2009-08-06_REVISION - C1982056 - Laserfiche WebLink

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C-1982-056 TR-67 PAR - M LT 06-Aug-2009 Page 3 of 3 face of the CRDA, as the critical section. The model predicts a FS of 2.3 for a failure surface confined to the coal mine waste (fully constructed pile configuration) and FS of 2.2 for a failure surface which also incorporates the upper portion of the reclaimed spoil. In the computer models presented by NWCC, a cohesion value of 0 psi and a unit weight of 115 pcf were assumed for the coal refuse material, which would be the "worst case" scenario. At internal friction angles of 25 and 30 degrees, lower factors of safety (1.17 and 1.45, respectively) were calculated for the outer face of the fill, indicating that a shallow failure on the face of the fill slope may possibly occur if insufficient compaction is attained near the edge of slope. A review of the_1995 CTL/Thompson study found in Exhibit 26A of the, current Permit reveals that internal friction angles of 30 to 35 degrees and unit weights of 80 to 100 pcf were used in the design of the existing 3 MM cubic yard CRDA. These less conservative values resulted in an allowable slope design of 2.8:1, as opposed to the 3:1 slope presented in the current revision application. The CRDA design proposed has been shown to be stable (with a Factor of Safety >1.5) with respect to any large, circular failure. However, the data provided by NWCC indicate that shallow failures on the face of the fill slope may be somewhat more likely to occur (although not with dire consequences). If, during Phase I or II construction of the CRDA, shallow failures are observed on the face of the fill with any frequency, the proposed slope configuration should be modified (flattened) to prevent such failures from occurring in future phases. Please provide additional language in Section 2.05 of the permit to address actions that will be taken, should there be shallow slope failures observed on the face of the CRDA fill. Rule 4.11- COAL MINE WASTE 4.11.4(3 - Disposal of Non-Coal Waste TCC does not propose to dispose of any non-coal waste in the CRDA. 4.11.5 - Dams and Embankments "Embankment" is defined as follows: "an artificial deposit of material that is raised above the natural surface of the land and used to contain, divert, or store water, support roads or railways, or for other similar purposes." The CRDA expansion is not being proposed for any of these purposes. Therefore, it is the Division's determination that the conditions of Rule 4.11 do not apply to this revision. This concludes my adequacy comments for TR-67. Please do not hesitate to contact me with any questions you may have. Cc: Sandy Brown