2011-02-14_PERMIT FILE - C1980007A (3)

Home Browse Search

Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 27 A low to high potential exists for rockfalls in the South of Divide area. Analysis of the terrain in g the South of Divide mining area reveals slopes that range from 30 to 80 percent along Minnesota Creek, the Dry Fork of Minnesota Creek and its tributaries, and in local areas 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, are located either in areas 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 are 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 area described in Section 10.1 (mostly in the SE' /. of Section 32, Township 13 South, Range 90 West). There are no roads or man-made structures in the area. 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 western edge (within the area of mining influence) of un -mined longwall panels E1 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 area 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 areas are 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 area 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 area, which has an estimated moderate to high rockfall potential, is within the area of mining influence of the western and southern part of un -mined longwall panels E8 and E9, respectively (located mostly in the W' /2 and S%, Section 5, Township 14 South, Range 90 West). 6. This area contains six rockfall areas that have locally steep ridges. The area is located near the headwaters of Deer Creek, Poison Creek, Lick Creek, and a tributary of Dry Fork. The areas, which have an estimated low to moderate rockfall potential, are located above, or partly within, the area of mining influence of un -mined longwall panels E5 through E8 (located in parts of Sections 3, 4, 9, and 10, Township 14 South, Range 90 is West). 831 - 032.810 Wright Water Engineers, Inc.