2010-06-02_REVISION - C1981019 - Laserfiche WebLink

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27. In question 3 the Division details the decrease in the amount of available spoil material due to the decrease in the swell factor from 20% to 15%. Since a majority of the South Taylor Operation will be a truck/shovel operation, this decrease will be amplified as mining progresses from the box-cut area to the east. The Division believes that this decrease in available spoil will manifest itself in Colowyo's inability to meet their approved post-mining topography as shown on Map 19B. Keeping in mind that the Division has already granted Colowyo a variance in AOC for steep slope mining in the South Taylor area, please detail how Colowyo will achieve their currently approved AOC with the spoil deficiency that has been identified. Although Map 19B has not been changed, based on the additional 10 million yards of proposed permanent spoil and the possible deficiency in spoil from the decrease in swell (from 20% to 15%), there will be at least a pit-wide decrease in elevation of at least 12 feet. Response: Please refer to the response to the above comment. Comments Regarding Shannon and Wilson Report 28. In the process of conducting the TR-81 adequacy review, the Division revisited the earlier Shannon and Wilson Study and Addenda Nos. 1 and 2. A discrepancy was noted in the transcription and utilization of data, as follows: Table B-1 in Appendix B presents a "Summary of Laboratory Test Results by Boring". Data in the Direct Shear "Cohesion" and "Friction Angle" columns, for Borings CO-3 and LW-1, appears to have been incorrectly transcribed from Figures B-4 and B-5, the original reports prepared by Knight-Piesold Consulting. Corrected friction angles for CO-3 and LW-1 would be 24 and 33 degrees, respectively. Similarly, the tabulated cohesion values for these two samples should actually have been 370 and 40 psf. The incorrect numbers shown in Table B-1 appear to have been used in the development of the Average Friction Angle and Cohesion values reported in Table C-1 for "Soil", which parameters were in turn used for input into the Slope/W program. Inclusion of these erroneous values may have resulted in Factors of Safety that were inaccurate (too high). Please review the information presented in Appendices B and C of the original Study, and revise as appropriate, including any conclusions or recommendations that may change as a result of the review. (We note that corrected friction angle and cohesion values from Boring CO-3 appear to have been used in the Shannon and Wilson study submitted with TR-81.) Response: Shannon and Wilson has issued an April 15, 2010 revision to their October 2009 Addendum No 3, and it supersedes all previous geotechnical studies, including tabulated material strengths and computer based slope stability analyses. Since the original July 2006 Geotechnical Study, additional strength tests have become available such as a triaxial strength test for the valley bottoms clay. Other typographical and/or data transpose issues in the various tables in the previous reports and addenda have all been addressed in the tables included in the Revised Addendum No. 3. Consequently, going back to correct the original report would serve to raise more confusion than it would resolve if the corrections have to do with material strength parameters, pile slope angle, or slope stability evaluations. The original report and the previous addenda do still contain useful information such as site geotechnical setting and other general considerations. Accordingly, we would prefer to leave all the previous reports as they currently exist, and address all slope stability issues and material properties via the Addendum No. 3 Revision.