2012-04-10_REVISION - M1981185 (43)

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The cut/fill area consists of colluvium deposits overlying Morrison Formation bedrock or exposed bedrock. The Jurassic Age Morrison Formation consists of siltstone and claystone containing thin beds of very fine - grained sandstone and minor conglomeratic sandstone. The strike and dip measurements indicate the strike of the bedding varies from north 80 to 88 degrees west, and dips steeply southeast between 28 and 35 degrees. This bedding slope is favorable. The native soil in the flat section consists of heterogeneous deposits of chiefly silty to boulder sediments deposited by or adjacent to glacial activity near the mouth of La Plata Canyon. The surface deposit consists of fine sandy silt material underlain by rounded cobbles and boulders in a sandy silt matrix. 3.2 Surface Conditions As shown on cross sections RWB -07 on Figure 3, surface conditions in the through -cut area consist of a sloping roadway surface with cut slopes on either side. The road surface transitions from a through -cut to a cut -fill at approximately Station 16 +50. The road surface is cut into the north slope with the outboard side of the bench to the south. There is existing cribbing to the south on the outslope approximately between Stations 18 +50 and 19 +50. The roadway then descends down the slope, crosses a drainage culvert, and then reaches a flat section near the La Plata River (Figure 2). 3.3 Subsurface Conditions Between Stations 15 +00 and 16 +50 materials exposed in the cutslopes consist of terrace deposits and colluvium. These materials consist of dense to very dense angular to subrounded cobbles from 2 to 6 inches in diameter in a silty sand matrix; occasional boulders up to 18 inches are also present in both the terrace deposits and colluvium. Test pits and borings in this through - cut area indicate these materials extend 3 to 4 feet below the existing road grades and overlays bedrock. Borings and test pits in the cut/fill area between Stations 16 +50 and 21 +00 indicate the roadway is blanketed by a layer of fill 5 to 13 feet thick. The fill material consists of lean gravelly silty clays with cobbles and large boulders overlying bedrock. Figure 3 presents representative cross section through boring RWB -04. A plan view of the cross section is shown on Figure 2. As shown, the road surface appears to be completely in cut, with fill placed over the rock bench. Boring RWB -01 indicates that the fill east of the drainage culvert is a loose to medium dense silty fine sand. Fill thickness varies from 14 feet in boring RWB -01 to essentially nil as the road slope descends to the flat section at approximately Station 20 +50. Test pits in the flat section indicate the native material is fine sandy silt overlying rounded cobbles in a sandy silt matrix. Groundwater was encountered in boring RWB -01 at elevation 8,622 feet (13.5 feet below ground surface), which appears to be consistent with the elevation of the La Plata River. Rule 6.5 Geotechnical Stability Technical Revision New Access Road April 2, 2012 Page 3