Numerous loess relic sites with cultural and historical values exist in the seasonally frozen ground region of Northwest China. Freeze-thaw action is an essential factor in inducing cracking and collapse of loess relic sites, and the creep behavior of loess also affects its long-term stability. Microbially induced calcium carbonate precipitation (MICP) technology has a promising application in earthen ruin reinforcement due to its environmental friendliness and good compatibility. To evaluate the feasibility of MICP technology for reinforcing loess relic sites in the seasonally frozen ground, triaxial compression tests, triaxial creep tests, and SEM tests were conducted on MICP modified loess after 0, 1, 3, 7, and 9 freeze-thaw cycles. Then, the changing laws of shear strength and creep properties of samples in the freeze-thaw conditions were analyzed. The results show that the MICP technology can enhance the mechanical properties and frost resistance of loess. The shear strength, cohesion, and long-term strength of MICP modified loess are enhanced by 27.8 %, 109 %, and 29.8 %, respectively, under 100 kPa confining pressure, and their reduction is smaller than that of the untreated loess after 9 freeze-thaw cycles; the internal friction angle fluctuates within 1 degrees. Finally, the reinforcement mechanism and freeze-thaw resistance mechanism of MICP technology were revealed. Microbially induced calcium carbonate can cement soil particles, fill interparticle pores, and inhibit the development of pores and cracks caused by freeze-thaw action. The results can provide a theoretical foundation and scientific basis for the long-term stability analysis of loess relic sites reinforced with MICP technology.

There are many drawbacks in traditional loess-strengthening technology. MICP (microbially induced calcium carbonate precipitation) technology provides a new approach to loess management, but there are few studies on loess solidification and a lack of engineering application research and verification. This study investigated the strength and microscopic mechanisms of loess solidified by the application of MICP technology combined with plant straw. The permeability conditions of loess for MICP technology were derived, and multiple sets of experiments were conducted using specific loess, Bacillus pasteurii, cementing solution, plant straw, and other materials. The experiments explored shear strength, unconfined compressive strength, microscopic properties, plant growth adaptability, and factors affecting bacterial growth. The results indicated that within the temperature range of 25-35 degrees C, the concentration and urease activity of Bacillus pasteurii were significantly affected by temperature, with the highest bacterial concentration observed at 30 degrees C. During scaled-up cultivation, increasing the inoculation ratio prevented a significant decrease in the urease activity of individual bacterial strains, and a 1% inoculation ratio was generally sufficient to meet the experimental requirements. When the loess density was 1.7 g/cm3 and 1.8 g/cm3, the cohesive force and internal friction angle in the experimental groups with added bacterial solution were increased by approximately 30% and 50% and 15% and 5%, respectively, indicating that MICP technology can significantly enhance the shear strength of loess.