Abstract: Laboratory consolidation test was carried out to study the influence factors of sand reinforcement by native microorganism. The native urease organisms in the soil were activated by using different injection methods with different concentrations of organic matters. After the urea hydrolysis ability reached the threshold value by the urease bacteria proliferated in the soil samples, the microbial species in the samples were identified and phylogenetic analysis was carried out, and then MICP cementaion treatment was performed. Biochemical changes in soil samples were monitored by cells density of urease bacteria and urea concentration. The reinforcement effects of soil samples were evaluated via unconfined compressive strength (UCS) and scanning electron microscope imaging (SEM). Results demonstrate that the reinforcement effect is strongly related to the way of injection and the concentrations of organic matter and urea. Bidirectional alternative grouting is beneficial to enhancing the uniformity of cementation. Biochemical analysis indicates that high concentration of organic matter (yeast extract) has adverse impact on the microbial diversity in soil, the activation of native urease bacteria in soil and the cementation effect. The residual concentration of urea in the sample with 0.35 mol/L urea was higher than that in the samples with 0.20 mol/L urea. In the present study, the soil column treated by 1.0 g/L yeast extract, 0.20 mol/L urea/calcium chloride solution and bidirectional injection achieved sound uniformity of cementation, with its carbonate crystals tightly connected and the UCS value reaching 1.55 MPa. The results conclude that the native microorganism in the soil can be activated effectively by a proper proportion of solution, hence strengthening the soil.

Key words: soil reinforcement, microbial induced calcite precipitation (MICP), native urease organisms, injection method, concentration of organic matter