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Shannon and Wilson first addressed the issue of subsidence due to underlying highwall <br />mine zones in their November 21, 2008 Addendum No. 2 to their Geotechnical Study for <br />Excess Spoil, South Taylor Area. At that time they concluded they did not believe the <br />presence of highwall mining below the piles would affect the piles' overall stability. For the <br />revised pile geometry that is the subject of this TR-81, they reconfirmed this conclusion with <br />regard to overall pile stability in the April 15, 2010 Revision to the October 21, 2009 <br />Addendum No. 3. <br />As shown on Map 208 and Map 45, the extension of the toe position of the permanent East <br />Taylor and West Taylor valley fills that is proposed in TR-81 does not significantly increase <br />the depth of permanent valley fill over the previously highwall mined areas beyond what <br />was proposed for PR-02, but it does add several hundred feet of temporary surcharge due <br />to the temporary spoil stockpiles associated with TR-81. In their April 15, 2010 Revision to <br />Addendum No. 3, Shannon and Wilson acknowledged that the highwall mining pillars were <br />probably not designed for this additional overburden load. However, they provide their <br />opinion that any settlement that might occur would most likely be observed as a gentle <br />subsidence trough on the face or top of the temporary spoil stockpile, and not as a <br />sinkhole. It should be noted that any such subsidence would be masked by the normal <br />compression settlement of the fill under its own weight, and may not be discernable at the <br />surface. Finally, any such settlement would occur during the time that the temporary spoil <br />stockpiles are in place. It would therefore be obliterated during final regrading of the <br />permanent valley fills when the temporary spoil stockpiles are hauled back the mine pit. <br />This topic is also addressed in the response to Item 7 (b), with reference to the addition on <br />the cross sections of the areas of highwall mining. <br />2. In the Division's incompleteness question 1, the Division questioned the change in <br />the swell factor, on Page ST/LW-Rule 4, Page 20, from 20% to 15%. Colowyo's response <br />was that the decrease in swell was due to the fact that the materials in the permanent spoil <br />piles are being moved by the truck/shovel operation rather than the dragline (which is the <br />way all of the material in the fills was moved in the past, as well). Colowyo has not <br />documented how this reduction in swell factor was derived. In the initial permitting of the <br />Colowyo Mine and in the permitting of the Streeter Fill as an experimental practice there <br />was a great deal of time was spent analyzing and documenting both the swell factor and <br />the mine achieving AOC in its backfill (since material was being placed in a permanent fill <br />and not being returned to the pit). With the submittal of TR-81 Colowyo has simply <br />changed the 30+ years of historic data associated with the mines swell factor with little <br />more than a statement that the mining method has changed. Please provide the Division <br />with detailed documentation regarding why the swell factor is being changed as well as <br />detailed data to support this change. This documentation and data needs to be included in <br />the permit as an exhibit and a detailed explanation of the swell factor change needs to be <br />included in the permit text. Additionally, Colowyo needs to specify what portions of the <br />mine this change in swell factor applies to. Please provide the Division with detailed <br />documentation and data for the change in swell factor in the South Taylor Pit from <br />20% to 15%. Please include this documentation in the form of an exhibit. Please <br />include a detailed description of the change in swell factor in the text, beginning on <br />page ST/LW-Rule 4, Page 20.