PERMFILE52001

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 6/2/2006
• Section_Exhibit Name: Exhibit 60B Subsidence Evaluation for the South of Divide Mining Area
• Media Type: D
• Archive: No

Subsidence Evaluation For
ExhibcT 608 South o/ Divide Mining Area
3.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE
Subsidence may be influenced by the local geology in the following ways:
1. 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-pillaz or limited panel-pillar 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 area, 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 propeRies of the rocks. These properties control the amount and cafe
of subsidence. Strong; brittle sandstones and siltstones tend to break and cave in large blocks
on the mine IIoor. 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 sofl, weak rocks.
3. Strati~aphic sequence. The stratigraphic distribution of rock units (stratigtaphic 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 compared to shales,
i whereas sandstones in the fractured zone above the caved zone may increase the height of
fracturing compared to shales. Conversely, the height of caving may be increased and the
height of fracturing decreased where weaker shale and claystones occur in the fractured zone
'above the coal seam to be mined.
ht 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. In the South
of Divide mining area, 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. A4oisture 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 are observed in the West Elk mining area: 1) subsidence cracks
and bulges, 2) constroction 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
831-032.690 Wnght Water Engineers, lnc.