- freely available
Minerals 2017, 7(3), 35; https://doi.org/10.3390/min7030035
2. Mine Conditions and Mining Techniques
3. Field Monitoring
3.1. Arrangement of Boreholes
3.2. Monitoring via Surface Borehole
3.2.1. Strata Movement and Fracture Propagation in Borehole #1 and #1’
3.2.2. Strata Movement and Fracture Propagation in Borehole #2
3.3. Underground Observation Borehole
3.4. Heights of Caved Zone and Fractured Zone
3.5. Ground Surface Subsidence
4.1. Movement of Overburden Strata and Fracture Propagation
- Initiation stage: This stage started when the working face was 100 m away from the borehole and ended when the working face was 20 m away from the borehole. This stage was characterized by cracks distributed on the ground surface, and fractures appeared on the loess layer and strata beneath it. The density and average size of these cracks increased as the working face moved forward. Surface deformation and local borehole wall exfoliation were observed. Additionally, axial and circular fractures were observed on near-surface strata, while deep strata were not significantly affected.
- Active stage: This stage started when the working face was 20 m away from the borehole and ended when the working face was 120 m past the borehole. Propagation of fractures in the overburden strata and movement of the ground surface were significantly accelerated. Mining-induced fractures appeared in the coal and rock ahead of working face within a certain range. When the working face reached the borehole, step subsidence was observed, resulting in severe surface ground deformation. Large-sized fractures and local damages were observed in the bedrocks beneath the epipedon. Separation and dislocation of the overburden strata were observed after failures of the immediate roof and main roof, and mining-induced fractures propagated. The overburden movement, which was characterized by the movement of the key strata, was transferred upwards to the ground surface gradually.
- Degradation stage: This stage started when the working face was 120 m past the borehole. During this stage, the movement of the overburdened strata degraded (there was a slow subsidence due to gravitation). The bed separation and fractures that had developed in the overburden strata were partially closed to compaction and the rate of surface deformation slowed down. Surface cracks and step subsidence were partially mitigated.
4.2. Distribution of Fractures in the Overburden Strata
- Field observations showed that the overburden strata generally experienced the phases of roof caving, generation of fracture, bed separation, dislocations, fracture propagation, surface subsidence, closing of fractures. The entire mining process can be divided into the initiation stage, the active stage, and the degradation stage according to activities of the overburden strata and propagation of mining-induced fractures.
- The advance affecting range and angle of the working face were 96.7 m and 71°, respectively. The caved zone height is 2.9–4.11 times of the mining height, and the length of fractured zone is 19.35–22.19 times of the mining height. The height range of three parts in fractured zone is 24–26, 40–45, 30–35 m. Significant fractures were observed in the bending zone. Step subsidence and cracks, which indicate severe damages, were observed on the ground surface above goaf.
- The borehole television approach is an effective and rapid way to observe the dynamic morphology of boreholes and facilitate the investigation of temporal and spatial evolution of mining-induced fractures in overburden strata during the mining process. Nevertheless, application of this approach has been limited by several issues. For instance, the resolution of the obtained images may be significantly affected by air humidity in the underground borehole. When the air is very damp in the boreholes, the observed images are often blurred. Also, observation cannot be achieved in the presence of wall damage or horizontal dislocation of the borehole, which lead to collapse and blockage.
Conflicts of Interest
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|Zone||Borehole #1||Borehole #1’||Borehole #2||Borehole #3||Borehole #4|
|Height/m||Ratio to Mining Height||Height/m||Ratio to Mining Height||Height/m||Ratio to Mining Height||Height/m||Ratio to Mining Height||Height/m||Ratio to Mining Height|
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