MICP restoration law of coal mining induced ground fissures and its influencing factors

The coal mining induced ground fissures in the loess covered area of northwest China cause shallow groundwater resource leakage and soil instability. Therefore, the experimental study was conducted on Microbial Induced Carbonate Precipitation (MICP) restoration of coal mining induced ground fissures in the loess covered area. Specifically, the characteristics of mining induced ground fissures were observed and their development characteristics were revealed based on Ningtiaota Coal Mine in northern Shaanxi. The coal mining induced ground fissures in loess were classified with consideration to the observed mine pressure. Based on the classification and characteristics of coal mining induced ground fractures, the mechanical and hydraulic parameters of two types of MICP-treated loess fissure samples were compared and tested using the unconfined compression tests, triaxial compression tests, variable head permeability tests and triaxial permeability tests. In addition, the influencing factors of MICP restoration for coal mining induced ground fissures were analyzed based on the pH value of the restoration body, MICP calcium carbonate production, and scanning electron microscopy test results. The results indicate that coal mining induced ground fissures in loess covered areas can be divided into two types: boundary ground fissures and internal ground fissures. In terms of boundary ground fissures, the optimal ratio of bacterial solution to cementitious fluid is 1.2∶1.0. For the internal ground fissures, the optimal ratio of wind-blown sand and loess to fill the fissures is 1∶1, and the optimal ratio of bacterial solution and cementitious fluid is 1∶1. The difference in the opening of different types of coal mining induced ground fissures in loess covered areas results in a 31.2% difference in the yield of calcium carbonate. Therefore, the optimal MICP restoration solution for boundary ground fissures has a larger proportion of cementitious fluid compared to internal ground fissures. When the ratio of wind-blown sand and loess in the filling material is 1∶1, the pH value of the mixture of filling material and restoration fluid is 9.2, and the yield of calcium carbonate in such pH value environment can reach 88%, promoting the efficient operation of MICP. The research results provide a reference for the restoration of coal mining induced ground fissures in loess covered areas.