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Second Adequacy Review Comments <br />Varra Coulson Mine Operations <br />Greeley, Colorado <br />Page 2 <br />Varra does not use typical slurry walls for its liners — rather it uses reworked native <br />clay /claystone /shale for lining materials. These material are generally exposed at the base of <br />the mine and are placed by pushing native materials up the side walls (and floor if needed) <br />with a bull dozer. The liner is compacted in place with the heavy equipment. The side -wall <br />liner thickness near ground surface can exceed 20 feet and a conservative value of seven feet <br />was used for the model. <br />Mr. Cazier's comments regarding Plate 1 and 2 are accurate and a reference to Plates A and B <br />was a typographical error. Only Plates 1 and 2, were in the attachment. <br />25. To be addressed by Varra Companies, Inc. (Varra). <br />The following addresses the stability analysis comments of February 14 by Mr. Peter Hays. All stability <br />analysis plates are presented in Attachment B. <br />1. The plan map used for the stability cross section is provided as Plate 3. <br />2. This response addresses items 2 through 6. The site lithology varies greatly across the site <br />however the soil profile submitted in the permit application accurately represents the average <br />soil profile and pit depth. The proposed pit bottom can consist of claystone, clay or sand and <br />gravel. The stability analysis used the soil profile from boring H -14 as the boring was located in <br />in or near the mine wall face with unconsolidated soils extending well below the pit bottom. <br />We have also generated a stability analysis, which accounts for a claystone /shale pit bottom <br />with the upper three feet composed of weathered material. The soil profile for this analysis is <br />depicted on Plate 4. <br />For both soil profile scenarios trial and error runs were made so that a safety factor of greater <br />than 1.3 was achieved for mine slopes in critical areas. In areas where cohesionless soils <br />extend well below the pit bottom, and using the most conservative soil properties, the mine <br />wall slope (bottom to top) was reduced to 3:1 for the first 10 feet, 2:1 for the next ten feet and <br />1.25:1 to the mine crest. The predicted safety factor for this scenario is greater than 1.4. The <br />model output is presented as Plate 5. <br />For the claystone /shale pit bottom scenario .a mine wall slope of 3:1 for the first 20 feet is <br />recommended. Keeping the remaining slope at a 1.25:1 achieves a good safety factor ( >1.4 <br />Plate 6) and allows for the maximum material extraction. For the bedrock pit bottom analysis <br />a boundary load of 2500 pounds per square foot (psf), which is a typical allowable soil bearing <br />pressure for sands and gravels, was simulated at a distance of 28 feet from the mine face <br />within the termination zone. The addition of a boundary load at this distance did not affect the <br />stability analysis. The boundary load simulation is depicted on Plate 7. <br />: AWU% 04") 14 ur Slar 4-4 url. 14 if 4 ;4 W11%. (/ %4 41 / )11 P 04 910 1 <br />