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r ' • • Balaz & Associates New Coal Waste Pile and Haul Road Project, Sanborn Creek Mine, Somerset, CO June 18, 1997 Page 3 Il is possible that the quality of borrow materials may be such that a Geogrid material will have to be either partially or completeh~ substituted for the geotextile fabric. To make this determination, either a lazge number of exploration pits and additional laboratory testing will be required or changes will possibly be made during actual conswction. The fill section at station 27+00 will probably require mechanically reinforced earth in the entire fill. Even though this slope is somewhat flatter than the section at station 8+00, it is anticipated the materials used for construction in this area may contain a significantly higher amount of silt and clay fines and the anticipated colluvial soils at the base and tce area will convol the design. The use of mechanically stabilized soils `forces the projected failure surfaces back toward or into the more competent Bowie Member. It is anticipated the more clayey collucial soil will be used to conswct this fill. The `external wck loading' creates potential failtre near the upper fill face, which can be resisted by the mechanically stabilized earth. The surface exposures oCsandstone and siltstone are expressed on the topography map as relatively steep slopes, which were initialh• avoided for the road alignment. ARer consideration of the increased amounts oC sandstone and siltstone beds in these areas, it is believed the majority of the cut slopes can be steepened to between 1:2 (H: V) to 1:4 (H:V) without sacrificing the integrity oCthese cut slopes. These cut slopes will have to be carefully obsen•cd during the conswction process to determine if any additional stabilization techniques, such as benching, (latter slopes, rock butvess/MSE walls or tie-backs in the worse case aro required. Item 18. Location and T}pe of Fill Slopes LincolnDeVon;andBalaz&Associateshavc jointly developed sheet OXWVF- I G to indicate typical cut and fill sections, slope stabilization and anticipated stationing along the proposed roads for review, bidding and construction purposes. Actual field conditions may require modifications oC this plan. Item 24. Scdimcnt Pond Edward M. Moms. PE, of Lincoln DcVorc was prescnt, during the excavation of an exploration vcnch within the proposed Scdimcnt pond on 9-1 G-9G. Il was our understanding that as this exploration vrnch was being logged by personnel of WESTEC, and that a log of the soil profile and recommendations for the construction of the sediment pond would be forthcoming from WESTEC. Following is our description of this exploration pit. Personnel of Lincoln DeVorc did not obtain any samples for testing and the following descriptions will be based on visual logging only. The exploration pit was situated near the in-flow point of the sediment pons, as indicated on the attached drawing. This drawing is based upon the Plan of the Scdimcnt Pond as depicted on the Plan OXWVF-03. prepared by Balaz & Associates. The total trench was approximately 50 feet long, including minor probing at the upsucam side. near the visible outcrop of the Rollins Sandstone. At the lovvcr, southern end of the vcnch. the naticc debris fan/colluvial material was encountered at a total depth of I S fcet and the Rollins Sandstone surface had fallen below the maximum reach of the backhce bucket. At approximatcl}' 10 feet below the ground surface. some iron/steel debris ryas encountered, indicating the original ground surface ma} have been approximately 10 feel bclo~v the prescnt ground surface. It is bclicccd that some oCthc `recent' fill in this gully is related to construction of the Railroad embankment. At a depth of 6 to 7 feet below the prescnt ground surface, a vrn rocki zone vvas encountered, which does not appear to be man-made fill but appeared [o be part of natural deposits of alluvium and colluvium. The type of svatification, the orientation of the rock and boulder