2008-12-05_REVISION - C1994082 - Laserfiche WebLink

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Yoast Mine 7R-38, Pond 012 Slide Remediation Page 2 modifications in TR-37. We are also aware that the final sedimentation contm1 plan for the Pond 12 watershed may be modified in the future, depending on the details of the final slide remediation plan. Please address "short and long term sediment control measures for the Pond 12 Watershed" for insertion m the permit, along with scheduhng commitments requested in Item 1, above. Include references as appropriate to pertinent sediment control plan maps and details (approved and "under review'), as well as measures that ha ve been implemented in the Lreld. Response: As the Division is aware that even before the NOV was written and as discussed above, SCC had developed and submitted a plan to provide long-term sediment control for the area by incorporating Pond 012A into the Pond 012 watershed. Since this sediment control system is currently inplace, it is also the short-term control measure. When the Division reviews/approves the plan in TR-37, it will be evident that the systems in place are sufficiently adequate to provide the required sediment control. This information was submitted (in March 2008) in TR-37 and will be incorporated into the permit when approved. It appears to be redundant to incorporate this information twice into the same document. However, in order to comply with the DRMS request, this list of sediment control measures is enclosed for insertion into Attachment 13-12B, Landslide Repair Plan. 3. The Division is concerned that the additional data anticipated this spring may not be sufficient to allow for completion of the required analy ri r and mitigation. Four of the eight test holes drilled between June 3db and July 2"d 2008 were "cased with slotted and solid 2-inch diameter PVC pipe so thatgroundwater levels and soil movements could be monitored after the drilling was completed. " The other four holes caved in, and test pits were backfilled. None of the four cased holes had exhibited slide movement as of the time the report was submitted, meaning that we do notyet know the location of the failureplane of the slide. This information, as well as observations ofgroundwater and surface water movements will be critical to developing remediation plans for the slide. The four holes that were fitted with casings are Test Holes 1, 3, 6, and 8. We question whether the four holes described will enable a sufficient understanding ofgroundwater and slide behavior, and we anticipate that the drilling of additional holes may be necessary. No drilling was conducted outside of the slide block itself; if a cutedrain is warranted to intercept groundwater above the slope, it will be necessary to know the subsurface conditions, e.g. depth to bedrock, in that area. Similarly, to accomplish design of any proposed dewatering activities, an understanding ofgroundwater conditions in the drainage upstream of the slide may need to be established Please provide explanation for inclusion in the permit, addressing how the data from four cased holes will be used in the analysis of the failure mechanism and design ofproposed remediation efforts, along with a description ofany additional field investigation methods that are likely to occur, in light of the apparent data limitations noted by the Division. Response: NWCC completed a Supplemental Subsurface Investigation at the site on November 20, 2008. The supplemental investigation consisted of the drilling of two additional test holes to the east of the slide area and upstream of the failure area. The test holes were advanced to depths of 45 and 50 feet beneath the existing ground surface and into the underlying bedrock materials. Both test holes were cased with slotted and solid 2-inch diameter PVC pipe. In addition, PVC casing was installed in new test holes drilled adjacent to Seneca Coal Company . P.O. Box 670 • Hayden, Colorado 81639 Telephone (970) 276-5219 • FAX (970) 276-5222