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Technical Revision No. 79 -PAR Responses April l8, 1997 Page 7 problems from water underneath the liner. MCC also believes the waste refuse and resulting leachate are non-toxic and will present no water quality problems. 28. MCC installed LRP-1 and LRP-4 during October 1995 and began monitoring the ground water elevations in January 1996. MCC has continued monitoring the piezometers on a monthly basis; and therefore, has accumulated over one year of data which covers the annual hydrologic cycle. Please see the discussion in question 2 regazding ground water levels. As previously stated MCC will install additional piezometers after construction of Phase 1 on the RPE. 29. Bedrock: Mesa Yerde Sandstone group: In the initial stages of developing the revision for the Refuse Pile Expansion (RPE), Harding Lawson Associates (HI..A) utilized a sandstone bedrock strength parameter of 200,000 psf cohesion @ 40° internal fiiction angle for analyzing the "micro" slope stability slide potential of the RPE as it would be developed. The bedrock strength parameter was taken from E. Hoek, J.W Bray, 1973. "Rock Slope Engineering", Institution of Mining and Metallurgy text book. Emphasis during the initial slope stability analysis' was directed toward potential soil failure within the new coal refuse pile as it would be developed. Existing natural slopes within the RPE footprint, where the new coal refuse pile would buttress, were also examined for micro slide potential. This initial set of slope stability runs utilized the PCSTABL computer program. In the first meeting with the CDMG, to present the draft of the RPE permit, CDMG was satisfied with the initial slope stability results of the RPE; however, the Division was concerned that a "macro" slide may occur through the entire slope of the West Flat Iron. HLA acquired a new slope stability computer program entitled GEOSLOPE. The program was used in the analysis of the macro slide through the West Flat Iron. In addition to the new computer program, HLA performed a laboratory rock strength testing program of recently acquired bedrock samples retrieved from the WEM exploration program conducted on the slopes of the West Flat Iron. The laboratory test program provided new bedrock strength data for the slope stability analysis (the lowest strength value was 30,000 psf cohesion @ 38° internal friction angle). This new bedrock strength pazameter was used in the "macro" slide analysis. To maintain continuity of the permit presentation, the RPE micro slides were ran again using the GEOSLOPE program and the new bedrock strength parameter. Analysis of the stability runs for both the macro and the micro slide analysis indicate that bedrock (even at a lower strength) does not control the failure. The failure sheaz planes for both the macro and the micro slide analysis were contained or propagated in the colluvial clay layer above the bedrock. A failure shear plane through the bedrock was not evident. Engineering Fill: The engineering 611 is limited to the embankments of the ponds near Hwy. 133. The ponds and embankments will be lined with HDPE to minimize foss of water. Subsequently, the embankment fills will remain in an unsaturated condition. The clay liner in the RPE foundation footprint will not be used and will be replaced with a PVC liner.