PERMFILE42013

• Permit No: C1980007
• IBM Index Class Name: Revision
• Doc Date: 6/13/2005
• Doc Name: Exhibit 60B Subsidence Evaluation For the South of Divide Mining Area
• Type & Sequence: PR10
• Media Type: D
• Archive: No

Subsidence Evaluation For
Exhibit 60B South of Divide Minino Area Paae 2 f
•
A low to high potential exists for rockfalls in the South of Divide azea. Analysis of the terrain in
the South of Divide miffing azea reveals slopes that range from 30 to 80 percent along Minnesota
Creek, the Dry Fork of Minnesota Creek and its tributaries, and in local aeeas along the main fork
of Lick Creek. Vertical displacement, tilt, and strain produced by mining may locally trigger
already unstable rocks to fall during, or shortly after mining.
The areas with steep slopes in the South of Divide mining area, which have the greater potential
for rockfalls, aze located either in aeeas with local access roads, which have only limited travel,
or in areas remote from any access roads or other man-made features. Based on a review of
aerial photographs and analysis of the USGS 7.5-minute quadrangles, there aze eight areas with
slopes ranging from 30 to 80 percent that contain local cliffs and ledges (small cliffs 5 to 10 feet
high). The seven general areas listed below (listed in an east-to-west, north-to-south direction)
have a low to high rockfall potential (Map 1):
1. Steep slopes (with an estimated rockfall potential ranging from moderate to high) located
north of Dry Fork and west of Minnesota Reservoir. However, no mining is planned in
this area, so this rockfall area would not be impacted.
2. Two steep ridges with cliffs and ledges, located above the northern part of longwall panel
E9 east of the landslide azea described in Section 10.1 (mostly in the SE'/4 of Section 32,
Township 13 South, Range 90 West). There aze no roads or man-made stmctures in the
• azea.
3. The south end of a steep ridge containing cliffs and ledges located north of Minnesota
Reservoir and Dry Fork and north of the confluence of Deer Creek and Dry Fork above
the westem edge (within the area of mining influence) of un-mined longwall panels El
and E2 (S1/2, Section 29 and the NW'/< of Section 33, Township 13 South, Range 90
West). The Dry Fork road is located 400 to 500 feet south of the nearest azea boundary.
4. A steep to moderately steep slope containing eight separate rockfall areas, located north
and south of Dry Fork and its tributaries. The estimated rockfall potential is low to
moderate. The rockfall aeeas aze located above longwall panels El through E4 (Sections
33 and 34 and the NE'/z of Section 35, Township 13 South, Range 90 West).
5. The area is located in the southwestern part of the South of Divide mining azea east of the
main fork of Minnesota Creek. While the majority of this area is located outside the
influence of mining, the northeastern part of this azea, which has an estimated moderate
to high rockfall potential, is within the azea of mining influence of the western and
southern part of un-mined longwall panels E8 and E9, respectively (located mostly in the
W'/z and S%s, Section 5, Township 14 South, Range 90 West).
6. This azea contains six rockfall aeeas that have locally steep ridges. The azea is located
neaz the headwaters of Deer Creek, Poison Creek, Lick Creek, and a tributary of Dry
Fork. The aeeas, which have an estimated low to moderate rockfall potential, aze located
above, or partly within, the azea of mining influence of un-mined longwall panels ES
831-032.620 Wright Water Engineers, lnc.