Caving structure and crack evolution of overlying strata in gob-side entry retaining with flexible formwork concrete walls

This study investigates the differences in roof caving structure and crack evolution between gob-side entry retaining and traditional coal pillar mining. Taking the 52605 and 52606 working faces of Daliuta Coal Mine as the engineering background, two sets of similar-material simulation experiments were conducted to reproduce the mining processes under both conditions. The movement and fracture evolution of the overlying strata were systematically recorded and analyzed. The results show that, under gob-side entry retaining with flexible formwork concrete walls, the crack development rate at the end of primary mining is lower than that during secondary mining. In contrast, under traditional coal pillar mining, the crack evolution patterns on both sides of the coal pillar are similar. Significant differences are observed between the two mining methods in terms of crack rate, crack type, caving range, and caving angle. Specifically, for gob-side entry retaining, the crack rate of overlying strata reaches 5.0756%, the caving range extends to within 50m, and the caving angle varies from 31° to 86.9°. For coal pillar mining, the crack rate is 2.8604%, the caving range is within 40 m, and the caving angle ranges from 50° and 52°, with shear cracks dominating along the caving direction. After mining with gob-side entry retaining, the roof strata on both sides of the concrete wall remain stable without sliding, forming a hinged structural system. In contrast, in coal pillar mining, the overlying strata on both sides of the pillar tend to fail together as a whole after extraction. These structural differences lead to distinct load transfer mechanisms, resulting in significant stress concentration effects on the concrete wall in gob-side entry retaining faces.