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
Exhibit fi08 South oiDivnde Mining Area Page 24
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2. Install, and periodically measure, survey monuments strategically located on the dam and
on the north and south edges of the reservoir to monitor any movement prior to, during,
and after mining in the area.
3. install seismometers in strategic places, such as in the dam area and toe areas of the
landslides, in order to detect vertical and horizontal ground motion prior to, during, and
after mining, pazticulazly during the mining of longwall panel E9.
11.2 Stock Wafering Ponds and U.S. Forest Servfce Water Resources
The stock watering ponds in the South of Divide mining area are located in debris flows or
colluvium derived from the debris flows (Dunnrd 1989). A total of 26 ponds have been mapped
in the South of Divide mining area. Ten of these ponds are also classified as U,S. Forest Service
water resources (Map 1). The debris flows consist of a heterogeneous mixture of clay derived
from the Wasatch Formation and boulders and gravels derived primarily from the Mount
Gunnison intrusive (ganodiorites and quartz monzanites). Based on observations made during
geologic mapping in the area, these debris flows are even less likely to be affected by longwall
mining than the alluvium The debris flows have a very low permeability and, because the clay
matrix is armored by the interstitial gravel and boulders, are resistant to erosion (the Deep Creek
Ditch locally flows in this material ai steep gradients). Based on dre above-mentioned
observations, no effects are expected whet ponds in the South of Divide mining area are
undermined. The clay-rich material that lines these ponds is expected to provide a seal against
• any subsidence effects.
Alo stock watering ponds in the Apache Rocks and Box Canyon mining areas have been
noticeably affected when longwall mining occurred beneath them. The ponds above mined
longwall panels 13-A, 15, 16, and 22 in Hotse Gulch, Sylvester Gulch (stock ponds P27-2 and
P27-3), and West Flatiron (stock pond P23-4) respectively, showed no noticeable water level
change when compared to pre-mining water levels (after taking seasonal precipitation levels into
account). These ponds, which are located in clay-rich (Wasatch clay) material in order to
minimise leakage, occur in overburden that ranges in depth from about 800 feet in Horse Gulch
to 2,200 feet in the stock pond P23-4 above the eastern end of mined longwall panel 22.
11.3 Streams and Ditches
A greater potential exists for longwall mine subsidence to impact ~mc in the South of Divide
mining area than in the Apache Rocks and Box Canyon mining areas. This is primarily because
a) the overburden depth is less than in the Apache Rocks and Box Canyon mining area,
particularly in the northwestern part of the area, and b) the coal extraction thickness is projected
to be greater for the E Seam than was mined in the B Seam (14 feet versus 12 feet)- Ahhough no
subsidence cracks are expected to affect streamflow in dre South of Divide mining area, based on
prior observations in the West Elk and Somerset mine areas, it is recommended that detailed
subsidence and surface flow measurements be done in order to verify this concept. The
measurements should be conducted particulazly in the northwestern part of the mine area where
coal extraction thickness is projected to be a maximum of 14 feet and overburden depth ranges
. from 375 to 400 feet.
831-032. b'90 Wright Water Engineers, lnc.