Characteristic and Mechanism of Roof Fracture Ahead of the Face in an LTCC Panel When Passing an Abandoned Roadway: A Case Study from the Shenghua Coal Mine, China

Due to the abandonment of roadways after the use of conventional mining techniques, fracture behaviours of the roof of a longwall top coal caving (LTCC) working face significantly differ from general coal mining behaviours. This study examines conditions of the no. 3 LTCC working face of the Shenghua coal mine. By theoretical analyses, analogue simulation tests and numerical calculations, we examined the roof failure characteristics, forces acting on supports, and action characteristics of abutment stress observed in front of a longwall face when an abandoned roadway was installed in front of a cave working face. We formed the following conclusions. First, due to effects of abandoned roadways and roof-caving regions in front of the working face, the roof of the working face exhibited advanced fracture characteristics. Second, stress exerted on the surrounding rock of the working face showed a different distribution pattern than that of the coal mining face, with supporting stress noticeably concentrated at the pillar. Third, as hydraulic supports have been subjected to the impact load of the fractured roof, supports have poorly adapted, with a maximum working resistance of 14,000 kN. These conditions may cause serious accidents due to the supports' failure or hydraulic cylinders' breakage. Fourth, based on a statically indeterminate equation and the superposition principle, the critical condition of roof fracturing occurring in front of a working face was measured, and the critical width of an abandoned roadway that spurs advanced fracturing in the working face of the Shenghua coal mine was calculated as 6.2 m. Last, to ensure the stability of the surrounding rock and panel supports, a pre-filling scheme of the abandoned roadway is proposed.