Analysis of Strata Stress and Its Effect on Lower-level Seam Exploitation under Room-and-pillar Shallow Coal Mining

To investigate the stress characteristics of the strata of coal 2-2 and its impact on the coal 3-1 exploitation under roomand- pillar shallow coal mining in Huoluowan coal mine of China, the theoretical, numerical and physical-modelling approaches were employed. The strata of coal 2-2 were categorized into four types, namely, stable, unstable, 20 m and 50 m coal pillars, respectively. The stress fields corresponding to various types of underlaying strata were revealed and the influence on the coal 3-1 mining was analyzed and verified by the physical model experiment. Results show that the stress of coal 3-1 is in the pressure-relief state under the stable and unstable pillars, 8% to 14.3% decreases of the initial vertical pressure. The vertical stress in coal 3-1 increases 5% to 32% under the 20 m and 50 m coal pillars, respectively. The periodic caving length of roof of coal 3-1 is 12 m to 14 m under the stable and unstable pillars, but it is 10 m to 11 m under the 20 m and 50 m coal pillars. The physical modelling demonstrates that the coal 3-1 is ejected when that is mined below the 20 m and 50 m coal pillars. The obtained conclusions have applicability in reference to the similar practice.