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Geoteclinical �ngineeriug Letter Alhussaini Slope Stabilization and Retainiug Walls Everett, Washinbtoil NGA File No. 867612 December 10, 2012 Page 7 wall area and that, a nlinimunl of one foot of tl�e native material be overexcavated froiu the wall � alig�uilent to key the wall into coinpetent ilative soils. Additional fill in�y need to be overexcavated dependi�lg o�� existiilg till conditions. The mediu�n dense or better native soils � should provide adequate support for thc planned i-etaining walls. Tlle fouiidations for tl�e retaining walls should extend through any undocumented fill, organic soil, or loose materiaLs, and � be keycd into the underlying medium dense or better native soils. Based on otu- explorations, we auticipate that the conlpetent native soils should be cncountered approximately 4.0 ro 6.5 feet ' ' ' ' ' ' ' � ' ' � 1 1 below tl�e existing ground surface. It is our opiniou that � Keystone Standard 21.5-inch Block wall with geogrid reinforced backfill is most suitable for the developu�ent given the site conditions. As discussed above, we anticipate that older undocumented fill soils will likely be encountered in the wall areas. If older undocunlented fill soils are encountered in the wall area t�hey should be overexcavated down to the competent uative soils. We recoinmend that level benches be graded iilto the native site slope to allow placeme�lt of tl�e block walls a�id tllls to be retained by the walls. We recouimend that the walls have a maximum exposed height of 5.0 feet. We aiiticipate tLlat the tot�al wall height may be up to 6.5 feet in oi-der to satisfy the recommeilded base embedment i�Ito the native soils. This is discussed further iu the Wall Desigu and Construction Recommend�ttions subsection o'I� this lettcr. Due to tl�e tight site constraints and tlie substantial amount of fill material that will need to be reinoved from the slope area prior to constr�iction of the walls, we stress that implementing proper planning and construction sta�ing techniques will be key to achieve a successful outcome. The entire recent fill needs to be removed fi-o�1i tlle sLope prior to attempting to build the block walls. NGA should be retained to review project plans prior to construclion and sLlould be retaiiled to observe wall eonstruction to verify wall installation is being perfonned in accordauce with the plans and our reconm7endations provided in this letter. Erosion Control and Slope Protection The on-site soils cail have a low to inoderate potential for erosion, depending on llow the site is �raded and l�ow water is allowec� to concentrale. Best' Management Pi-actices (BMPs) sllould be NELSON GEOTECHN/CAL ASSOCIATES, INC.