Problem:

A coal mine located on the east coast of the United States was facing problems with their crib system. Although this system was adequate, many problems were attributed to the crib blocks. Due to the time period between crib installation and actual mining, the cribs would dry out and shrink, requiring them to be re-tightened several times which involved many man-hours of labor. The large number of crib blocks that had to be stored on the surface, transported into the mine, and eventually installed in the tailgates presented significant material-handling problems.

From a roof control standpoint, due to the passive nature of cribbing, its effectiveness in controlling longwall abutment pressures had come into question. Under new ventilation requirements, and with the trend toward longer and wider panels, cribbing in tailgate entries is restricting air flow.

Cost analysis has shown that the price of wood, and specifically all crib products, has increased dramatically in recent years. However, at the same time, the availability and quality of the material has decreased. Thus tailgate support has become more expensive in both terms of material and labor costs. Based on these safety and cost considerations, a better method for supplemental tailgate entry support clearly was needed.

Solution:

Keystone Mining Services knew that the type of support system for this project would have to effectively control not only the vertical pressure, but also the horizontal stress components. After considering all of these factors, a dynamic or fully tensioned truss system was chosen as the support system for this project. The truss plan was designed to function in conjunction with the primary support plan.

To monitor the effectiveness of the support plan and to obtain as much data as possible for future design considerations, an extensive instrumentation plan was designed and implemented with the assistance of the West Virginia University Mining Department. Initial data was collected from the instrumentation at the time of installation, with subsequent readings taken as the retreating longwall face approached and mined through the test area. A complete photographic log was also maintained. The face successfully mined through the truss test area. There were no roof control problems in the tailgate entry and the truss systems successfully controlled abutment pressures; thus the tailgate entry remained open throughout the mining period. On the longwall face, no roof problems or adverse pressure was detected in front of the shield tips near the tailgate.

Based on these test results, plans are now being implemented to further utilize this truss plan in lieu of cribbing for tailgate support. In addition to the obvious safety benefits of the truss plan, cost analysis indicates an average of $50,000 to $80,000 per panel. This figure only reflects material costs and does not reflect total labor and outby material handling costs. However, it does appear that material handling cost savings will be significant as only one supply car of trusses is required to support an equivalent length of mine entry that would require ten supply cars of cribbing.