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<br /> <br />Mitigation of the Impacts of Mining Operations; Subsidence Rule 2.05.6 <br />l4. Twentymile Coal Company submitted revised pages 2.05-181.5 with discussion of undermining <br />residences in Section 22, with regard to the Northem Mining District. The only residences the <br />Division is aware of in Section 22 are located in the S 1/2 S 1/2. These residences overlay the Eastern <br />Mining District and do not belong in the discussion of Northem Mining district. Please remove <br />reference to the residences as being discussed in the Northem Mining District. On this same page <br />there is a sentence describing, "The Trout Creek Sandstone outcomes in the EIvID..." Please keep this <br />discussion applicable to the NMD, and if'`outcomes" should be "outcrops", please make this <br />correction. <br />1 ~. During discussion of subsidence impacts on revised page 2.05-181.6 (rev. 2/24/99), Twentymile states <br />that "no significant impact to the road [Routt County Road 33] resulted..." from undermining the <br />road. Cracking and buckling of the road were observed, however, as discussed in Division inspection <br />reports and in Twentymile's semi-annual subsidence reports. The Division considers cracking and <br />buckling of the road significant impacts. Please include a discussion of observed impacts to the road <br />in this discussion. <br />16. Review of Exhibit 7F-1 finds a description of Western Mining District longwall panel 6 as east of the <br />"current study area". This was correct with regazd to the Southwest Mining District, but this Exhibit <br />is discussing the Northem Mining District and particulazly rockfall hazard predictions for longwall <br />panel 12R. Please make it clear which "study azea" is being discussed. <br />17. Exhibit 7F-1 predicts 30 inches of subsidence of the Twentymile Sandstone outcrop overlying <br />longwall panel 12R. This prediction is significantly different from observed and predicted subsidence <br />overlying EMD panels. Subsidence measured over the center of longwall panel 6 in 1998 at station <br />108 was 66 inches, and station 112 was 49 inches. Please explain why the predicted subsidence in the <br />center of panel 12R is significantly less than that of previously measured subsidence in nearby <br />longwall panels. <br />18. Review of TCC's subsidence reports finds that all current predictions refer back to subsidence <br />measurements collected at the northwest end of the Western Mining District in 1994. The mine plan <br />has progressed into a different portion of the structural basin, panel widths have been substantially <br />increased, and mining conditions have vaned since the data were collected above the Westem Mining <br />District. To assure accuracy of subsidence predictions, in accordance with Rule 2.0~.6(6)(c), the <br />Division considers that subsidence monitoring, oriented transverse and longitudinal to a longwall <br />panel axis, and verification of the angle of draw, needs to be conducted at the eastern end of longwall <br />panel 12R in the Northern Mining District. Please provide a subsidence monitoring plan for the <br />NMD. <br />19. The Twentymile sandstone outcrops in section 13 and 14 of TSN, R86W. Map 23 (certified 9/10/99) <br />indicates that proposed longwall panel 12 R will undermine this outcrop in this location. Exhibit 7F-1 <br />states on page 6, "cliff failure occurred several months after undermining where the cliff had been <br />completely undermined." Exhibit 7F-l recommends "temporary closure of County Road No. 33 for <br />short periods of time", for protection of public using the roadway. The Division agrees that the public <br />must be protected from any possible risk due to rockfall along Routt County Road No. 33. <br />C:/jhb/C82056/PROS/jhb adq I.doc 3 3/10/00 <br />