Stress and deformation evolution characteristics of gob-side entry retained by the N00 mining method

The N00 mining method, as a novel coal mining technology, differs greatly in both the mining process and key technologies from traditional coal mining methods. To maintain entry stability under such new mining conditions, the stress and deformation process of the rocks surrounding the entry were analyzed based on the Mohr-Coulomb theory in this paper. The following results are concluded: the surrounding rocks of the entry formed with the N00 mining method usually experience eight different stress states. Consequently, the entry deformation also goes through eight development stages. In these eight stages, under the primary stress state (stage I), the entry deformation is negligible. After coal seam excavation is started (stage II), the surrounding rocks of the entry will immediately show a trend of large deformation. Next, after roof support (stage III) and roof cutting (stage IV) are implemented, the entry deformation velocity will decline obviously. However, the entry will continue to deform for a long time when the roof rock stratum at the back of the working face begins to move (stage V). The entry deformation tends to be stable until the roof stratum stops moving (stage VI). When the adjacent working face is mined (stage VII), the entry at a certain distance in front of the working face will once again demonstrate prominent deformation. After the adjacent working face is extracted, the entry will be in the goaf (stage VIII) and become damaged. Furthermore, numerical modeling and engineering measurements are adopted to analyze the deformation and stress evolution process of the surrounding rock. The same results as provided by the theoretical analysis are obtained. The above research results show that there are three potential critical stages for the entry formed by the N00 mining method. To ensure entry stability, the clarification of these three critical stages, timely support and roof cutting are vital measures that can be taken. At the same time, roof temporary support and advance support are essential in the process of roof movement and during the mining of the adjacent working faces.