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Morrison Knudsen Corporation <br />• January 7, 1997 <br />Page 2 <br />Portal Road Cuts: Our original report (Job No. 5002-60-2655) recommended cut slopes no <br />steeper than 2:1 (horizontal:vertical). However, the report stated that with an increased risk of <br />shallow slope failure, erosion-related maintenance problems and increased difficulty supporting <br />vegetation, cut slopes could be as steep as 1.5:1 where located well above ground water levels <br />in the very stiff to hard clay. Due to the location of the conveyor and the topography of the site <br />several tight switchbacks were necessary along the portal road. The majority of the road cuts <br />along these switchbacks were at inclinations of 1.5:1 and extended from about 10 to 150 feet <br />laterally from the road. Cuts steeper than 1.5:1 were not identified during our review of the <br />proposed haul road alignment. <br />Two locations along the portal road were determined to have the most critical cross-sections. <br />The [road cut near Sta 61+00 was determined to have We steepest overall slope with a cut of <br />1.5:1 extending about 70 feet laterally from the road. At this location no fill was placed down <br />slope of the road and the natural ground surface also had a slope of about 1.5:1. The second <br />cross-section analyzed was at Sta 76+00 which had a 1.5:1 cut slope extending about 150 feet <br />laterally from the road. This location was determined to have the largest overall cut uphill of <br />the road. The natural ground surface down slope of this section of the road had a 4.5:1 slope. <br />• Based on our review of the portal road cross-sections, it was determined that the majority of the <br />sections had cut slopes of 1.5:1 and Fill slopes of 2:1 and therefor a hypothetical cross-section <br />with this geometry was analyzed. The portal road cross-sections analyzed during this study are <br />presented on Figs. 1 through 3. <br />Topsoil Pile: Grading plans indicate that topsoil will be stockpiled about 1/4 mile uphill of the <br />base facilities on a gently sloping ridge. The pi]e is expected to be from 10 to 50 feet high with <br />side slopes of about 2.5:1. The north side of the ridge has the steepest existing slope varying <br />from 1.3:1 to 2:1. Across-section in this area was analyzed where the largest amount of topsoil <br />(50 feet) is expected to be placed and is labeled as Section 1 on Fig. 4. The second cross- <br />section analyzed followed the general southern trend of the slope and also represented an area <br />where about 50 feet of topsoil is estimated to be placed. This section is labeled Section 2 on <br />Fig. 4. The topsoil pile cross-sections analyzed during this study are presented on Figs. 5 and <br />6. <br />Stability Analysis: The stability of the portal road cut slopes and the existing slopes beneath <br />the topsoil pile is dependent upon their ability to resist shear stresses. These shear stresses are <br />generated from internal body forces created by the weight of the slope itself. The sections <br />described above were subjected to stability analyses using current state-of-the-practice methods. <br />The method deemed appropriate to analyze the slopes was a conventional circular stability <br />analysis. The computer program Geoslope 4.0 was used, and is based on ttte program <br />• STABLES which was developed at Purdue University. <br />