2014-03-06_REVISION - C1996083 (3)

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Bowie No. 2 Mine C 1996083 Technical Revision No. 84- Second Adequacy Review Page 7 of 15 BOWIE NO. 2 MINE — SUMMARY OF RECENT GEOTECHNICAL ANALYSES FOR PERMITTING OF COAL MINE WASTE DISPOSAL AREAS STABILITY ANALYSES Buckhorn Geotech has prepared a series of geotechnical reports addressing the various disposal areas for coal mine waste at the Bowie No. 2 Mine. The initial reports prepared in 2006 and 2007 for TR -42, TR -44 and TR- 45, were the most comprehensive in nature. For TR -56 (2008) and TR -64 (2009), the complexity of the analyses was reduced somewhat based on the dissipation of pore pressures that had been observed with the initial series of Vibrating Wire Piezometers (VWPs). The newest series of stability analyses, beginning with TR- 75 in 2012, are noticeably simpler in nature, evaluating fewer variables and assuming that no phreatic surface is present. A brief summary of the pertinent revisions and related Buckhorn Geotech stability analyses for the mine's Coal Mine Waste Disposal Area Nos. 2, 4, and 3 is provided here. Gob Pile #2 • TR -42 (WD) - Steepen Upper Slopes from 5h: 1v to 4h: 1v Buckhorn Feb -2006 (this report is referenced by many later Buckhorn studies) Total Stress (Undrained) and Effective Stress (Drained) analyses were conducted • TR -56 — Steepen Upper Slopes from 5h:1v to 3h:1v Buckhorn Sep -2008 Pore pressures observed to be dissipating, therefore Undrained analysis not appropriate (pp. 6 & 10) Evaluated 2 phreatic surfaces: one as indicated by pore pressure readings from piezometers, and the second at a conservative, higher level • TR -76 — Connect Gob Piles #2 & #4, increase pile heights Buckhorn Nov -2012 No phreatic surface modeled, since "ultimate pile stability assumes dewatering which is reasonable given the decreasing pore pressures... over the past 3 years" • TR -81— Revise bench spacing, steepen one section (between #2 and #4) from 3h:1v to 2.5h:1v Buckhorn Jul -2013 See comment from TR -76, above, re: phreatic surface (none modeled) Recommendations regarding keying -in the piles as they are joined • TR -85 (Currently in Review) — Temporary Drying Area on top of valley fill portion Buckhorn Sep -2013 & Dec -2013 Gob Pile #4 • TR -44— 2h:1V excavation slopes; outer slope 2.5h:1v first 50'; 15' bench; 3h:1v; pile height 118' Buckhorn Feb -2007 Total Stress (Undrained) and Effective Stress (Drained) analyses • TR -64— Increase pile height from 118' to 150'; assumed excavation slope 1.3h:1v Buckhorn May -2009 Pore pressures dissipating, therefore used only Effective Stress; adjusted parameters Evaluated 2 phreatic surfaces: 1) assumed, and 2) increased, to evaluate sensitivity Considered both block and circular failure surfaces, and three 4' angles (30, 32, & 34) • TR -75 — Stability of As -Built Configuration Buckhorn Sep -2012 — Pile height 165'; no phreatic surface CIRCES Cost Estimating Soffi are