2011-05-04_REVISION - C1982057 - Laserfiche WebLink

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Pond 016 Slide Remediation Plan Page 2 submitted to NWCC's laboratory for maximum dry density and optimum moisture content (standard Proctor) testing in accordance with ASTM D698/AASHTO T99. 3. Given the high clay content of the native materials, and the proposed relocation of Road J/K across the Pond 016A fill, compaction should be a minimum of 95% Maximum Dry Density (NWCC recommends 90%), with a moisture content between 0 and 4% above Optimum Moisture Content (NWCC recommends +14%). It is critical that Pond 016A be backfilled and compacted sufficient to prevent, to the extent possible, the infiltration of additional moisture into the fill. Lift thicknesses of 12 " to 18 " proposed by NWCC may be too great to achieve the desired densities. Response: The fill materials placed in Pond 16A will not be "pond" embankment fill materials. They will be placed in the abandoned pond as Access Road embankments (embankments). The permit specifies that the embankment fill materials placed in the access road embankments be compacted to a minimum of 90 percent of the maximum standard Proctor density and with a moisture content to achieve acceptable compaction. The permit also states that the fill materials will be placed in lift thicknesses not exceeding 12 inches prior to compaction. NWCC recommends that the embankment fill materials be compacted to at least 90%. NWCC also recommends that the moisture content range of the fill materials range from minus 4% to plus 4% of the optimum moisture content at time of placement. NWCC believes that acceptable compaction can be obtained at this range of moisture contents. The maximum lift thickness should be kept to 12 inches unless SCC can prove that acceptable compaction can be obtained with lift thickness greater than 12 inches but less than 18 inches. 4. The abandoned wells located at the base of the Pond 016A embankment were damaged in the original slide (May 2003) that occurred at the inlet and access road to Pond 016 The slide bent the and in some cases broke the casings below ground surface (about 40' bgs). After SCC consulted with Halliburton on-site, service rigs and drill rigs were employed to try and either straighten or remove the damaged casings. The effort failed; SCC was able to get a variance from the State Engineer's Office from their abandonment requirements because it was not possible to run tremmie pipe down to properly abandon them. Pond 016A was built in 2004, and SCC abandoned the wells in 2007. The wells were cemented in from 40' bgs to ground surface but continue to flow (we believe subsurface from the point of the broken casings). To rectify the situation, SCC installed a substantial French drain to collect the seeping water and direct the flow to the Pond 016 inlet which is located, downstream of the Pond 016A dam. The site was capped and backfilled after installing the drain. Given the condition of these wells, it is imperative that the abandoned wells and French drain not be disturbed. Please modify the Remediation Plan to clarify that grading will not take place in the immediate vicinity of the abandoned wells to ensure that repair activities do not disturb the wells or drain. Response: Grading and seeding will have to be conducted in the vicinity of the abandoned wells; however, extreme care will be taken not to cause any disturbance to the wells or drain. Seneca Coal Company a P.O. Box 670 • Hayden, Colorado 81639 Telephone (970) 276-5219 a FAX (970) 276-5222