_GENERAL DOCUMENTS - C1981017 ‎(142)‎

• Permit No: C1981017
• IBM Index Class Name: GENERAL DOCUMENTS
• Doc Name: Bid Documents ‎(IMP)‎
• Permit Index Doc Type: General Correspondence
• Media Type: D
• Archive: No

APPENDIX A
Continuous stress relief is imperative for safety
in the Coal Basin mines . The depth of the mines , nature of
the strata, and interaction of previously mined areas
interact with abutment pressures from the workings themselves ,
resulting in stress-created outbursts , rockbursts and bumps .
The dangers of stress are compounded by the methane gas which
is contained in the coal and which is released suddenly,
in high concentrations, by outbursts and bumps . High con-
centrations of methane, as is well known, can create explosions .
The Coal Basin coal mines are the deepest in North
America, at depths of 2 , 500 - 3 ,000 ft. The weight of the
overlying mountain is , in itself, a source of heavy pressure.
The coal itself is very soft in both seams, having
a Hardgrove grinding index of over 100 , and a compressive
strength in the range of 600 to 1100 psi. At the same time,
the rock strata above and below the seams is very strong.
The rock intervening between seams has a compressive strength
which ranges from 8 ,569 to 24 , 900 psi . Pressures manifest
in the weakest rock, namely the coal .
The coal is, as mentioned, gas bearing. It is also
non-permeable, so the gas is liberated only when the structure
of the coal itself is broken. This occurs in mining, and
also when the coal fails because of stress, as happens in an
outburst. The effect of a breakage in the coal, from what-
ever the source, is the sudden release of methane in con-
centrations which can only be controlled by the flow of air.
A constant and heavy current of air is directed at the
working face, and equipment is calibrated to stop when
concentrations of methane are liberated. However, un-
controlled outbursts release methane suddenly, in potentially
explosive concentrations which cannot be as well monitored
or controlled. We believe that an uncontrolled outburst
was the source of methane which caused the mine disaster
of 1981, and precautions against such occurrences have the
highest priority, and are conducted on an ongoing and
continuous basis in our mines .
In addition to the natural conditions of depth,
strata, and gas, stress is created by the workings themselves .
Abutment zones , which are areas of high stress , surround
every opening or hole in the ground , and also are found around
faults and other irregularities in the earth ' s strata.
Abutment pressures are a constraint factor which must be dealt
with in the mines . If the zone of abutment pressure is too
close to a working face, an unplanned outburst can occur while