2008-03-21_PERMIT FILE - C1980007A (5)

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West Elk Mine • South of Divide and Dry Fork Mining Area -Predicted subsidence impacts for the South of Divide mining area has been described in detail above. The greatest surface impacts are expected to occur along the precipitous slopes and cliffs that flank Minnesota Creek, Lick Creek, and other tributaries. Though unlikely, the worst possible consequences foreseen are that cracks could locally form and be as much as 25 to 50 feet deep above chain pillars and barrier pillars in bedrock on the precipitous slopes, ridges, and/or cliffs that flank Minnesota Creek, Lick Creek, and other tributaries, and may locally accelerate the naturally-occurring rockfall and landslide process. Some cracks are expected occur on the Dry Fork access road, however, in the area near the confluence of Deer Creek and Dry Fork. Maximum vertical displacement ranging from 8.4 to 11.2 feet, tilt from 1.3 to 6 percent, and horizontal strain from 0.7 to 4.2 percent are projected in the Dry Fork valley above the western limits of longwall panels E2 and E3. Based on subsidence observations by Mr. Dunrud (as discussed in Maximum Depth of Surface Cracks), no cracks are expected to occur in either alluvium in the Dry Fork and Lick Creek drainages or stock watering ponds and drainage diversion ditches. In addition, no cracks are predicted to occur in colluvium more than about ten feet thick. Mr. Dunrud has estimated the alluvial cover in Dry Fork and Lick Creek to be approximately 25 to 75 feet thick (Section 11.3, Exhibit 60 B). The monitoring suggested above will compare results expected by the author, and actual field results. . Potential Impacts front Local Seismic Activity Earth tremors have been recorded or felt by local residents in the Somerset area since the early 1960s. The tremors commonly are the result of coal mine bumps and rock bursts, which are spontaneous releases of strain energy in highly stressed coal and rock. In the Somerset Mine area before closure, the bumps and rock bursts were common in room-and-pillar mining areas where stresses concentrated within isolated pillars and blocks of coal (called bump blocks). Earth tremors have continued sporadically in the Somerset Mine area since the mine was closed. Tremors generated by bumps and rock bursts in the Somerset Mine area attain magnitudes that have shaken structures in the West Elk Mine area and have been felt sometimes by West Elk Mine personnel. These local tremors may affect underground workings, landslide or potential rockfall areas, particularly during prolonged periods of increased precipitation. It is noteworthy, however, that the Rulison nuclear shot in 1969, which produced a tremor with a Richter magnitude of 5.2, was many times greater than the magnitudes of any recorded coal bump. To Mr. Dum-ud's knowledge, the Rulison nuclear shot did not trigger any known landslides, rockfalls, did not affect the Somerset Mine, neither did it impact reservoirs, ponds, nor streams in the South of Divide mining area. Earth tremors have been recorded or felt by local residents in the Somerset area since the earl. 1960s. The tremors conni-nonly are the result of coal mine bumps and rock bursts, which are spontaneous releases of strain energy in highly stressed coal and rock. In the Somerset Mine area before closure, the bumps and rock bursts were connmon in room-and-pillar mining areas where • stresses concentrated within isolated pillars and blocks of coal. Earth tremors have continued sporadically in the Somerset Mine area since the mine was closed. 2.05-159 Revised June 2005 PRIO, Rev. Alarch 2006; Alay 2006 FRIO, Nov. 2006TRIO7,.9pril 2007TR1O8' Sep. 2007 PR12; Feb. 2008 PR-12