Instability mechanism of surrounding rock in gob-side entries with narrow coal-pillar in extra-thick coal seams and push-pull anchorage control

This study addresses the problem in the control of large deformation in surrounding rock of gob-side entries with narrow coal pillars during the fully-mechanized top coal caving mining of extra-thick coal seams. The instability mechanism of surrounding rock and the application of push-pull anchoring technology were studied using theoretical analysis, numerical simulation and field testing. Specifically, we established a mechanical model of an overlying right triangular block to unveil how the structural state of this block affects the stability of surrounding rock. Secondly, a numerical model was developed to systematically analyze the influence of overburden movement on the distribution of the deviatoric stress field in the surrounding rock, revealing the stress evolution characteristics of gob-side roadways with narrow coal pillars. Finally, a coordinated control strategy integrating zonal differentiated high-strength support and push-pull anchorage technology was proposed and implemented in field applications. The research results indicate that, under the combined effects of rotation, subsidence, and migration of lateral key blocks in the main roof, severe deformation and failure occur in the shallow surrounding rock of the roadway, resulting in insufficient accumulation of distortion energy. Consequently, high distortion energy is transferred to the deep roof on the solid coal side, forming a strong compressive strain zone. The shallow plastic surrounding rock in the coal pillar rib, solid coal rib, and upper shoulder region constitutes the dominant deformation control area, with strain types exhibiting pronounced dynamic evolution characteristics. This result can effectively improve the problem of controlling the fracturing and large deformation of the surrounding rock in narrow coal pillar roadways and provides a reference.