In order to obtain the minimum mining height that can play an effective protective role in the mining of the non-full coal protective layer in the Hongyang No. 3 coal mine and improve its economic benefits, the relationship between the mining height and the pressure relief of the protected layer is studied. Theoretical analysis is used to establish a calculation model of the goaf stress distribution law, with the mining height as a variable. The calculation model research results show that the mining height adjusts the goaf stress distribution by adjusting the range of the “three zones”. The force of the falling zone and the frustration zone on the goaf is approximately trapezoidal geostatic stress, and the roof stress in the vertical projection area of the trapezoidal waistline is not transmitted to the goaf. The development heights of the “two zones” are different at different mining heights, and the corresponding pressure-relief ranges are different from the waistline vertical projection. The curved subsidence zone transmits stress to the goaf through the fissure zone and the caving zone below, which can be calculated by the elastic foundation beam model. The falling zone is the elastic foundation, and different mining heights have different foundation coefficients. With the increase in mining heights, the foundation coefficients first decrease and then tend to be stable. The pressure-relief range of the stress transmitted from the curved subsidence zone to the goaf first increases and then tends to be stable. According to the calculation model, the minimum mining height for effective pressure relief of the upper protective layer of thin coal in the Hongyang No. 3 coal mine is 2.5 m, which can effectively relieve the pressure of the protected layer with the floor layer spacing of 48 m.

In order to quantitatively evaluate the degree of protection in protective layer mining and provide guidance for the design of a secondary outburst elimination scheme, a variable weight-projection gray target dynamic evaluation model for the effectiveness of protective layer mining is established. The improved order relation analysis method was used to determine the subjective weight of each index toward the decision-making goal based on numerical diversity characteristics, and the initial fixed-weight calculation for mixed multi-attribute metrics was processed through the degree of index action. The variable weight function was used to dynamically adjust the fixed weight through the penalty and incentive index methods. Four indexes (gas content, gas pressure, coal seam permeability coefficient, and expansion deformation) were selected, the outburst elimination and anti-reflection were taken as the guide, and the critical value of each index for eliminating burst and the critical value of pressure relief were taken as the positive and negative bullseyes. Based on the variable weight-projection gray target decision model, the distance between the two target centers of each scheme was calculated; at the same time, the variable weight vector changed dynamically with the evaluation scheme to achieve the dynamic quantitative evaluation of the degree of protection. Additionally, compared with the calculation results of fixed weights, it was found that the variable weight-projection bullseye distance can more accurately reflect the dynamic control effect of differences in numerical combinations of multi-attribute indexes in different decision schemes based on the degree of protection of protective layer mining. Taking a mine in PingMei as the engineering background, Ding protected the Wu area, and the degree of protection in the Wu group coal seam reached 116.29%, eliminating the outburst risk of the coal seam. The Wu protected the Ji group coal seam, with the degree of outburst risk in the Ji group being reduced by 14.27, and the Ding + Wu group protected the Ji group coal seam, with the degree of outburst risk of the Ji group being reduced by 20.71%, but not eliminated. The evaluation model quantifies the degree of protection of protective layer mining, and provides a theoretical basis for further assessing whether the working face should strengthen the enhancing permeability or whether it needs to be used in tandem with high-strength outburst elimination methods.