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C-1981-019 TR-81 <br />PAR - mlt <br />16 Feb 2009 <br />Page 5 of 7 <br />Please review Table A-3 entries for CMS Cohesion and Factor of Safety and revise as <br />appropriate. <br />5. Note No. 1 on Figure 4 refers to Appendix B. The report supplied to the Division <br />included only Appendix A. Please provide Appendix B for the Division's review. <br />6. On Figure 5, Curve No. 4, representing friction and cohesion for CO-3, has been <br />extrapolated to become Curve No. 6, the Lower Bound for stability analysis. We <br />question whether an actual test result is appropriate for use as a "Lower Bound", in that <br />the number of samples analyzed is not excessive, and there may well be other soils that <br />are "worse" upon which the fill may be constructed. <br />Please provide additional rationale for the selection of the Lower Bound for the clay <br />overburden. <br />Rule 2.05.3(6) - Overburden <br />7. Section 2.05.3(6)(c) lists the requirements for the geotechnical investigation of a <br />proposed disposal site. Bedrock characteristics; adverse geological conditions; locations <br />of springs and seeps; and rock drain material characteristics have been successfully <br />addressed in previous S&W studies for the South Taylor area. With respect to (iii), <br />potential effects of subsidence, S&W issued an opinion in Addendum No. 2 (Nov-2008) <br />that highwall mining will not significantly affect spoil foundation conditions and should <br />not reduce the global stability of the spoil fill. With TR-81, the dimensions (lateral and <br />vertical) of the temporary and excess spoil fills have been enlarged. The issue of <br />potential subsidence has not been addressed in the Oct-2009 S&W study. <br />Please address the potential effects of subsidence of the subsurface strata due to <br />highwall mining activities, considering the proposed configurations of permanent and <br />temporary excess spoil fills. <br />Rule 4.09 - Disposal of Excess Spoil <br />The first paragraph of the Conclusions and Recommendations section of the October 2009 S&W <br />study (page 4) concludes that the Regulations do not specifically address temporary slopes. <br />Rule 2.05.3(6) requires that temporary overburden disposal (storage) structures comply with the <br />applicable performance standards of Rule 4. "Applicable" sections of Rule 4 are those that are <br />not limited specifically to the "final" excess spoil configuration. The requirements of Sections <br />4.09.1(7) and 4.09.2(1) that a long-term static safety factor of 1.5 be ensured are therefore <br />considered to be applicable to the Temporary Excess Spoil and valley fills at South Taylor. This <br />comment is provided for purposes of clarification only. No response to this item is required, <br />S& W has used a long-term static factor of safety of 1.5 in the design of the temporary excess <br />spoil fills.