2014-02-26_REVISION - C1996083 (2)

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Page 6of10 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 Buckhorn Nov -2012 — Segments with 2h:1v okay; regrade any that are steeper Gob Pile #3 • TR -45 —Approved Design: 3h:1v slopes, with 15' bench; pile height 65' Buckhorn Mar -2007 Total Stress (Undrained) and Effective Stress (Drained) analyses Evaluated circular, block, and high groundwater options; various slopes and heights • TR -84 (Currently in Review) — Steepen to 2.5h:1v; increase height to 122' Buckhorn Oct -2013 & Jan -2014 PORE PRESSURES A total of ten Vibrating Wire Piezometers (VWPs) have been installed at Gob Pile #2 since June 2005. Over time, five of these were damaged, and are no longer in service. Of the five that continue to function, three (VWP -05, -09 and -10) are located near the toe of the waste pile and two (VWP -06 and -08) are within the higher valley -fill portion of the pile. Pore pressures are measured at least quarterly, and are reported in the operator's quarterly Coal Mine Waste Bank Reports. Pore pressures at the toe of the pile are consistently low and steady over time, ranging from 1 to 2.5 psi. Higher in the pile, where the thickness of the coal mine waste is greater and the path for dewatering is significantly longer, the pore pressures have remained high, between 14 and 16 psi. The slow pace at which pressures are dissipating (or not) in the upper reaches of the pile has not been adequately addressed in the more recent Buckhorn studies, which have in fact assumed that no phreatic surface exists. A brief summary of the pore pressure history for each VWP is described here, with noteworthy elements highlighted. A Location Map and graphical representation, extracted from the 4t" Qtr 2013 Coal Mine Waste Bank report, follow after, for ease of reference. Jun -2005: VPW -01 thru -05 installed VWP -01 Jun -'05 to Oct -'06 VWP -02 Jun -'05 to Mar -'11 VWP -03 Jun -'05 to Mar -'11 VWP -04 Jun -'05 to May -'06 VWP -05 Jun -'05 to present Jun -2009: VPW -06 thru -08 installed Peaked at 8.2 psi May -'06 Peaked at 3.2 psi Apr -'06 Peaked at 12.2 psi Jan -'06 Peaked at 9.3 psi Apr -'06; Peaked at 3.5 psi Dec -'07 Final: 7 psi, decreasing Final: 2.5 psi, steady Final: 9 psi, steady Final: 9 psi, decreasing Currently 2.5 psi, steady VWP -06 Top Jun -'09 to present Peaked at 16 psi Mar -'13 Currently 15.5 psi, decr VWP -07 Top Jun -'09 to Mar -'11 Peaked at 21 psi Dec -'09 Final: 16 psi, increasing VWP -08 "Top Jun -'09 to present Peaked at 15 psi Dec -'09 Currently 14 psi, steady