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Abstract In ground control theory，the supporting action of a coal seam under a tight roof has often been simplified to that of an elastic foundation，which ignores the influence of the decreasing bearing capacity in the plastic zone of the coal seam near the working face. In order to solve this problem，in this research，the coal seam was divided into two regions：a plastic zone near and an elastic zone ahead of the working face. A nonlinear expression for calculating the supporting force in the plastic zone of the coal seam was proposed based on the fact that the variation of the coal seam supporting force is continuous and the maximum supporting force occurs at the boundary between the plastic and the elastic zones. The calculation method to determine the maximum supporting force and two methods to assess the accuracy of this value are presented. The supporting mechanism of the elastic and plastic zones and the hard roof mechanical behaviour were analyzed by extending the traditional assumption that the roof is fully supported by an elastic foundation. An equation for roof deflection above the plastic zone of the coal seam was derived and constants were obtained for all five parts of the equation. A numerical example has demonstrated that with increasing step size of gob area，the deflection of the roof，the maximum supporting force in the plastic zone，and the bending moment of the roof between the working face and the middle of the gob area all increase，while the supporting force near the working face decreases slightly. However，the maximum bending moment ahead of the working face is always greater than that in the middle of the gob area. Therefore if failure occurs because of too large a bending moment，the fracture lines will be in the plastic zone of the coal seam ahead of the working face. Compared with the traditional approach of assuming a fully elastic foundation，our analysis has led to the following new conclusions. The roof and the coal seam work together to support the weight of overlying rocks. The supporting force of the coal seam at the working face assuming a fully elastic foundation is several times greater than that obtained when the plastic zone of the coal seam is considered. Consequently，when fully elastic foundation is assumed，the roof deflection is small due to small bending moments in the roof，and the range of significant deflection in the roof is also small. However，when the plastic zone is considered，the supporting force of the coal seam near the working face reduces significantly and therefore the roof will take more of the weight of the overlying rock，resulting in much greater deflection in more areas of the roof. This leads to significant increases in both the maximum bending moment and the distance from where it occurs to the working face，which is directly related to the fracture failure in the roof ahead of the working face. The predicted distance based on the plastic zone approach in this case is greater than 4 m，which agrees well with in-situ observations of the fracture failure in the roof ahead of the working face，suggesting the bending force and deflection of the roof calculated based on the proposed approach are much closer to reality.
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2015, Vol. 34 
