2013-09-19_PERMIT FILE - C1981019A (2)

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• Colowyo Coal Mine Attn: Mr. Tay Tonozzi April 15, 2010 Page 6 Using the lower bound strength and including a 20- foot -thick layer of water are conservative assumptions in our opinion. Analyzing the slope with lower bound strengths and /or conservative water levels results in a calculated FS or 1.3 or greater. Considering lower bound conservative conditions provides a check that if one of the strength parameters is lower than estimated a reasonable FS remains. Highwall Mining In our Addendum No. 2 of our 2008 report we stated — "Two areas of highwall mining are planned below excess spoil fills. The highwall mining configurations (web widths, barrier pillar criteria, extraction ratios, etc.) were designed by Agapito Associates, Inc. (AAI) in their report `Geotechnical Design and Operational Considerations for Highwall Mining, West Pit South, F12 and E2 Seams' (November 23, 2005), and Addendum (April 13, 2007). • In their report, AAI stated on page 10, `Web pillars are designed to remain stable and preclude subsidence.' The planned excess spoil fill is generally confined in a valley and the out - slopes at the downstream toes are relatively flat at 3.0:1 (H:V). In our opinion, the highwall mining will not significantly affect spoil foundation conditions and should not reduce the global stability of the spoil fill." • The locations of the highwall mining have not changed; the location /thickness of spoil fill above has changed. We do not believe AAI designed the highwall mining pillars to support deep spoil fill above the mined areas. However, it remains our opinion that subsidence of the highwall mining areas will not adversely affect the global stability of the proposed spoil fills. There is a possibility that subsidence of the highwall mined areas under deeper zones of fill could occur and may result in subsidence at the surface of the fill. We did not perform a specific analysis of the location, extent or degree of settlement/subsidence which might occur. Based on our experience with abandoned mine subsidence and because deep air or hoist shafts are not present, as in more typical abandoned mines, we do not expect sinkhole propagation though the excess spoil to the surface to occur. If subsidence of the highwall mined area were to occur, we expect trough -like zones of subsidence /settlement at the surface of the spoil fill. These could affect surface grading and surface water drainage, but we do not expect it will have adverse effects on the global stability of the fills. Exhibit 21, Item 1 Revision Date: 12/06/10 Addendum No. 3- 6 Revision No.: 'VR -?,A