Deeper cover equates to higher stress levels, which in turn require particular measurements to ensure the stability of the ground, the agency said. For example, because vertical stress goes up at a rate of approximately 1.1 pounds per square inch (psi) for each overburden foot, the psi of a 1500ft-deep operation is 1650 before mining starts.
Once development commences, that original distribution is impacted, creating higher stress on the rock that surrounds pillars and entries.
The best practices MSHA cited specifically were those related to geology, multiple seam mining and retreat mining, mine planning/pillar design and longwall mining.
Geology
Ground control can be influenced by the high stress that comes with deep cover and various unstable ground conditions can result, depending on local geology.
"If the roof and/or floor geology is weak, roof falls or excessive floor heave are likely to occur. Mine planning can mitigate some problems but additional roof support (eg, bolts and surface control measures) also may be required," said MSHA.
Even if the roof and floor are strong, there is a possibility of coal bumps (bounces). While planning is important for all mines, it is especially vital for strong strata, multiple seam and retreat mining.
It also said that rib sloughing is a potential for operations mining in high seams. "The direction of the face cleat with respect to entries and crosscuts often influences the type of failure, creating, for example, vertical slabs of coal along pillar ribs or triangular slabs from pillar corners."
Multiple Seam Mining
There are concentrations of vertical stress in mine entries that are above or below other workings, most prevalently in retreat mining when gob-solid boundaries and isolated barriers are involved.
Vertical stress from deep cover can create multiple seam interactions, even when high interburden thicknesses exist. Mining direction can also play a role in the intensity of interaction of multiple seams; gob to solid, for example, will typically result in lower concentrations than vice versa.
"The type of remnant pillar structure (gob-solid boundary or isolated barrier) in overlying or underlying workings influences the degree of multiple seam interaction," MSHA noted. "Isolated barriers cause more ground control problems than gob-solid boundaries."
Retreat Mining
"The extraction of coal pillars/panels in retreat mining operations creates abutment stresses adjacent to gob areas. Under deep cover, special precautions are required to accommodate these elevated stress levels."
There are two categories of precautions in room and pillar scenarios: global stability (the prevention of failure from bumps, collapses and squeezes) and local stability (the prevention of working section roof falls).
"Global stability is addressed through proper mine design. Local stability is addressed through the installation of roof bolts, use of standing support such as mobile roof supports or posts, and an adequately sized final pillar stump," it said.
Barrier stability between panels and those next to the retreating gob line must be examined. Features such as faults, sandstone channels and higher jointing zones should be planned and mapped prior to retreating in order to permit for additional roof support or changes to the plan as needed.
Also, because retreat areas have the same abutment stresses as longwall headgate and tailgate entries, more roof support (such as longer and stronger bolts) should be looked at. The more cover that is encountered, the higher the magnitude of stress and loner front abutment, making the need for supplemental roof support higher.
Mine Planning/Pillar Design
Planning is crucial to all mines, but especially so in operations working under deep cover. The agency said operators can benefit greatly from the use of design programs such as the Analysis of Retreat Mining Pillar Stability (ARMPS) and numerical modeling software, like LAMODEL.
"Regardless of the particular software employed, care must be taken to ensure that appropriate input values are used," it noted. "Some programs have 'built-in default input values' [and] a decision to use mine specific information instead of the defaults should be weighed carefully.
"In addition, actual in-mine conditions and ground control history should be used to validate/calibrate any analysis."
The agency recommends an evaluation of other nearby mines' ground control histories to be most prepared, especially for those mines just beginning development.
Longwall Mining
Longwall mines are also getting deeper in overburden levels, and for those operations special considerations should also be taken to be aware of the potential for elevated stress.
For example, chain pillars on gate entries must have been designed properly for stability of roof and floor and to alleviate bumps/bounces. Yield pillars have been beneficial in some scenarios to keep bumps at bay, but in the deepest mines barrier pillars have helped to limit stress between active panels.
"Installing guards on longwall face equipment has proven effective in reducing injuries due to forcible ejection of coal from the face," the agency said of another best practice.
"This added protection includes belt guarding hung from the longwall shields, metal guarding attached to the panline, face sprags on longwall shields, and deflector plates installed on the shearer."
Also helpful for deep mines can be PPE, or personal protective equipment, items including body armor, helmets and face shields. Mines should also review administrative controls that will keep workers out of certain areas that are historically bump-prone.
Control stations can also be moved or additional stations installed to reduce workers' exposure to hazards in higher-risk areas. This includes permitting crews to perform work from a remote and safer area; MSHA also outlined one mine example that is installing cameras and lighting at the longwall shearer that, if successful, would allow for remote operation.
