em of sub-level caving with crushing of coal by overlaying strata: Coal seam is preliminarily broken in upper section by means of sub-level caving (using the same sub-level caving supports as for the operation below, but without the rear conveyor. Bottom section is mined by the basic system of sub-level caving as illustrated above. Typical seam thickness is up to 18m and above.17��3 Sequence of working
A. The sub-level caving face operates in the bottom of the coal seam to be extracted.
B. The front coal is extracted using a shearer or by blasting in the conventional manner to a depth of 1 or 2 meters.
C. The rear coal is allowed to cave ready for loading.
D. There is provision in the support to give access to drill and fire the roof coal where the coal does not readily break up for loading.
E. A rear conveyor within the support transports the caved coal.
F. Once loading has been completed the rear door is closed and the sequence recommences [11].
17��4 Suitable Conditions of Sub-Level Caving
A. Thickness of coal seams: In general, the sub-level caving is suitable for the thickness of coal seams between 6��12m. If the thickness of the coal seam greater than 12m, the top slice must be extracted in advance.
B. Dip angle of coal seams: In general, the most suitable dip angle for using sub-level caving is less than 10��in the gently inclined seams, In China, sub-level caving has been tried in coal seams with 20��dip angle, but the tries are unsatisfactory.
C. Roof strata conditions: The roof of the coal seam is not too hard and it can cave following the top coal drawing.
D. The strength of coal: The coal is easy to break and can draw immediately following the supports advancing.
E. There are no severe geological disturbances, spontaneous combustion, and gas emission.
Although the sub-level caving in gently inclined extrathick coal seams [12] was well practiced in many coal mines in China, some problem still need to be studied, such as the most suitable application conditions, the measures of increasing the coal recovery rate, the methods of controlling spontaneous combustion [13] and dust etc. It is expected that this mining technology will get certain development under suitable conditions.
Sub-level caving is also a very effective method for mining steeply inclined extrathick coal seams at present. The sub-level caving by horizontal slices [14] has been successfully applied in coalmines with steeply inclined extrathick coal seams in China . The coal seam is divided into many horizontal slices (sub-level) with thickness of 7��10meter. In the bottom of a slice, a face with height of 2.5 meter is extracted by blasting or shearer, and advances along the strike [15] of the coal seam. The top coal on the face will be drawn down immediately after the supports move forward.
NOTES TO THE TEXT
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