Physical model experiment and numerical analysis on innovative gob-side entry retaining with thick and hard roofs

Gob-side entry retaining is a common method in underground coal mining. Conventional gob-side entry retaining (CGER) always causes stress concentration and large deformation around roadways under the condition of thick and hard sandstone roofs. Thus, an innovative method of no pillar gob-side entry retaining (NPGER) in longwall mining is proposed in this paper. First, a physical model was established to explore the mechanism of the new method. The modeling results indicate that the progressive caving of the strata in the middle of the stope is consistent with CGER. The major difference between NPGER and CGER is that CGER collapsed with a structure of a long arm beam, but NPGER collapsed with a structure of a short arm beam near the roadway. In the end, a new roadway with a gangue rib was formed when the immediate roof and main roof slipped along the pre-cutting plane. Subsequently, the numerical simulation software Universal Distinct Element Code (UDEC) was adopted to establish the numerical model of the 8501 working face in Tangshangou coal mine. The NPGER process is simulated. The evolution of the roof structure and stress characteristics were clearly demonstrated. The results showed that the roof structure noticeably changed from a long beam to a short beam after roof fracturing. The retained roadway was under low stress conditions, and the stress was transferred to the deep rock. In the end, the NPGER method was applied in the field, and the results showed that the retained roadway could meet the requirements of the next working face, which may provide guidance for longwall mining with thick and hard roofs.