Simulating field grouting to improve the shear strength of consolidated clay by microbially induced carbonate precipitation

Soft clay layers in urban areas pose challenges due to low strength and high compressibility, complicating urban construction. Microbially induced carbonate precipitation (MICP) has demonstrated its ability to enhance soil strength in sandy soils. However, research on the utilization of MICP for clay has been limited. Thus, this paper introduced a new method that combines MICP with grouting. The treated MICP solutions were injected into the consolidated clay samples in a triaxial system to simulate the improvement of grouting in the consolidated soil layer in the field. A series of modified triaxial tests, fluorescent nucleic acid stains, solution/soil pH tests, and microscopic observation tests were conducted. Results indicated a 1.7 times increase in soil strength with bioaugmentation treatment on clay under 150 kPa consolidation stress. Additionally, the weight of CaCO₃ increased by 3.2 g, achieving a precipitation ratio of 58 %. The injected bacteria were observed in the bottom-most and center of the sample, confirming their mobility in the clay. The enhancement of shear strength on treated clay involved not only urease-producing bacteria but also environmental chemical reactions and natural bacterial effects. Furthermore, a microstructure changes of MICP in clay was observed, wherein CaCO₃ effectively filled the pore spaces and bonded particles together to enhance strength. Based on the results, through MICP treatment with injection method, consolidated clay enhances its mechanical behavior and microstructural formation to improve the shear strength.