Stability mechanism and control of roadway protection by narrow protective coal pillars in main roadways under dynamic pressure disturbance

Under the condition of narrow protective coal pillars, the stability of the surrounding rock of a main roadway is affected by the coupled effects of mining-induced stress disturbance from the working face, the lateral overhanging roof structure, and coal pillar stability. However, existing studies lack systematic research on the coordinated control of deformation and instability of the surrounding rock of main roadways and the key parameters under the condition of narrow protective coal pillars. Taking Working face 2208 of Shanxi Coal Transportation and Sales Group Jixian Shengping Coal Industry Co., Ltd. as the engineering background, a mechanical model of the surrounding rock of a main roadway under dynamic pressure disturbance was established based on the theory of nonuniform stress fields, and the influence patterns of dynamic pressure disturbance coefficients on the expansion of the plastic zone in the surrounding rock of the main roadway were analyzed. The expansion of the plastic zone in the surrounding rock of the main roadway showed the characteristics of "directionality dominated by dynamic pressure and scale controlled by strength". The vertical and horizontal dynamic pressure disturbance coefficients had the most significant influence on plastic zone expansion, followed by cohesion and the internal friction angle. Based on the pressure arch theory and the functional attributes of protective coal pillars, a reasonable width of narrow protective coal pillars was determined to be not less than 6.85 m, and the width of the narrow protective coal pillars was comprehensively determined to be 8.0 m based on engineering practice. The control mechanism for maintaining main roadway stability through mining-induced stress blocking, roof cutting and pressure relief of the lateral overhanging roof, and coordination with narrow protective coal pillars was analyzed. The horizontal distance from the left boundary of the hydraulic-fracturing weakened zone to the solid coal rib of the main roadway was comprehensively determined to be 20.5–25.7 m, the roof-cutting angle of pre-splitting blasting was not less than 9.33°, and the roof-cutting height was not less than 14.4 m. A FLAC3D numerical calculation model was established, and the effects of hydraulic-fracturing weakening and roof cutting by pre-splitting blasting on mining-induced stress blocking, reduction of the peak stress of the coal pillar, and deformation control of the surrounding rock of the main roadway were analyzed. As the distance between the working face and the main roadway decreased, the stress-blocking effect of the hydraulic-fracturing weakened zone became stronger. The greater the height of the hydraulic-fracturing weakened zone was, the smaller the peak stress of the coal bodies on both sides of the main roadway was. The greater the roof-cutting height of pre-splitting blasting was, the smaller the peak stress of the protective coal pillar was. Under the control of hydraulic fracturing and pre-splitting blasting, the deformation rate and deformation amount of the surrounding rock of the main roadway decreased. Based on the control strategy of reducing the degree of mining influence and optimizing the roof structure, a coupled control technology of "hydraulic fracturing weakening of hard rock strata + roof cutting and pressure relief by pre-splitting blasting" was proposed and applied in field practice. The field monitoring results showed that when the working face advanced to the final mining line, the maximum roof-to-floor convergence of the main roadway was 441 mm, and the maximum convergence between the two ribs was 319 mm, which effectively ensured the stability of the surrounding rock of the main roadway.