(SR) 6. Ground Control
#4 seam of the Smoky River Coalfield underlies both the Smoky river valley and the Sheep Creek valley. The 5B-4 Mine underlies the Smoky River valley. In the past, the Mines roof below the Smoky River valley was always more troublesome than its counterpart below the Sheep Creek valley. Roof reinforcement is needed in general at the 5B-4 Mine.
(SR) 6.1 Ground Control Method
Mechanical roof bolts had been used in the past. Due to numerous roof falls in the #5 Mine and #2 Mine, resin bolt trials were introduced in 1975. Roof stability is regularly monitored by 2- point telltales installed about 40 ~ 60 m (130 ~ 200 feet) apart. Roof support density increases for the poor roof condition. The support is denser at multiple opening intersections.
Roof stability at the portals seems more problematic than in the crosscuts and panel entries. The cause of this phenomenon is not yet determined. The local structure could be the reason.
Prior to 5B-4 Mine, all the Mines had entries 6.7 m (22 feet) wider and hence the heavy roof support was required. In 5B-4 Mine, the entry width is reduced to 4.8 m (16 feet) nominal. More details are given in Figure SR8. The support design has two aspects as follows:
1. Support type:
(a) Mechanical roof bolt
(b) Resin rebar bolt
(c) Steel screen retained by the roof bolts
(d) Plastic mesh retained by the rib bolts
The designed bolt strength is 62 MPa (9000 psi) on average.
2. Roof support density:
(a) Normal roof condition in the long term Entries:
3.6 m (12 feet) long resin rebars are used 4 pieces per row across the cross-section of the roadway at an interval of 1.2 m (4 feet) in addition to the side wall bolting. The entire roof is covered by steel screens.
(b) Poor roof condition in the long term Entries:
The number of bolts is increased from 4 to 6 per row. All the other features of 2(a) above remain unchanged.
(c) Normal roof condition in the short term roadway:
Instead of 3.6 m (12 feet) long resin rebars, 2.4 m (8 feet) long mechanical bolts are used. The roof is covered by the steel mesh.
(d) Poor roof condition in the short-term roadway:
6 pieces of bolts are used per row. All other descriptions in 2(c) above remain unaltered.
(e) Additional support on a regular basis:
1 or 2 pieces of 1.5 m (5 feet) long mechanical rib bolts are installed per row with plastic mesh.
(f) Intersection of the multiple openings:
The same support system is used as described in 2(a) above.