Abstract: Strengthening red clay with microorganisms is an innovative, environmental friendly and economical method. We prepared two kinds of microorganism-strengthened soil samples according to the optimum curing condition by selecting bacillus pasteurii and iron bacteria to strengthen soil with activated carbon as carrier. Tests on the basic physical and mechanical properties of microbe soil samples unveiled that the density, shear strength and unconfined compressive strength of microbe soil samples increased whilst porosity decreased, indicating that microorganisms remarkably promoted the physical and mechanical properties of red clay. In order to further study the solidification mechanism, we examined the microstructure and material composition of the samples via scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). We found that bacillus pasteurii induced the formation of calcium carbonate to fill the pores of soil and form the cemented and solidified soil. Calcium carbonate is mostly concentrated in the vicinity of activated carbon and presents a massive structure; iron bacteria produce iron-based compound to fill the soil pores, and the iron-based compound is mostly lamellar or layered structure. In short, the curing effect of bacillus pasteurii for red clay is better than that of iron bacteria. The research finding is of vital reference value for understanding the mechanism of microbial reinforcement of red clay and the selection of bacterial species.

Key words: red clay, microbial curing, physical and mechanical properties, microstructure, action mechanism