Floor Failure Evolution Mechanism for a Fully Mechanized Longwall Mining Face above a Confined Aquifer

Table 1

List of detection parameters.

No.	Mining position (m)	Detection results
1	814	The electrical characteristics are chaotic, and low resistance characteristics appear near the borehole.
2	807	Two obvious electrical changes: (1) a 20–45 m hole depth and a 0–10 m vertical distance to the borehole; (2) a 40–60 m hole depth, a 20–30 m vertical distance to borehole, and a 0–5 m vertical distance to coal seam floor.
3	802	One obvious electrical change: a 45–65 m hole depth, an approximately 15–30 m vertical distance to the borehole, and an approximately 0–14.3 m vertical distance to the coal seam floor.
4	796	Two obvious electrical changes: (1) 5–30 m hole depth, 0–10 m vertical distance to drill hole, and 0–14 m vertical distance to floor; (2) 50–70 m hole depth, 15–35 m vertical distance to drill hole, and about 0–17 m vertical distance to coal floor.
5	793	One obvious electrical change: a 45–65 m hole depth, a 20–40 m vertical distance to the drill hole, and a 0–5.2 m vertical distance to the floor.
6	783	Two obvious electrical changes: (1) at a hole depth of 45–55 m, the location of the work water and the greater grounding of the metal body led to a significant reduction; (2) at a 60–75 m hole depth, resistivity was significantly higher than that previously measured. The resistance value suddenly changes from 100 Ω·m to 1000 Ω·m. Based on the mining conditions, it is judged that the floor is broken due to the mining effect. There was no water in the initial stage or slight water cutting. The characteristics of the rock formation clearly were highly electrical.
7	762	One obvious electrical change: a 45–80 m hole depth, a 12–40 m vertical distance to the drill hole, and a 0–24.6 m offset distance from the coal floor. The site was judged to be the floor rock layer destroyed by mining.
8	682	One obvious electrical change: a 25–80 m hole depth. The site was judged to be the floor destroyed by mining.
9	557	Within the same range as the previous change, the rock formation in the initial stage was severely broken by mining. It gradually became compact and stable. The resistance was generally low. There was a certain degree of water absorption, and the distribution was more uniform.