Experimental study on MICP restoration law and influencing factors of coal mining ground fissures

The coal mining ground fissures in the northwest loess covered area cause shallow water resource leakage and soil instability. Therefore, the experimental study on MICP (micro induced carbon precision) restoration of coal mining ground fissures in the loess covered area were conducted. Taking the Ningtiaota Coal Mine in northern Shaanxi as the research background, the characteristics of mining ground fissures were observed and the development characteristics of mining induced ground fissures were revealed. Based on the observation of mine pressure, the types of ground fissures in loess mining were classified. Based on the classification and characteristics of coal mining ground fractures, the mechanical and hydraulic parameters of two types of fractured loess MICP restoration samples were compared and tested using unconfined compression tests, triaxial compression tests, variable head permeability tests, and triaxial permeability tests. Based on the pH value of the restoration body, MICP calcium carbonate production, and scanning electron microscopy test results, analyze the influencing factors of MICP restoration for coal mining loess ground fissures. The results indicate that coal mining ground fissures in loess covered areas can be divided into two types: boundary ground fissures and internal ground fissures. Under the condition of boundary ground fissures, the optimal ratio of bacterial solution to cementitious solution is 1.2:1. Under the conditions of internal ground fissures, the optimal ratio of sand and loess to fill the fissures is 1:1, and the optimal ratio of bacterial solution and cementitious solution is 1.0:1. The difference in the opening of different types of coal mining ground fissures in loess covered areas results in a 31.2% difference in the yield of calcium carbonate. Therefore, the optimal MICP repair solution for boundary ground fissures accounts for a larger proportion of the cementitious fluid components compared to internal ground fissures. When the ratio of windblown sand and loess in the crack filling material is 1:1, the pH value of the mixture of filling material and repair fluid is 9.2. The yield of calcium carbonate in this pH value environment can reach 88%, promoting the efficient operation of MICP. The research results provide a reference for the repair of coal mining ground cracks in loess covered areas.