Failure analysis and control mechanism of gob-side entry retention with a 1.7-m flexible-formwork concrete wall: A case study

Gob-side entry retention (GER) improves coal resources recovery rate and solves the gas problem at the working face and is thus of great importance. In consideration of the existing GER results that lack a failure mechanism for GER with double roadways and roadside backfilling body (RBB) width below 2 m, taking the first test section of GER with double roadways of No. 3 coal seam of the Hexi mine as the research object, the uniaxial compressive strength of each age concrete is obtained through laboratory tests. Numerical simulation and theoretical calculations are used to comprehensively determine the optimal width of the flexible-formwork concrete wall of GER. The distribution and evolution of plastic zone and vertical stress of surrounding rock in different positions are simulated after mining for a certain distance. The results demonstrate that the plastic zone of surrounding rock of the roadway evolves morphologically as asymmetrical ellipse -> asymmetrical circle -> small semicircular arch -> large semicircular arch. The maximum depths of plastic zone and the peak vertical stress in solid coal side are 3.56 m and 3.50 m, respectively, and the roof and solid coal present obvious asymmetric failure characteristic. The flexible-formwork concrete wall is completely in the plastic state, and the peak stress is mainly concentrated in the middle of RBB. Based on the above analyses, a zoning asymmetric coupling control technology named narrow flexible-formwork wall with steel bar reinforcement along single prop wall + strong double-row single props + high-strength and high-preloading asymmetric anchor cable is proposed, and its support mechanism is clarified. The on-site monitoring results demonstrate that the convergence of surrounding rock of GER is effectively controlled.