2008-03-21_PERMIT FILE - C1980007A ‎(8)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 3/21/2008
• Section_Exhibit Name: Exhibit 60E Subsidence Evaluation for the South of Divide & Dry Fork Mining Areas
• Media Type: D
• Archive: Yes

• 3. South of Divide mining area (Map 1):
a. An extensive landslide is located south of Minnesota Reservoir above the northern part of
projected longwall panel E9, in the N'/2 of Section 32, Township 13 South, Range 90 West
(Dunrud 1989). Overburden depth to the E Seam ranges from 500 to 550 feet. The average
slope of the slide is roughly 20 percent. The landslide contains more cracks and scarps in
the upper part and more depressions and bulges in the lower part than were present in the
early 1960s (based on image data from July 1963 aerial photographs).
¦ Based on a stereographic review of July 2004 vertical aerial photographs,
renewed activity occurred locally in western part of the landslide areas north and
south of Dry Fork during wet periods in the 1980s (1984 to 1987) and the mid
1990s (1994-1996). The Dry Fork road was taken out one half-mile west of the
Minnesota Reservoir dam by this renewed movement in 1987 (Map 1).
b. Landslide located near the north shore of Minnesota Reservoir in the SW% of Section 29.
This slide is located on the border between the Apache Rocks and South of Divide mining
areas.
c. Landslide area located on the Dry Fork road in the approximate center of Section 31,
Township 13 South, Range 90 West. Two small landslides are located to the southeast in
the SE% of Section 31 and the SW% of Section 32.
d. Landslide located near the southwest corner of un-mined longwall panel E8 in the N'/2 of
Section 8 and the S'/2 of Section 5, Township 14 South, Range 90 West.
4. Dry Fork mining area (Map 1):
• a. Landslides were identified on the Dry Fork mining area in the Final Environmental Impact
Statement prepared for the U.S. Department of Agriculture - Forest Service in August 2005.
The report identified three active landslide areas in the southeast corner of the area. These
are outside of the projected area that will be longwall mined and should therefore be
unaffected by mining. Two small landslides (slope failures) were identified by a Tetra Tech
geologist during a site inspection on September 20, 2007. These are located just above the
Deep Creek Ditch. The first landslide is located above the bleeder entries along the south
side of panel E6 while the second landslide is located beyond the angle of draw and should
not be affected by mining.
10.1.1 Effects of Subsidence and Mine-induced Seismic Activity on Landslides
Some of the most important information regarding mine subsidence and mine-induced seismicity
was obtained from observations of active landslides on Jumbo Mountain above longwall panels 8
and 9, which were mined during the mid 1990s. Landslide movement occurred during unusually wet
periods before mining, during mining, and after mining and subsidence was complete. The
landslides located north and south of Minnesota Reservoir are similar to those on Jumbo Mountain.
Both occur in surficial material (rocks, gravel, sand, silt, clay, and soil) and local outcrops of bedrock
that have slumped and flowed downhill during periods of increased saturation. Cracks, bulges, and
depressions or troughs, and springs were locally observed in both landslide areas.
It is important to note that no earth tremors (seismic activity) were felt by Dunrud (2006, p. 20) in all
of his annual traverses and observations made above the longwall mining areas in the Jumbo
Mountain, Apache Rocks, and Box Canyon mining areas during the last ten years (1996-2005
inclusive). For example, no tremors were felt during the annual traverse above longwall panel 13 in
1999, when the mining face was located directly beneath one of the subsidence observation points.
• This point was located approximately 1,200 feet vertically above the active mining face, and
2,800 feet north of Minnesota Reservoir.
Tetra Tech - 090717/P 20