Abstract: Based on numerical simulation, a new model has been established on the effectiveness of protective mining due to the influence of the mining thickness of the upper protective coal seam, the thickness of the protected coal seam, and interburden rock strata properties. The results have shown that if the mining thickness of the upper protective coal seam increases but still is less than the occurrence thickness of the lower protected coal seam, or if the occurrence thickness of the protected coal seam increases but the mining thickness of the upper protective coal seam remains the same, the protective effects will increase gradually. But once the mining thickness of the upper protective coal seam is larger than the occurrence thickness of the protected coal seam, the protective effect will not change basically.Furthermore, the harder the interburden rock stratum between coal seams is, the less the plastic deformation and stress reduction of the lower protected coal units will be, and therefore the worse the protective effect will become. Finally, the stress reduction would be larger if the differences of the neighboring rock stratum are large in coal and rock mass, and both the longitudinal squeezing caused by imbalanced vertical stress and the transverse shearing caused by imbalanced horizontal stress are the important precipitating factors that lead to the eccentric instability of coal and rock units under the gob and then result in the plastic deformation and failure.

Key words: coal thickness, interburden rock properties, upper protective mining , changes of geostress and effects of plasticity, numerical analysis