Hydraulic behavior of unsaturated soils under microbially induced calcium carbonate precipitation–vegetation combined treatment

Vegetation and microbially induced calcium carbonate precipitation (MICP) techniques are considered promising approaches for enhancing slope stability under extreme rainfall conditions. However, limited research has been conducted to elucidate the combined effects of these two techniques on soil hydraulic behavior, particularly with respect to water retention and infiltration. Using soil column experiments conducted under controlled evaporation and rainfall conditions, this study investigates the effects of MICP and vegetation (vetiver grass) treatments on the hydraulic behavior of treated soils across different planting densities. Volumetric water content (VWC), matric suction, and infiltration rate were continuously monitored. Soil–water retention curves (SWRCs) were fitted using the van Genuchten model, and microstructural changes were examined via scanning electron microscopy (SEM). The results indicate that the coupled interaction between MICP and vegetation significantly modifies soil hydraulic properties. Vegetation enhances evaporation-induced suction, whereas MICP treatment improves water retention capacity and reduces the infiltration rate by 15.8%–20.8% through calcium carbonate precipitation. Notably, a high planting density (V1) leads to root decay and macropore formation, thereby increasing the infiltration rate. In contrast, a moderate planting density (V2) promotes healthier root development and more stable hydraulic behavior. It was also found that MICP treatment suppresses plant transpiration, possibly due to the inhibitory effects of ammonium byproducts on plant metabolism. These findings highlight the importance of balancing vegetation density and MICP concentration to optimize the hydraulic performance of reinforced soils.