Microbial solidification of sand has obvious effects: energy-saving and environmental protection. It is a green and sustainable soil consolidation technology with low energy consumption, which meets the needs of high-quality development of modern economy and society. However, when clay is doped in sand, clay has an uncertain influence on the effectiveness of the microbial solidification of sand. Therefore, triaxial consolidation undrained tests before and after microbial solidification of sands with different clay content are carried out in this paper. The effects of clay content on the solidification effect of sands are compared and analyzed. The variation laws of shear strength, unconfined compressive strength, internal friction angle and the cohesion of sands with different clay content before and after microbial solidification are discussed. The failure modes of sand samples were studied and the influence mechanism of clay on the microbial solidification of sand was revealed from a micro perspective. The test results show that the failure strain and unconfined compressive strength of microbial-induced calcium carbonate precipitation (MICP) treated samples increase first and then decrease with the increase in the clay content. The unconfined compressive strength is the highest when the clay content is 9%, and the samples with low clay content (3~9%) can still retain good integrity after being destroyed. As the content of clay in the sand–clay mixture increases, the internal friction angle of the sample decreases and the cohesion increases. After MICP treatment, the internal friction angle and cohesion of the sand increase first and then decrease with the increase in clay content. There are three main contact modes between sand-clay-CaCO₃. When clay content is low, clay plays a filling role. The contact mode between sand-clay and CaCO₃ is mainly between sand particles and calcium carbonate and between clay particles and calcium carbonate. When clay content is high, the contact mode between particles is mainly between clay particles and calcium carbonate. Higher clay content wraps sand particles, prevents contact between calcium carbonate and sand particles and reduces the strength of sand.

Microbial-induced carbonate precipitation is an environmentally friendly foundation treatment technology that effectively improves soil engineering performance. The various nutrient components of liquid curing compounds significantly influence the curing effect. On the basis of penetration, dry density, water absorption, and unconfined compressive strength tests, this study showed the effect of nutrient solution composition, including urea, calcium chloride, sodium bicarbonate, ammonium chloride, and nutrient broth, on the physicomechanical properties of bio-cemented sand. The morphological differences of calcium carbonate precipitates under nutrient solution composition were compared through scanning electron microscopy (SEM). Results showed that the curing effect of compound nutrient solution was improved compared with the basic nutrient solution (urea and calcium chloride). Among the individual components added, ammonium chloride had the most remarkable effect, followed by sodium bicarbonate and nutrient broth. Among the paired components added, sodium bicarbonate + ammonium chloride had the most significant effect, followed by sodium bicarbonate + nutrient broth and ammonium chloride + nutrient broth. The strength of bio-cemented sand cured with compound nutrient solution containing five components could reach 3.43 MPa, which was 1.92 times higher than the strength of the basic nutrient solution. As shown by the SEM image, the calcium carbonate precipitation in the solidified sand was distributed in the clearance of sand particles, effectively bonding the sand particles. The calcium carbonate obtained by the composition of the compound nutrient solution precipitated the sand particles, and some of the sand particles were wrapped. Moreover, the amount of precipitation was evidently greater than that of the basic nutrient solution. Compared with the basic nutrient solution, the compound nutrient solution effectively reduced the apparent porosity and average pore size of the sand. Thus, the curing effect of the compound nutrient solution was better than that of the basic nutrient solution.

The stability of tunnel is greatly affected by the changes in volume and strength of swelling loess caused by humidification. Quantitative reflection of the influence of humidification on stability in mechanical model has important engineering significance for the design of support structures. Based on the fact that parameters related to swelling and strength are functions of water content, the water content was introduced into the classical surrounding rock–support model, and a safety factor was employed as the index of stability. Thus, a stability analysis framework of swelling loess tunnel was established by considering the swelling and softening of swelling loess. In an engineering case, the theoretical models presented in this paper were introduced to investigate the change rules of surrounding rock deformation and tunnel safety factor, as well as the sensitivity of the safety factor of tunneling in swelling loess. Analysis results showed that the change rules of surrounding rock response curves and safety factors obtained by the models were consistent with the actual situation, and the predicted maximum radial displacement of free face was close to the measured value. These indicate that the models are applicable to the stability analysis of initial support of swelling loess tunnel.