and decreased on average by 43.74%, respectively; while those for EICP spray treatments increased on average by 29.51% to 50.76% and decreased 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 effect of increasing runoff and reducing sediment had an increasing trend followed by a reduction with the increase of cementing solution concentrations, and cementing solution concentration of 1.5 mol/L performed the best effect. However, there were no significant differences in total runoff and erosion among EICP spray treatments with different cementing solution concentrations (P>0.05), indicating the low concentration (0.5 mol/L) was good enough for controlling sheet erosion. The erosion sediment exhibited a distinct sorting phenomenon with 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 spherical calcium carbonate precipitates and urease filaments appeared in the gaps and on the surfaces of soil particles after EICP spray treatment, which not only reinforced the soil and enhanced its erosion resistance leading to reduce erosion, but also reduced infiltration thereby increasing runoff production. The results of this study will help elucidate the sheet erosion reduction mechanism of EICP and provide theoretical support for soil and water conservation measures in the TGRA.