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Geotechnical E�lgineering Letter Alhussaini Slope Stabilization and Retaining Walls Everett, Washington NGA File No. 867612 December 10, 2012 Page 11 The total height of the planiied walls is expected to be up to 6.5 feet, including a ininimum reconliilended eilibedment of 1.5 feet into competent soils. We have provided w111 designs for up to an 8-foot higl� i-etainin� wall with Keystone block facing. We recomn�end that walls greater Chan four Ieet in total height (exposcd l�cibht plus wall embedment) be constructcd utilizin� geogrid reinForced backfill. Walls less lhan [our i�ct in total height can be constructed without beogrid reinforced backtilL The wall detail and design parameters along with consti-uctio�l notes for walls greater than or enual to four feet in total height are shown on Figure 8. We have assumed that the i-etained flll zones will consist of gra�iular material compacted to structural flll specifications. The fill should be placed level behind tlle walls and exte�id back into the slope. The draii�age system, as indicated on the detail, should be installed along the base of the blocks and behind the Keystone block facing. The block facing should consist of Keystone Standard 21.5-inch Block facing. The block i'acing sllould be pLaced on a mi��imuni of 4-inch thick crushed rock leveling pads placed over competent native soils, or stiuctural t111 material }�repared under the supervision of NGA. The subgrade should be ii-ee of any undocuuzeuted fill or topsoil and be comp�etcd to a non-yielding condition before placing the blocks. Where loosc undocumcnted till soils arc encountered at the planned relaining wall subgrade elevation, the subgradc should be over-excavaled by a minimuin of one foot and replaced with 1-1/4-inch crushed rock compacted to structural fill specifications. We also recommend that aLl undocumented lill found within the planned reinforced till areas be overcxcavated prior to construction of the walls. The wall and reinforced fill areas should also be graded to level bencl�es prioi- to wall and reinforced fill conslruction. Since the walls will be terraced, we recomme�ld that the lowest block cetainiug wall be constructed to completion prior to beginning construction of the upper wall. All tiers should be separated by a minimun� horizontal distance of 20 feet. A drainage blanket of 12 inches of fi-ee-draining crushed rock should be placed between the blocks and tLle retained fill zone. The block cavities should also be tilled with the crushed rock. A rigid, perforated drainpipe embedded iu a niinimum of 1-foot of pea gcavel �ind wrapped in � NELSON GEOTECHNICAL ASSOCIATES, INC.