Parameter design of coal pillar in highwall mining under action of dynamic-static load

In view of the application of end slope shearer mining technology to recover a large amount of residual coal, the determination of reasonable width of supporting coal pillar is a key factor whether it can be safely and efficiently popularization and application, especially considering the influence of blasting vibration on the stability of supporting coal pillar. Based on the southern end slope at the open-pit coal mine of Pingshuo, field vibration test, theoretical analysis and numerical calculation were used to study the web pillar stability in open-pit highwall mining and its parameter design under the action of triangular load and blasting vibration on the side slope. Based on the theory of limit balance and the mutation theory, the stress distribution at the coal pillar was analyzed, combined with Mohr-Coulomb failure criterion. Besides, the ultimate strength function expression of coal pillar under the influence of mining height, mining width, load stress of overlying strata, cohesion and internal friction angle of coal pillar is established. The calculation formula of the maximum allowable plastic zone width and rational width of web pillar under different safety reserve factor conditions are established. The three-dimensional simple harmonic vibration response model of the supported coal pillar was established, and the blasting parameters such as the amount of single shot, elevation difference and horizontal distance of the blast center were studied on the response of the maximum instantaneous dynamic stress of the coal pillar, which revealed the influence mechanism of the blasting dynamic load effect on the width and stability of the plastic zone of the supported coal pillar and proposed the design method of the parameters of the supported coal pillar under the blasting dynamic load. The results show that the blasting vibration has a greater influence on the stability of coal pillar, and the instantaneous maximum dynamic stress response of coal pillar under the blasting dynamic load is positively correlated with the amount of single shot, and negatively correlated with the elevation difference and horizontal distance. With the increase of the maximum instantaneous dynamic stress response of coal pillar, the width of plastic zone of coal pillar increases proportionally, and the safety factor of coal pillar decays in an approximately linear pattern. The width of coal pillar under dynamic-static load is determined to be 5 m, and its reasonableness is verified by engineering practice.