2015-07-21_REVISION - C1982056 (2)

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Twentymile Coal, LLC. Page 10 July 21, 2015 28. Revised page 2.05-184.11 indicates mining of the WCR will result in undermining and subsiding the Yampa Valley Electric Line serving the mine. Exhibit 7K submitted with PR10 indicates that three powerlines will be impacted by subsidence. Please update page 2.05-184.11 to discuss all three powerlines that will be impacted by mining of the WCR. a. TC's response: The revised and updated subsidence evaluation and associated text revisions provided as a supplemental submittal on March 19, 2015 address this Comment. Please note that while the subsidence evaluation does address potential subsidence effects to all three powerlines, as noted, only the Yampa Valley Electric line will be affected by those activities proposed under PR10. b. Division's response: Item resolved. 29. As indicated in the previous item, Exhibit 7K indicates mining and subsidence will impact three powerlines. The monitoring plan for the powerlines discussed on revised page 2.05-191.9 only discusses the Yampa Valley Electric Line. Please revise this section to discuss all three powerlines. a. TC's response: Please refer to the previous response to Comment 28. b. Division's response: Item resolved. 30. In the NW 1/4 of Section 13, T5N R86 West, a very steep slope with potential rock outcrops is located above RCR33. Subsidence from the east end of the WC-5RT panel will impact this area. Please evaluate the danger of rock fall caused by the subsidence of this slope and its potential impact to motorists on RCR33. Please revise the subsidence control plan to address this issue. a. TC's response: The slope angle for the referenced slope is consistent with natural slopes in similar materials in the area, and the slope shows no evidence of instability or movement. The referenced rocky outcrops are thin, relatively high on the slope, and dip into the slope at a low angle. The generally stability of this area and these materials is evidenced by the lack of movement or rockfall in the road -cut which is due west a short distance from the referenced area. Given the generally stable configuration and lack of movement of the existing slope, the potential for rockfall to occur and affect the nearby County road is negligible. In the unlikely event that minor rockfall were to occur, the rock outcrop is high enough on the slope and the run -out on the slope is long enough that there is little potential that any rock material would reach the road. As a precaution, however, TC will place signs adjacent to the road right-of-way in this area warning motorists of potential rockfall. b. Division's response: The Division agrees that placing signs at the entrance of County Road 33 into the subsidence area would be a good practice. Also, the Division requests that visual monitoring of the area in question be conducted during the time periods TC will be monitoring County Road 33 for subsidence. A map is enclosed that outlines the area of concern. Please revise the subsidence monitoring plan submitted for County Road 33 to include conducting visual monitoring of this area and a discussion of what actions will be taken should unstable conditions and rockfall be observed.