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1 � � � ' � ' ' 1 ' ' , , ' ' ' ' ' Geotechnical Eugineel-ing Lerier Alhussaini Slope Stabiliaatiou and Retaining Walls Everett, Washington NGA File No. 867612 December 10, 2012 Page 6 slope during our site visit on Novenlber 20, 2012. Tllis activity appears to be localized witlun the upper new fill material placed on the slope, but could extend deeper into the older fill soils. Tl�e native core of the site slope is in(=erred to consist of inedium dense or better glacial soils. It is our opi�iion that wl�ilc tllerc is a high potential for failures witl�i�i tlie loose s�irGcial soils aild undocumented till on the steep slope as l�as alrcady been observed, the poteutial for deep-seated slope failures tl�at would involve the dense to very dense material that fornzs the core of the slope �uider current site conditions is low. 7�IIis potential can be elevated if immediate measul-es of slope stabilization ai-e not takcn. Proper site grading drainage, retaining wa11 constructio�i, aud vegetation �nanageinent as reconunended in this letter should inlprove current stability conditions. CONCLUSIONS AND RECOMMENDATIONS It is our opinion that the fill material tllat was recently placed on the steep slope below your residence is cun-ently unstable and could ex�erience sigiuficant movement and abrupt failures if immediate measures of stabilizi�ig the slope are not implemented. We recoinnle�Id that at a �niniulurn, a11 of tl�e recently placed till on the steep slope area be rellloved and tlle surface of tlle slope be compacted to an unyielding conditiou. AfCer all of the recently placed fill has been re�noved I'rom the slope aild the resulting slope face conzpacted to a tlrnl condition, we recommend that either tlle slope be coveced with a heavy-duty erosion control n�aterial and vegetation, or a tei-��aced i-einfoi-ced-fill block wall system as previously proposed be consh-ucted along the upper poi�tion of the slope aloilg with additionll erosion control nieasures. The impleinentation of any wall system on tl�e slope would need to be perinitted by the City of Everett. It is our opinion from a geoteclvlical standpoint that the site ean be compatible with a terraced block retaiuing wall and reinforced backfill construction. Our explorations indicated that the planiled wall �reas are generally underlain by loose to medium dense undocunzented fi1L soils with conipetent native glacial deposits at depth. We anticipate that tlle pl�nned wall fou�idation subgracles will likely eucounter loose to mediuul dense older till materials. To adequately support the proposcd walls, we recomniend that all undocumented till soils be removed fro�n the NELSON GEOTECHNICAL ASSOCIATES, INC.