Change in Primary Roof Support System at Quarto Mining Company's Powhatan No. 4 Mine Results in Improved Safety, Productivity and Costs
Organization: International Conference on Ground Control in Mining
Jan 1, 1986
The Powhatan No. 4 Mine is located in southeastern Ohio along the Ohio River in Monroe County. The mine produces approximately 3.2 million tons of steam coal annually from the Pittsburgh No. 8 seam. The Pittsburgh No. 8 seam in this area dips to the southeast in varying amounts from 10-feet to 40-feet per mile. The seam is made up of a main bench coal (4' to 5f' ) containing several thin and variable partings and a rider ("roof") coal (0" to 18") which is separated from the main bench by a parting (0" to 18") that is reasonably consistent in occurrence and position. Strata overlying the seam varies from a bedded to nonbedded calcareous mudstone-claystone (6' to 12') in the north and east regions of the mine to a sandstone (Pittsburgh Sandstone C10' to 40'1 in the south and west regions. The Redstone Limestone (10' to 18' ) overlying the mudstone-claystone in the north and east thins and rises toward the area of sandstone deposition in the south and west. From its opening in April of 1971 until late 1981, primary roof support in the long-1 ived entries at Quarto Mining Company's Powhatan No. 4 Mine had been achieved with 8-foot and 10-foot. 518-inch mechanical roof bolts using a Single expansion shell. These bolts were installed so that the expansion shell achieved a competent anchorage in the limestone. However, as the mine developed westward, the limestone began to trend upward becoming inaccessible as an anchorage medium at the 8-foot and 10-foot horizons. Anchorage achieved in the underlying mudstone-claystone strata was failure-prone due to weathering (after exposure to air and moisture) and inconsistencies in the anchorage horizon, leading to an increase in the incidence of reportable roof falls at the mine. Responding to the threat to safety and productivity posed by the increase in roof falls. Mine Management through its Safety and Engineering Departments, sought solutions to the problem. In seeking a solution to the problem, the safest and en engineering groups examined the reportable roof falls to determine the cause of those failures. Their examination pinpointed two items which were found to be a common link in the majority of those falls; namely, anchor integrity and geologic trends. Analysis of the roof falls indicated that due to effects of air and moisture on the anchoraae zones in the mudstones and clay- itones, the integrity of the anchorage over time could not be maintained. Geologic trends noted included the upward trend of the -main limestone as mining progressed westward, inconsistencies in the amount of roof coal and draw slate in the immediate mine roof and increasing evidence of heavily slickensided immediate mine roof. In the absence of the protective barrier provided by the roof coal, the immediate strata and particularly the draw slate, is susceptible to deterioration due to the weathering effects of air and moisture and thus is prone to "eat out" around the roof bolts and cause roof falls. The presence of heavily slickensided roof creates a need for increased contact surface such as that provided by plank and header blocks to aid in holding up the roof and tends to make the behavior of such roof very unpredictable. Having determined the causes of the roof falls. Mine Management began to identify the kind of roof support which would best solve these problems, while at the same time providing the mine with a safe, cost effective and efficient roof support system. This paper will provide an analysis of how this decision was reached, the background that was researched to aid in forming the decision, the considerations that played key roles in the formulation of the plan, the mechanics of following through and implementing the new roof support system, and an assessment of the resultant gains in safety, productivity and improved costs derived from the implementation of the new roof support system.