(Q) 6. Ground Control
In both 2 North and 4 South Mines, the roof strata is mainly mudstone. The roof and walls are generally in good condition and indicate no sign of failure. Pillars are in good condition and need no artificial support for reinforcement. However, the Comox formation has many steep faults, which necessitate artificial support of timber cribs at faulty areas.
In the case of the 2 North Mine in the #1 Seam, ground control aims at preventing the undesirable roof falls due to faults. In the case of the 4 South Mine in the #3 Seam, ground control aims mainly at the controlled roof fall and preventing the surface subsidence.
The overlying strata are regularly reinforced by bolting. The warning timbers are installed at the intersections while depillaring. Figure Q8 shows the sequence of face advance and roof bolting simultaneously. The exposed roof is reinforced with the roof bolts within 24 hours.
Roof support at 2 North Mine is provided by 2.4m (8 feet) long, 19mm (0.75 inch.) diameter, torque-tension bolts. The bolts are installed with a 0.6m (2 feet) of resin anchor resulting in almost 0.9m of encapsulation. Welded wiremesh is installed occasionally to prevent roof spalling at some places (Figure Q9). Breaker posts are installed at the roadway intersections (Figure Q7).
In the depillaring districts, 1.8m (6 feet) long and 19mm (0.75 inch.) diameter rebars with a threaded end are bolted on a 1.2m x 1.2m (4 feet x 4 feet) grid. Point anchorage with a single epoxy resin cartridge (35 second set) and 15.2cm to 20.4cm (6 inch. to 8 inch) plate secures the bolt in competent strata. The bolt is torqued to 24.2 to 27.6 kg-m (175 to 200 ft-lbs).
The main entries and headings for long term access are reinforced by 2.4m (8 feet) long and 19mm (0.75 inch.) diameter rebars bolted on a 1.2m x 1.2m (4 feet x 4 feet) grid. In addition, they are supported with full wire screen.
Roof support for 4 South Mine is similar to 2 North Mine with some differences as follows:
(a) 1.8m (6 feet) long roof bolts are used instead of 2.4m (8 feet) long in main headings.
(b) 4 bolts are used per row across the roadway instead of 5 bolts. That gives a 5 feet x 5 feet grid pattern.
(c) No wiremesh is used in the roof support of the 4 South Mine.