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.: .. DIVISION OF MINERALS AND GEOLOGY Ur'parv ncnl ul Nalu ral hr~soun rs l i l 4 Shrrnytn SL, Ruont ! I S Drnvr•r. (nluoidn HIL'01 I'hunr: 1411 1174LL-,I iL 7 I':\A 1 411 q tV I! II 1110 November 12, 1996 Mr. Norm Every Mountain Coal Company P.O. Box 591 Somerset, CO 81434 RE: Technical Concerns -Lower Refuse Pile Technical Revision No. 78 -Stability Analysis Addendum West Elk Mine - C-80-007 Dear Mr. Every: I~~ DEPARTMENT OF NATURAL RESOURCES Roy Romer Gnvemo~ lames S Lochhead E+ecu~ive Di.ecmr hnchacl B Long Drvismn Dicecmr The Division of Minerals and Geology (DMG) has reviewed the Slope Stability Analysis of Piezometric Surface (Lower Refuse Pile) received August 21, 1996 from Mountain Coal Company (MCC). This report was submitted in response to DMG adequacy comments dated June l9, 1996. [n response to the June 19, 1996 adequacy comments, MCC commissioned Harding Lawson Associates (HLA) to revise their original stability analysis of [he Wes[ Elk Mine's Lower Refuse Pile reconfiguration. The revised analysis applied a perched water table assumption to the earlier configuration. The soil material strength parameters and distributions were unchanged, with the addition of the perched saturated zone. In completing this analysis, HLA used [he GEO-SLOPE analytical software, rather than the PCSTABLS used for the original analysis. GEO-SLOPE can accommodate the perched water table assumption, whereas PCSTABLS cannot model the perched water table assumption. The methodology and the modeled distributions appear to have been appropriately completed. As in the earlier analysis, three critical cross sections were analyzed, and the waste material was modeled using four different material strength assumptions to anticipate possible variations in the refuse. One section was modeled for each of the east, north, and northwest facing slopes of the reconfigured refuse pile. One material assumption case, the ]0% fines material parameters for Profile 3 (the east facing slope) was determined to have a static slope safety factor of 1.45. HLA's report concludes; "It is HLA's opinion, based on long term working and monitoring experience (from July, 1981 through July, 1996) with [he Lower refuse Pile project that the 1.45 factor of safety is within the margin of tolerance for non-failure of the slopes at the LRP." HLA's opinion may or may not prove to be correct. However, the 1981 through 1996 period of observation dealt largely with lower sloped pile configurations under lesser levels of perched saturation. HLA further recommends monitoring of the underdrain outflows, already required by MCC's approved permit. Finally, if drainage increases, HLA recommends the installation of a permanent set of slope stability monuments. The safety factor of 1.45 does not satisfy [he regulatory requirement. The substandard safety factors can be addressed through several possible approaches: iiiiiiiiiiiniii iii STATE OF COLORP~UU