2019-03-22_REVISION - C1982056 (5)

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Geotechnical! Environmental Engineering • MaterWsTesting March 20, 2019 Twentymile Coal Company Nick Aromando 29515 County Road 27 Oak Creek, CO 80467 Job Number: 18-11283 Subject: Subsurface and Surface Drainage Recommendations, Wash Plant Thickener Tank, Twentymile Coal Mine, Routt County, Colorado. Nick, This report presents NWCC's recommendations for improving the subsurface and surface drainage conditions beneath and around the exterior of the Thickener Tank, currently under construction at the Wash Plant and within the Twentymile Coal Mine in Routt County, Colorado. NWCC previously prepared a "Subsoil and Foundafion Investigation " report under the above job number, dated October 12, 2018. Site Observations: As of this date, approximately 4 to 8 feet of imported, granular fill materials have been placed within the tank excavation. It should be noted that the excavation, prior to fill placement was widened so that a minimum of 4 feet of granular fill materials would be placed beneath the tank. The fill materials placed in the center of the tank excavation will be removed so that finished elevation of the fill materials matches the conical shape of the tank. These fill materials were initially placed to provide more uniform fill placement and better compaction with larger vibrating compactors. Based on our observations made at the project site during the fill placement, it appears that 2 to 3 feet of relatively clean, crushed limestone aggregates ranging from 2-6 inches in size were imported and placed within the lowest part of the excavation. These aggregates were approximately 20 feet in width and extended from the approximate center of the tank to outside the northwest corner of the tank. Approximately 2 to 3 feet of imported pit run sand and gravels were then placed over the crushed limestone aggregates and approximately l to 2 feet of imported base course aggregates were placed over the pit run sand and gravels. It should also be noted that prior to the placement of the fill materials, the subgrade soils exposed at the base of the tank consisted of natural clays. Groundwater was not observed in the excavation as it was completed. Subsurface Drainage Recommendations: Based on our observations, made during the fill placement, it appears the imported aggregates placed over the natural clays consisted of granular soils that should drain to the base of the excavation. We believe that the crushed limestone aggregates placed at the base of the tank excavation and over the natural clays should collect and hold any subsurface drainage that enters the tank (970) 879-7888 • Fax (970) 879-7891 2580 Copper Ridge Drive • Steamboat Springs, CO 80487