Experimental Study on Microbially Induced Carbonate Precipitation Reinforcement of Silty Sand in an Artificial Seawater Environment

Microbially induced carbonate precipitation (MICP) is an eco-friendly technique for weak soil reinforcement. In this study, Sporosacina pasteurii was used to strengthen silty sand after multigradient domestication in an artificial seawater environment. The efficiency of MICP was investigated by carrying out a series of macroscopic and microscopic tests on biocemented silty sand specimens. It was found that the salt ions in seawater impacted bacterial activity. The best activity of the bacterial solution in the seawater environment was achieved after five-gradient domestication, which was approximately 8% lower than that in the deionized water environment. The significant effects of domesticated bacteria on silty sand reinforcement were demonstrated by the content of precipitated carbonate and the unconfined compressive strength (UCS) of the treated specimens. The seawater positively impacted the MICP procedure due to the roles of calcium and magnesium ions, indicated by the X-ray diffraction spectra. The scanning electron microscopy (SEM) results showed that carbonate precipitations distributed primarily on the surfaces and near the contact points of the soil particles, contributing to the soil strength. The cementation solution concentration and injection rate significantly influenced the content and distribution of carbonate precipitations and UCS of the biocemented silty sand, and the values corresponding to good reinforcement efficiency were 1.0 mol/L and 1.0 mL/min, respectively. The results of consolidated undrained triaxial tests showed that the mechanical properties of treated specimens were influenced by biocementation cycles. It was found that the stress–strain behavior of biocemented samples changed from strain hardening to strain softening when the number of reinforcement cycles increased. The peak strength of silty sand was increased by 1.9–3 times after 5 times MICP treatment. The effect of biocementation cycles on the shear strength parameters could be represented by relating the effective friction angle and effective cohesion of biocemented silty sand to the carbonate content.