(SR) 4. Mine Design
The original Room and Pillar mining design was to drive parallel sets of 3-entries along the strike from the portal to the property boundary for long-life panels leaving barrier pillars of 50 m (150 feet) inbetween. The 3-entry system seemed to have many disadvantages. A 5-entry system with short-life panels was adopted in the early 1980's (Figure SR7). At present, 4-entry system is used for panel development due to following advantages:
(a) Better ground control in the roof reduces the artificial support requirement.
(b) Less air is required to ventilate the 4-entry system as compared to 5-entry system.
Barrier pillars left between the adjacent panels are 50 m (150 feet) wide.
The 5B-4 Mine has 3 portals 30 m (100 feet) apart at the outcrop of the #4 seam, located somewhere half way up the mountain.
Main entries are developed following the seam. The #4 seam has inclination varying from 6° to 12° in this area. Entries are 3 m (10 feet) high and 6 m (20 feet) wide. Development pillars are 30 m (100 feet) centre to centre as above.
Panel entries are driven up the dip where seam inclination is less than 16° Seam thickness varies from 3 m (10 feet) to 5 m (17 feet). 3 m (10 feet) high and 5 m (17 feet) wide entries are driven along the roof. Pillars in all panels are 25 m (83.33 feet), centre to centre. Crosscuts are driven as follows:
(a) Crosscut interval is 30 m (100 feet) in the panels #1 South, #2 South and #3 South.
(b) Crosscut interval is envisaged 60 m (200 feet) in the future panel #4 South as per Analysis of Retreat Mining Pillar Stability (ARMPLS).
3-entry panel system was practiced before. At present, 5-entry system is in use. Irrespective of these development systems, the depillaring follows the same pattern as given below (Figure SR7)
- After completion of the panel development, the depillaring commences from the farthest end of the panel. Depillaring proceeds towards the mains down the dip of the seam.
- The developed pillars are split along the strike to form blocks approximately 12 m (40 feet) wide.
- These blocks are mined in the above depillaring sequence using an open-ended christmas tree cut sequence.
- There are 6 m (20 feet) passes taken on each side of a conventional continuous miner.
- The final block within the pillar under depillaring is left unmined in the gob.
- Occasionally, stump pillars are left in the block under depillaring.
In the 3-entry system, the entry and the crosscut used to meet at acute angle (¹90°, Figure SR7). Consequently, the maximum roof span at the intersection would become large. The same figure reveals that in the 5-entry system, an entry and a crosscut meet at a right angle. The openings are 6 m (20 feet) wide, giving a maximum roof span at the intersection 8.5 m (28.28 feet) approximately.
In #3 portal, the 3-way intersection rendered better roof stability than the 4-way intersection in #4 and #5 portals. However, the latter provides more traffic convenience.
An extended cut is considered to be any cut done by the continuous miner greater than 6m (20 feet) from the last full row of roof bolts. In Smoky River property, the practice of extended cut is successful everywhere during depillaring in the 5B-4 Mine except for the fracture zones.
The experience with U.S. Mines indicated in the past that the CMRR value should to be more than 50 for successful use of the extended cut. The CMRR rating for Smoky River roof strata is 50 or less as seen in (SR) 5.2 below.