Best Practices Ground Control for Deep Cover UG Coal

Ground Control for Deep Cover Coal Mines
Coal mines in many regions of the United States are operating under
increasingly greater depths. These greater depths translate to higher stress
levels that require special precautions to ensure ground stability.
Vertical stress in coal measure rocks tends to increase at a rate of about 1.1 psi
for every foot of overburden. Thus, at a depth of 1500 feet, the vertical stress
before mining is about 1650 psi. The development of mine entries disturbs the
original stress distribution and at greater depths concentrates a high stress in the
rock surrounding the entries and in the coal pillars.
Geology - high stresses associated with deep cover can create a variety of
unstable ground conditions. The nature of the instability is largely dependent
on local geologic conditions.
9 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 (e.g. bolts and surface control measures) also may
be required.
9 If the roof and floor geology is strong, conditions may be conducive to coal
bumps or bounces. Mine planning is important in all deep cover
operations but it is critical in strong strata and even more so in multiple
seam and/or retreat mining scenarios.
9 If the coal seam is high, pillars are prone to rib sloughing. 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 – mine entries that are developed above or below other
workings may be exposed to vertical stress concentrations. These
concentrations are most prevalent when gob-solid boundaries or isolated
barriers associated with retreat mining are encountered.
9 Deeper cover translates to higher total vertical stress levels that can create
multiple seam interactions even when interburden thickness is substantial.
9 The direction of mining can influence the degree of multiple seam
interaction. Mining from the gob to the solid generally results in lower
stress concentrations than from the solid to the gob.
9 The type of remnant pillar structure (gob-solid boundary or isolated
barrier) in overlying or underlying workings influences the degree of
multiple seam interaction. 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.
Precautions for room and pillar retreat mining can be divided into two main

categories: global stability (prevention of pillar failure due to bumps, collapses
and squeezes) and local stability (prevention of roof falls in the working area).
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.
9 In room-and-pillar retreat mining operations, it is imperative to consider
the stability of barriers that separate panels in addition to the stability of
pillars adjacent to the retreating gob line.
9 Geologic features such as faults, sandstone channels and zones of
increased jointing that concentrate stresses or fracture the roof should be
mapped on development prior to retreat mining activity. This will allow for
the timely installation of additional roof support or changes to the retreat
mining plan.
9 Since retreat sections are subjected to abutment stresses just like longwall
headgate and tailgate entries, the level of roof support installed should be
enhanced (e.g. longer bolts, stronger bolts and/or a denser pattern).
9 As cover increases, the magnitude of the stress increases and the distance
that front abutment stresses transfer outby the pillar line also increases. It
is important that supplemental roof support be installed prior to any stress
increase, ideally upon development.
Mine Planning/Pillar Design – mine planning is important in all mines but
especially so in deep cover operations. The use of empirical design programs
such as the Analysis of Retreat Mining Pillar Stability (ARMPS) and the use of
numerical modeling software such as LAMODEL can be a great asset.
9 Regardless of the particular software employed, care must be taken to
ensure that appropriate input values are used. Some programs have “builtin default input values” (e.g. coal strength and rock density). 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.
9 A review of the ground control history of nearby mining operations under
similar ground conditions can be invaluable, especially for new mines or
for existing mines expanding into new reserves.
Longwall Mining – longwall mining under deep cover also requires special
precautions to be taken in response to the elevated stress levels encountered.
9 Gate entry chain pillars must be properly designed to achieve roof/floor
stability and mitigate bumps. In some instances, yield pillars have been
effective to deter bumps. However, in the deepest operations, barrier
pillars have been required between panels to limit stress levels in the work
areas.
9 Installing guards on longwall face equipment has proven effective in
reducing injuries due to forcible ejection of coal from the face. 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.
9 Personal protective equipment such as helmets with face shields and
body armor (e.g. chest protector and shin guards) can provide personnel
with an additional level of protection.

9 Administrative controls that keep personnel out of certain bump prone
locations during the mining cycle can be implemented. One example of
such a precaution is to not allow personnel in the headgate or tailgate entry
(for a specified distance outby the longwall face) when the shearer is
cutting within a designated distance of the headgate or tailgate entry.
Another example is to keep personnel on the longwall face a specified
distance away from the shearer while it is cutting, unless they are located
behind the shearer, as in the case of the shearer operator.
9 Relocation of operator control stations or the installation of additional
control stations can reduce exposure of personnel to high-risk locations
during regular operations and maintenance procedures by allowing tasks to
be performed from a remote (safer) location.
9 The feasibility of installing cameras and lights on the shearer is being
evaluated at one deep cover longwall operation. If successful, this would
allow for remote operation of the shearer from a control station outby the
longwall face.

_______________________________________________________________
U.S. Department of Labor
Mine Safety and Health Administration
Richard Stickler, Acting Assistant Secretary
Visit the MSHA home page at www.msha.gov