_INSPECTION - C1994082 (40) - Laserfiche WebLink

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September 20,2010 C-1994-082/Yoast Mine DTM need to be seeded this fall. Some riparian tree and shrub planting was conducted during the period of inspection along YPM-6-2 and YPM-6-3. A small patch of narrow leaf cottonwood was planted along the west side of upper YPM-6-3; the trees were planted with weed fabric strips and were watered in. A number of willows were planted in the relatively flat segment of upper YPM-6-2. The slide repair cutslope will be broadcast seeded this fall. Final topsoiling and seeding in the slide project area will be conducted in 2011, following completion of the trench drain installation. SLIDES and DAMAGE -Rule 4.12: -Remedial grading and drainage work associated with repair of the Pond 012 slide was nearing completion. The slide is located on the steep hillslope immediately to the northeast of Pond 12, at the lower end of the former Haul Road"D". The site has exhibited stability problems for many years, and prior attempts to stabilize the slide by regrading have not been successful. The remediation plan currently nearing completion was based on geotechnical evaluation and hydrologic investigations conducted by Northwest Colorado Consultants,Inc. (NWCC), a geotechnical/environmental engineering firm, for Seneca Coal Company, and approved under Technical Revision 46 (TR-46) in early 2010. The TR-46 plan involved removing unstable colluvial mass from the slope,and creating a final slope as steep as 0.5h:1v in competent material,combined with intercepting sub-surface flow within two trench drains containing slotted PVC pipe embedded in screened gravel. The primary trench drains include one drain(Trench), near the toe of the steep south facing cutslope,and a second drain(Channel) located along the lower segment of permanent drainage channel YPM-6-3. The YPM 6-3 channel configuration with riprap is actually constructed within the larger drain trench structure, with the slotted PVC pipe embedded in gravel beneath the riprap. Designs are shown in plan view and cross section on Sheets 1 and 2 of Attachment 13-12B. By the end of the inspection on 9/24/10,the plan had been largely completed, with the exception of the trench along the toe of the cut slope. Excavation of the trench in this location was nearing completion(and excavation apparently was completed soon after the inspection), but on the advisement of Brian Len, P.E.,the project geotechnical engineer for NWCC, finalization of the trench drain with PVC and gravel installation will be suspended until after the 2011 spring runoff. During closeout discussion with operator representatives on 9/24,Mr. Len stopped by the office to report project observations he had made earlier that afternoon. Mr. Len recommended that the trench drain installation be delayed,to allow for observation of trench functioning during high groundwater conditions in the spring, and also to allow for cleanout of any surficial deposition from the freshly exposed cut slope that may occur during spring melt,prior to final completion of the trench drain. Based on my observations during the inspection,the remedial project appears to have been conducted to date in accordance with the plans approved in TR-46. The cut slope had been excavated into what appears to be competent shale material,with some weathered sandstone along the crest of the slope. Excavation of the trench drain along the toe of the cut had been largely completed, and the Channel 6-3 segment, including two short tributary drain segments depicted on Design Sheet 1,appeared to have been completed in accordance with approved design. Broadcast seeding of the cut slope is to occur this fall (upper and lower segments of the cut Number of Partial Inspection this Fiscal Year: 2 Number of Complete Inspections this Fiscal Year: 1 Page 8 of 13