Experimental study on optimization of parameters for microbial improvement of expansive soils based on response surface methodology

Expansive soils, known for their susceptibility to early damage due to their swelling and shrinkage properties, pose a significant technical challenge in geotechnical engineering. This study explores the optimization of microbial-induced improvement of these soils, emphasizing the pivotal role of temperature, pH, and time on both microbial growth dynamics and the soil’s swell-shrink behavior. A series of microbial growth and expansive soil swell-shrink tests were conducted, employing response surface methodology (RSM) to develop regression models that delineate the optimal conditions for microbial solution concentration and the free swelling ratio. The findings indicate that temperature and curing time exert a more pronounced influence on these parameters than pH. Notably, microbial solution concentration exhibits a peak with increasing temperature, while the free swelling ratio inversely declines. The curing time’s effect is characterized by an initial peak in microbial solution concentration followed by a gradual decrease, with the free swelling ratio of the soil consistently diminishing. The optimization analysis pinpoints the optimal conditions at 32 °C, pH 6.5, and a curing duration of 48 h, where the reduction in both microbial solution concentration and free swelling ratio is maximized. Under these conditions, microorganisms are used for improvement, the microbial solution’s OD₆₀₀ value peaks at 1.996, significantly reducing the expansive soil’s free swelling ratio from 169 to 64.3%.