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 ror
Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 8
• • Eastern Panels. The eastern panels, which trend about N80°W, will range from about critical
to subcritical in width in the west ends of panels 1, 2, 3, and 4 SE (numbered from north to
south) to subcritical in the eastem part of the panels. Maximum subsidence (Sm=a•t) above
panel centers is projected to range from 4.8 to 6 feet (0.4t to O.St) beneath areas of greatest
overburden depth to 7.2 to 9.6 feet (0.6t to 0.8t) neaz the head of Sylvester Gulch. The
maximum subsidence above the chain pillars (Sro a~,t) is projected to range from 2.4 feet
(0.2t) in areas of thinnest overburden to 6.0 feet (O.St) feet in areas of thickest overburden.
The Marine Sandstone located beneath the D-Seam is about 100 feet thick in the eastern
panel area. This sandstone may reduce the subsidence factor, compared to the western panel
azea, where the sandstone is less than 50 feet thick.
• Western Panels - E-Seam ~Llining. For E-Seam mining, all three westem panels (numbered
from south to north) will be of supercritical width (i.e., mining width greater than critical;
critical width is the mining width needed to cause maximum subsidence) in areas near the
head of Pond Gulch, in the Horse Gulch area, and the two unnamed draws east of Horse
Gulch. Only in areas of the higher ridges adjacent to these draws will the panels be of
subcritical width (mining width less than critical). With a projected longwall panel width of
950 feet, assuming an I1-foot coal-extraction thickness and chain pillar dimensions and
geometry similar to the current West Elk mining area, maximum subsidence is projected to
range from 6.6 to 8.8 feet (0.6t to 0.8t).
• Maximum subsidence is expected to be closer to 6.6 feet (0.6t) beneath the ridges and closer
to 8.8 feet (0.8t) beneath the draws (Table 2). Maximum subsidence above the chain pillars
(SAP) is predicted to range from 0.6 feet (O.OSt) where the overburden is thinnest (in the
draws) to about 3.6 feet (0.3t) beneath the ridge areas. Maximum subsidence may be less in
the northeastern part of the panel area where the Mazine Sandstone beneath the D-Seam is
about 100 feet thick.
• Western Panels -Combined E- and B-Seam Mining. For E-Seam mining and subsequent B-
Seam mining, the three mining panels will range from supercritical to critical in the draw
areas, previously mentioned in the E-Seam mining section, to subcritical in the ridge areas
adjacent to these draws. Because the total extraction thickness will average 23 feet (E-Seam.
1 l feet; B-Seam, 12 feet), the total maximum subsidence (vertical displacement) afrer mining
both seams is projected to range from about 13.8 to 18.4 feet (0.6t to 0.8t) (Table 2).
Box Canvon Mining Area. Overburden depth above the projected longwall panel centers in the
Box Canyon mining area ranges from about 750 feet beneath Box Canyon and the unnamed
gulch west of Box Canyon to about 2,300 feet beneath West Flatiron (see Map 14 for details).
With a projected longwall panel width of 1.100 feet, assuming acoal-extraction thickness of 1Z
•
831-032.181 Wright Water Engineers, Inc.