(Pr) 6. Ground Control
Ground control problems at Prince are related to its unique sedimentary and structural geological history:
(i) Roof falls have occurred in areas of shale and mudstone roof adjacent to the pinchout zones of paleo river channels.
(ii) Elevated levels of horizontal stress has had significant adverse effect on the convergence of the roof and the floor in longwall gateroads.
(iii) In the vicinity of MacKenzie Syncline, some anomalous strata movement events have occurred. This area must be watched closely in other areas of the mine where the synclinal axis is projected.
(iv) Extensive areas of Prince Colliery roof has poor rock mass ratings. (CMRR > 30).
(Pr) 6.1 Ground Control Method
Ground control at Prince Colliery is maintained in the following ways:
(i) Well lagged steel sets are used to support the roof.
(ii) To avoid the ground control problems associated with elevated horizontal stress, longwall panels are aligned closely to the direction of principle horizontal stress.
(iii) Because there are extensive areas with low roof rock ratings, the practice of extended cuts, and place changing have not been used.
Primary roof support at Prince Colliery is provided primarily by steel sets. These supports consists of steel beams (W6 x 25) supported by legs (W6 x 15). These have capacity of 12 tonnes yield and 18 tonnes ultimate load for a 4.3 m (14.1 feet) span (assuming uniformly distributed load and a 45º load triangle).
Most slope intersections are supported with square steel sets and large collar beams for breakoffs. All slopes are supported with 5.5 m (18 feet) steel arches.
Steel flat beams are spaced 0.9 m (3 feet) apart in the gateroads and installed at 0.6 m (2 feet) spacing in poor conditions. There are presently no areas where roof bolts constitute the primary roof support.