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C-1981-019 TR-81 PAR - mlt 16 Feb 2009 Page 3 of 7 Using the Slope/W program, S&W determined a Factor of Safety (FS) of 2.2 for the West and East Valley Fill outslopes (and 1.7 for areas as steep as 3H:1 V). For the East Valley Fill, with Temporary Spoil surcharge, the estimated FS dropped to 1.6. Under seismic conditions, the FS for fill outslopes was reduced to 1.5 and 1.3. Addendum No. I to the Study was dated October 26, 2006 (with PR-02). This study included additional sampling and testing of soil samples from a test pit excavated in the E Fork of the E Fork, and revised the Main and Lateral Drain designs per DRMS comments. With this addendum, Figures 2 and 5 were revised,.and Table B-1 of Appendix B was modified to include laboratory results from tests of the material sampled from the test pit. The Study was further modified under TR-77 by Addendum No. 2, dated November 21, 2008. Valley drain locations were revised in response to the establishment of the ST up-dip box cut and the southward extension of the West Pit. Figure 5 was revised once more to reflect these changes and two new Highwall mining areas. S&W also considered the potential effects of the proposed highwall mining on the stability of the spoil, and concluded that Highwall mining will not significantly affect spoil foundation conditions and should not reduce global stability of spoil fill. Addendum No. 3, dated October 21, 2009, analyzes outslopes for the West (2.911: IV) and East (2.5H:1 V) permanent and temporary spoil fills that are steeper than the 3.8H:1 V originally permitted. Stability analyses were also conducted on temporary spoil fills that are not placed upon a permanent fill. Additional laboratory testing and analysis was conducted on the spoil; the surficial soils upon which the permanent valley fills will be constructed; and the carbonaceous mudstone exposed beneath the lowest mined coal seam in the pit floor. Comments on S&W Studv 1. In the process of conducting the TR-81 adequacy review, the Division revisited the earlier S&W Study and Addenda Nos. 1 and 2. We note a discrepancy 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 B4 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