The U.S. Mine Safety and Health Administration agrees with a recommendation made in a National Institute for Occupational Safety and Health (NIOSH) report that operators of deep cover room-and-pillar retreat coal mines should conduct regular burst hazard assessments for any areas where retreat mining is proposed and the depth of cover exceeds 1,000 feet.
That is one of the points that MSHA made in a July 11 Program Information Bulletin sent out to the coal industry about this often-controversial practice, also known loosely as “pillar robbing.” In a room-and-pillar mine it is often the case that the operator, after moving through an area and leaving behind a checkerboard of mined out rooms and supporting pillars, will try and remove some or all of various pillars, which is a delicate operation that can literally bring down the roof if not done properly, particularly in areas of deep cover with higher roof stresses. But this coal can be produced very cheaply, since most of the major mining work has already been done, and will mean a higher percentage of recovery for the coal reserve in question (unmined pillars of coal stay there forever). This practice got a harsh spotlight shone on it last decade, particularly after a multi-fatality accident in 2007 at the Crandall Canyon mine in Utah.
The bulletin said assessments of burst hazards should be guided by an experienced ground control professional. Once zones of elevated risk of bursts have been identified, the risk can be reduced by limiting mining within that zone. The NIOSH report concluded that, within an identified zone of elevated burst risk, no combination of currently available mining sequences, administrative procedures, or monitoring techniques can be relied upon to reduce the risk posed by coal bursts during pillar recovery to an acceptable level, MSHA noted.
MSHA said it also agreed with the three specific recommendations made in the NIOSH report to limit the risk of violent coal bursts during retreat mining operations:
- At depths exceeding 1,000 feet, retreat mining should not be conducted without properly designed barrier pillars.
- At depths exceeding 1,000 feet, pillar splitting should not be conducted on the pillar line.
- At depths exceeding 2,000 feet, pillar recovery should not be conducted.
“The 2007 Crandall Canyon mine disaster resulted in the deaths of six miners engaged in coal pillar retreat mining operations under more than 1600 ft of cover,” said the bulletin. “In addition, two mine employees and one MSHA inspector received fatal injuries during mine rescue efforts, and six additional rescue workers (including another inspector) were injured. Subsequently, Congress directed NIOSH to conduct a study of the recovery of coal pillars, particularly at depths greater than 1500 ft. The NIOSH report, released in 2010, discusses the conditions under which retreat mining is used in the US, and the procedures currently used to ensure miner safety during retreat mining. The report also includes recommendations to enhance the safety of miners working where retreat mining is utilized.”
MSHA said it will be considering these findings when evaluating roof control plans involving pillar recovery under deep cover. Nearly all of the 42 deep-cover mines in the U.S. that have extracted pillars under depths greater than 1,000 feet are located in Central Appalachia, with a few others located in Utah and Colorado.
The most significant hazards faced by deep cover retreat miners include: ground falls, including large “roof falls,” smaller “rock falls” that occur between roof supports, and “rib falls” that come from the side walls of the mine; pillar failures that can affect large areas of a mine, but are usually non-violent “squeezes” that occur slowly and seldom result in injuries to workers; and coal bursts, which cause coal to be ejected into the mine with enough energy to injure or kill miners. Since 2007 there have been no fatal roof falls during pillar recovery in the U.S. (at any depth), compared with an average of about two per year during the previous dozen years, MSHA noted.
The three key technologies for improving the safety during pillar retreat mining are: leaving an engineered final stump; using Mobile Roof Supports (MRS); and using longer and stronger roof bolts, particularly in intersections. Other best practices for retreat mining safety include: conducting a geologic assessment of the entire panel before retreat mining commences; measuring intersection spans, and installing additional support in any that are significantly wider than usual; conducting pre-shift and on-shift examinations that include thorough assessments of geologic conditions; using test holes to determine if there is roof separation, and monitoring them during mining to see if conditions worsen; and monitoring the pressures and loading rates of MRS to provide information on roof stability.
“Roof control plans for retreat mining should also incorporate work procedures, including the location of miners while coal is being mined or loaded, and best practices for using MRS units,” said the MSHA bulletin. “Pillar design, including the use of properly sized barrier pillars, is the most effective technique for minimizing the risks of pillar failure and coal bursts under deep cover. The NIOSH report states that the current MSHA guidelines regarding the technical and engineering data related to pillar design that mine operators must submit as part of their Roof Control Plans have been successful in placing pillar design practice in U.S. underground coal mines on a solid engineering foundation that significantly reduces the risk of pillar failure and its attendant hazards to coal miners. The NIOSH ground control software packages Analysis of Retreat Mining Pillar Stability (ARMPS) and Analysis of Multiple Seam Stability continue to provide a high level of protection against pillar failure. The LaModel program also has a long history of successful application at coal mines throughout the world. Under a NIOSH contract, researchers at West Virginia University developed standardized LaModel procedures that are closely linked to ARMPS and its comprehensive case history database. Other pillar design methods have been successfully employed, but to be reliable they must be thoroughly validated for the underground environments to which they are applied.”