PERMFILE110491

• Permit No: C1980007
• IBM Index Class Name: Revision
• Doc Date: 12/2/2004
• Doc Name: Exhibit 60B Certification 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
3.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE
Subsidence maybe influenced by the local geology in the following ways:
Geologic 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 asroom-and-pillaz or limited panel-pillaz may be required. Joints often
control the way in which the roof rocks break, cave, and fracture, both underground and at the
surface during mining and subsidence. In relatively flat-lying, unfaulted coal seams like the
South of Divide mining azea, there is latitude to develop the most efficient layout and method
to recover a maximum amount of the coal resource with a minimum of impact.
2. Strength and behavioral orooerties of the rocks. These properties control the amount and rate
of subsidence. Strong, brittle sandstones and siltstones tend to break and cave in lazge 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
caning 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 (stratigaphic 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 caving compazed to shales,
whereas sandstones in the fractured zone above the caved zone may increase the height of
fracturing compazed to shales.
In addition, the lithology of the overburden rock may control the subsidence factor. The
subsidence factor may be less where the overburden contains a greater proportion of thick,
strong sandstones, and greater where the overburden contains thin, weak shales. 1n the South
of Divide mining azea, the fast 200 to 300 feet of rocks above the E Seam consist primarily
of siltstones, shales, claystones, local lenticulaz sands, and coal seams.
4. Moisture content. Wet or saturated conditions in the mine roof and overburden tend to
reduce the bulking 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 overburden rocks, the greater the subsidence factor.
4.0 FIELD RECOGNITION OF SUBSIDENCE AND NON-SUBSIDENCE
FEATURES IN THE WEST ELK MINING AREA
Four different types of features aze observed in the West Elk mining azea: 1) subsidence cracks
and bulges, 2) construction cracks, 3) desiccation cracks, and 4) gravity-induced tension cracks.
They can be distinguished easily in some azeas-where, for example, no mining has occurred in
that area. However, in other azeas they maybe difficult to distinguish, such as in azeas that have
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