2012-11-29_REVISION - C1980007 - Laserfiche WebLink

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Interoffice Memorandum To: Jim Stark From: Zach Trujillo Subject: West Elk – MR 389 Date: 11/28/2012 Jim – I have reviewed Mountain Coal Company’s (MCC) Minor Revision 389 assessing whether the modified Phase 1B buttress will provide sufficient support of the West Elk Mine’s Refuse Pile Expansion – East (RPEE) with additional refuse placement. MCC is currently approved for refuse placement on RPEE up to an elevation of 6,775 ft. with the currently approved Phase 1B buttress. Due to lack of sufficient durable rock fill material, MCC has proposed to delay complete placement of the Phase 1B rock buttress and modifying it to allow refuse placement on RPEE to an elevation of 6,930 ft. Barr Engineering Co. has submitted seepage and slope stability modeling and assessment for the modified Phase 1B buttress and refuse placement on RPEE to an elevation of 6,930 ft. Barr’s modeling results concluded an overall static factor of safety (FoS) of 1.58 and overall seismic FoS of 1.31. As per rule 4.10.1 and 4.10.4(2), coal mine waste banks shall have a minimum static FoS of 1.5 which has been met by Barr’s modeling. Regarding seismic activity, the Division has no rule to a minimum FoS for coal mine waste banks. General engineering practices for embankments and slopes require a minimum FoS of 1.1 under seismic conditions. Bar’s model reported a 1.31 while under horizontal seismic acceleration. After reconstructing the scenarios (static and pseudo-static) in the Galena slope stability program, the results met all requirements by the Division and general engineering practices for long term stability. Additionally, I performed a stability analysis on the upper bench and slope of the RPEE which was not considered in Barr’s assessment. Results from the Galena slope stability model are below. Galena Slope Stability Results Slope Lower Upper Static FoS 1.72 1.94 1 Seismic Fos 1.41 1.62 1) Horizontal Seismic Acceleration = 0.0698g Results between Galena and Barr’s modeling varied most likely due to the lack of complexity allowed by Galena and assumptions. Most noticeably, Barr’s model was able to take into consideration of surface runoff and seepage while the Galena model cannot. It is my opinion that Barr’s analysis most accurately reflects the final construction of the modified Phase 1B buttress with refuse placement up to an elevation of 6,930 ft. Results shown by Barr’s analysis of the RPEE and modified buttress met all construction requirements by the Division. Sincerely, Zach