PERMFILE42013

• Permit No: C1980007
• IBM Index Class Name: Revision
• Doc Date: 6/13/2005
• Doc Name: Exhibit 60B Subsidence Evaluation For the South of Divide Mining Area
• Type & Sequence: PR10
• Media Type: D
• Archive: No

Subsidence Evaluation For
Exhibit 608 South of Divide Mining Area Page 2
3.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE
Subsidence maybe influenced by the local geology in the following ways;
Geoloeic structure. Attitude of the bedrock, faulting, and jointing may control mine layout
and mining method. In steeply dipping, faulted coal beds, for example, a certain mine layout
and method, such as room-and-pillar or limited panel-pillaz may be required. Joints often
control the way in which the roof rocks break, cave, and fractwe, both underground and at the
surface during mining and subsidence. ffi relatively flat-lying, unfaulted coal seams like the
South of Divide mining area, there is latitude to develop the most efficient layout and method
to recover a maximum amount of the coal resowce with a minimum of impact.
2. Strength and behavioral properties of the rocks. These properties control the amount and rate
of subsidence. Strong, brittle sandstones and siltstones tend to break and cave in large blocks
on the mine floor. The bulking factor is greater for strong rocks than it is for soft, weak
rocks. The greater bulking factor of strong, caved material commonly reduces the height of
caving and the subsidence factor over soft, weak rocks. Conversely, the height of fracturing
often is greater for strong, brittle rocks than it is for soft, weak rocks.
3. Stratigraphic sequence. The stratigraphic distribution of rock units (strarigraphic sequence)
influences the effects of mining and subsidence. For example, strong and brittle sandstones
in the mine roof, as discussed above, can reduce the height of caning compazed to shales,
whereas sandstones in the fractwed zone above the caved zone may increase the height of
fracturing compazed to shales. Conversely, the height of caving may be increased and the
height of fracturing decreased where weaker shale and claystones occw in the fractwed zone
above the coal seam to be mined.
In addition, the lithology of the overbwden rock may control the subsidence factor. The
subsidence factor may be less where the overbwden contains a greater proportion of thick,
strong sandstones, and greater where the overbwden contains thin, weak shales. In the South
of Divide mining azea, the first 200 to 300 feet of rocks above the E Seam consist primarily
of siltstones, shales, claystones, local lenticular sandstones, and coal seams.
4. Moistwe content. Wet or satwated conditions in the mine roof and overbwden tend to
reduce the bullring factor of the caved roof rocks. Therefore, the subsidence factor commonly
is greater under wet conditions than it is in dry conditions. In general, the greater the
saturation of the mine roof and overbwden rocks, the greater the subsidence factor.
4.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE
FEATURES IN THE WEST ELK MINING AREA
Fow different types of featwes aze observed in the West Elk mining area: 1) subsidence cracks
and bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension cracks.
They can be distinguished easily in some areas-where, for example, no mining has occurred in
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