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Backfill Slope Design Lateral Pressure (pcf) 1.5:1 63 1.75:1 49 2:1 44 be placed in layers less than 8 inches thick, moisture conditioned to within 3 percent of the optimum moisture content, and be compacted to at least 90 percent relative compaction per ASTM D1557 -09. Oversize rocks should be removed during fill placement and stockpiled at a geotechnically stable location for future use as slope revetment or rip rap as necessary. During placement, the fill should be benched (Figure 4) at least 3 feet into the existing slope. The resulting bench should be smooth and non - yielding, and slope to drain away from the portal. The buttress fill should then extend upslope to the level of the Incas Access Road above. Upon completion, resulting slope should be trimmed of loose material and planted with ground cover appropriate to the area. The fill materials are highly erodible and thus should be protected until vegetation becomes established. Surface water runoff should be intercepted and diverted from the top of the buttress using v- ditches or graded berms. Concentrated flows should be collected in a lined ditch or closed pipe and discharged away from the slope face into natural drainages. Energy dissipators should be provided as necessary to prevent concentrated flows from causing erosion. To prevent the development of hydrostatic pressures within/behind the buttress, subdrainage should be installed as shown in Figure 4. The keyway subdrain, like the keyway itself, should extend to dense /stiff colluvium or rock over the entire width of the buttress. The geotechnical engineer should observe the keyway bottom to check that suitable bearing materials are exposed. If seepage zones or signs thereof are encountered during buttress construction, intermediate subdrains should be installed to intercept these zones. 5.2 Retaining Walls If existing slopes above the portal preclude a buttress sloping a 1.5:1 or flatter extending from the portal to the Incas Access Road, retaining walls will be required. Retaining walls should be designed to resist lateral earth pressures imposed on them from the sloping buttress fill. Use the lateral earth pressures summarized in Table 1 for design. From Air Force AFM 88 -3, Chapter 14. Equivalent fluid pressure Rule 6.5 Geotechnical Stability Technical Revision Chief Portal Table 1. Design Lateral Earth Pressures Retaining wall foundations can be supported on footings bearing on properly compacted fill or rock. Footings bottomed on rock or compacted fill can be designed using an allowable March 16, 2013 Page 4