Experimental study on fracture characteristics of mining-induced surface soil solidified by MICP technology

Mining-induced fractures in the overlying soil of thin bedrock easily lead to the loss of surface water resources and pose risks to underground production safety, traditional fracture repair methods suffer from low anti-seepage performance. To address this issue, a fundamental experimental study was conducted on the repair of surface fractures in goafs using the microbially induced calcium carbonate precipitation (MICP) technology. First, soil samples were collected from the surface fracture zone of a goaf in a coal mine for MICP solidification tests to analyze the effects of different MICP parameters on the solidification performance of test specimens, and the optimal MICP parameter ratio was determined. Then, fracture specimens with different characteristics were prepared for a second round of MICP solidification tests. Finally, the influence law of fracture characteristics on solidification performance was obtained through unconfined compressive strength tests and seepage tests. The results show that the order of factor importance affecting fracture solidification performance is as follows: cementation solution concentration, bacteria solution-to-cementation solution volume ratio, bacteria solution mass fraction, with the optimal ratio being 1.25 mol/L for cementation solution concentration, 1∶2 for the volume ratio of bacteria solution to cementation solution, and 100% for bacteria solution mass fraction. With the increase in fracture dip angle, the compressive strength of solidified specimens generally exhibits an “N”-shaped variation trend of first increasing, then decreasing and increasing again. For specimens with the fixed fracture dip angle and width, the compressive strength of those solidified by MICP slurry filling is 10%-12% higher than that of specimens filled with plain soil slurry and approximately 20%-30% higher than that of non-filling specimens. The permeability coefficient of specimens after solidification is reduced by about three orders of magnitude compared with that before solidification, with the minimum value reaching 8.89×10⁻⁷ m/s (for a fracture width of 4 mm), indicating a significant improvement in permeability. Scanning electron microscopy (SEM) and quantitative image analysis reveal that the MICP technology converts large soil pores into a dense microporous structure via calcium carbonate precipitation. Calcium carbonate crystals significantly improve soil compactness by filling pores and bridging soil particles, and their cementation effect results in the synchronous enhancement of mechanical strength and anti-seepage performance of the soil.