Preliminary study on microbially modified expansive soil of embankment

The improvement of expansive soils has crucial academic research significance and engineering application value. The regular practice using chemical improvement methods may be helpful to improve the expansive soil; however, at the cost of inconvenient material acquisition, troublesome mixing methods, expensive construction, and polluting the soil environment. To this end, based on the microbial induced calcium carbonate precipitation (MICP) technology, we carry out a series of soil modification tests on the expansive soil for the roadbed filling in Nanning, Guangxi. Considering the problematic behaviors of the expansive soil and its significant adverse effect on engineering, this paper focuses on the soil improvement; particularly, we aim to study the improved soil's physical properties, limit moisture content and swelling and shrinkage rates under the action of dry and wet cycles. It is found that the swelling index of the improved expansive soil has been decreased significantly, the free expansion rate drops to 14.9%, such that the samples fall into the category of non-expansive soil. The liquid limit and plastic limit of soil samples decreased to 47.2% and 20.4%, respectively, after 6 times of microbial treatment. Moreover, after several high-pressure consolidation tests under dry-wet cycle conditions, the hydrophilic capacity of the improved expansive soil is significantly weakened, and its corresponding water absorption induced swelling rate and drainage-induced shrinkage rate are significantly reduced. The above research results show that it is feasible to use the MICP technology to improve the swelling and shrinkage characteristics of the expansive soil; in particular, after six treatments with the microbial solution, all indices of the expansive soil achieve the best improvement effect.