PERMFILE123352

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 60 Subsidence Evaluation for Apache Rocks Mining Area & Box Canyon Lease Tract
• Media Type: D
• Archive: No

Subsidence Evaluation For
Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 2
150 foot centers) and one row of yielding pillars approximately 40 feet wide by 130 feet long (50
x 150 foot centers). These planned mine and pillar designs are subject to change pending
geotechnical analysis and mining experience from the new areas.
3.0 GEOLOGIC FACTORS INFLUENCING SUBSIDENCE
Subsidence may be influenced by the local geology in the following ways:
1. Geologic structure. Attitude of the bedrock, faulting, and jointing may control mine layout
and mining method. [n steeply dipping, faulted coal beds, for example, a certain mine layout
and method, such as room-and-pillar 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
Apache Rocks and the Box Canyon mining areas, 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 properties of the rocks. These properties may 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. 71te 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. Stratieranhic seauence. The stratigraphic distribution of rock units (stratigraphic 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,
whereas sandstones in the fractured zone above the caved zone may increase the height of
fracturing compared to shales.
In addition, the subsidence factor may be less where the overburden contains a greater
proportion of thick, strong sandstones compared to thin, weak shales. A unit that may reduce
the subsidence factor is the locally thick Lower and Upper Marine Sandstones that underlie
the D- and E-Seams. These sandstones are about 100 feet thick in the eastern panel area and
the eastern part of the western panels of the Apache Rocks mining area; they are
approximately 100 to 125 feet thick in the Box Canyon mining azea and the northwestern part
of the current mining area.

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.
831-032.181
Wright Water Engineers, Inc.