Surrounding rock activity law and support optimization of double flexible formwork wall gob-side entry retention in stratified mining coal seams

Abstract

To ensure gas control in the layered mining of extra-thick coal seams in Baijigou Coal Mine—in addition to reducing the loss of coal resources and amount of roadway driving—this article proposes the gob-side entry retention technology of flexible formwork concrete, and retains the 010202 working face four-layered return air roadway as the 010203 working face gas drainage lane. By combining theory and field practice, a flexible formwork concrete wall + anchor cable joint support scheme is proposed to strengthen small coal pillars. Further, the design scheme for the aforementioned framework at the bottom of the layered mining region of an extra-thick coal seam is proposed. Then, field tests are carried out, and the surrounding rock activity law of the double flexible formwork wall entry retention in a layered mining coal seam is obtained. The results show that (i) 010202 the whole process of overburden collapse to stability at the stratified mining face is completed 60--200 m behind the face and (ii) the roadway deformation of the extra-thick coal seam in the Baijigou Coal mine can be divided into four stages, and the mining pressure of the gas drainage roadway basically presents a dynamic change of “pressurized stage – peak stage – stabilized pressure stage”. Based on this, three mechanical stages of roadway deformation are derived: (1) δz< Nmz<< Nwz; (2) Nmz < δz << Nwz; and (3) Nmz < δz< Nwz. Three repeated mining support optimization schemes are provided, one for each stage. Finally, the deformation and damage of the surrounding rock are effectively controlled, and the requirement that the gas drainage roadway is still intact under the influence of the 010203 working face mining is realized. This study has certain reference significance for gob-side entry retention in the bottom layer of stratified mining in extra-thick coal seams.