Evolution of the Overlying-Strata-Bearing Structure and the Permeability Distribution of the Broken Interlayer in Shallow Multiseam Mining

Abstract

Due to the particularity of the occurrence characteristics of shallow multiseam mining, underground mining space is vulnerable to obvious roof behaviors and serious gas anomalies, which are closely related to the destruction and migration characteristics of the overlying strata, the characteristics of the overlying-strata-bearing structure, and the permeability distribution of the broken interlayer under the influence of the overlying-strata-bearing structure. However, the related research is still insufficient. In this work, the discrete-element method (DEM) numerical model was used to simulate the entire process of the continuous mining of a shallow multiseam. In addition, the gas migration and its distribution in the lower mining field, under the influence of low-oxygen (O2) gas intrusion into the upper goaf, and the air leakage flow in the working face were studied using a Particle-Flow Code (PFC)–COMSOL coupling method. The results showed that the main feature in the evolution of the overlying-strata-bearing structure was the dynamic change in the supporting relationship between the collapse area and the surrounding rock, which changed from complete support to local force conduction, and finally complete separation under continuous mining in the upper and lower coal seams. The load distribution of the coal-seam floor presented a continuous change from surrounding rock to inside the goaf, with the breaking angle of the overlying strata being the key influencing factor. The conjoint analysis of stress and porosity indicated that the load under the overlying-strata-bearing structure determined the permeability distribution of the broken interlayer, which gave the gas in the goaf obvious distribution characteristics.