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-_ <br />SHANNON <br />ALASKA <br />CALIFORNIA <br />6WILSON., INC. <br />COLORADO <br />GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS <br />FLORIDA <br />MISSOURI <br />OREGON <br />WASHINGTON <br />November 30, 2012 <br />Revised January 4, 2013 <br />Colowyo Coal Company, L.P. <br />5731 State Highway 13 <br />Meeker, Colorado 81641 <br />Attn: Mr. Juan Garcia <br />RE: ADDENDUM NO. 4 <br />GEOTECHNICAL STUDY FOR EXCESS SPOIL <br />SOUTH TAYLOR MINE AREA, COLOWYO MINE <br />MOFFAT AND RIO BLANCO COUNTIES, COLORADO <br />This report presents Addendum No. 4 to our Geotechnical Study, South Taylor Excess Spoil Fills <br />(Shannon & Wilson, 2006). This addendum addresses the stability of the planned spoil fill that <br />is identified as the West Taylor Fill on Map 45 of the permit documents (Figure 1). <br />SUMMARY <br />The results of our stability analyses are summarized on Table 1. In our opinion, the minimum <br />FS of 1.3, calculated when using low -bound strength estimates, is acceptable for the planned <br />temporary excess spoil fill. Using mean strengths based on laboratory testing, FS values in <br />excess of 1.8 were calculated. The permanent portion of this fill, after re- grading to the Post <br />Mining Topography (PMT), will be considerably flatter with resulting higher FS. These flatter, <br />slopes were analyzed in our study for the permanent excess spoil fill (Shannon & Wilson, 2006) <br />and meet the minimum FS of 1.5 for permanent excess spoil fills (Colorado Division of Minerals <br />and Geology, 2005, Rule 4.09.2 (1). <br />1060 BANNOCK STREET, SUITE 200 <br />DENVER, COLORADO 80204 <br />PHONE 303.825.3800 <br />FAX 303.825.3801 <br />www shannonwilson com <br />23 -1- 01105 -600 <br />