Distribution law of floor stress during mining of the upper protective coal seam.

Floor stress distribution is the main index for evaluating the mining effect of upper protective coal seam. However, parameters currently used in theoretical analysis of mining-induced stress lack practicality. Therefore, in this article, a model for calculating floor stress during mining of upper protective coal seams was established based on the theory of elastic mechanics. Subsequently, the stress induced by the abutment pressure at any point in the five parts of the floor was derived. Moreover, the distribution characteristics of the horizontal, vertical, and shear stresses of the floor during mining of the upper protective coal seam were elaborated. The results show that with the continuous mining of the working face of the protective coal seam, the vertical stress of the floor strata experiences three stages, that is, rapid increase, abrupt stress relaxation, and gradual recovery to the in situ stress. With regard to the morphology, the floor strata recompress or expand in the vertical direction. Vertical and horizontal stress are relieved in the shallow part of the floor in the goaf behind the working face, and there is an abrupt reduction in the increase of concentration degree of vertical and horizontal stress in the floor strata in front of the working face. High shear stress occurs underneath the goaf near the working face. The isoline of the shear stress is distributed in a bubble shape and is oblique to the goaf. These research achievements can provide some theoretical basis for understanding the gas drainage during the mining of the upper protective coal seam.