1001 39TH ST 2016-01-01 MF Import

DAVID L. NELSON AND ASSOCIATES, INC. Consulting Engineering Gcology November 27, 1991 Project No. 10591 Page 5 mitigated by design or a setback used to locate structures a�,uay from areas expected to slough with time. ^_'he steeper gradients to the nor'_�� and somewhat to the west on site could be susceptible to sha�'.cw sloughing failures downslope, if ground water seepag+: (springs) were to occur in this vicinity. This could occu7: if the Whidbey Formation is at shallow deptti near the base of the ravine . 47e did not encounter any obvious signs of outcropping ground water within the steep slopes. Provisions for maintaining �.xisting slope stability, including drainage control, earthwork (filling and grading) control and struct�ire placement, are presented herein to reduce potential risk of �;l�pe failures . Springs/Ground Wate: Seepage Areas These two elements mentioned in the ordinance are similar conditions, which are discussed together in this letter. During our evaluation of the site, we observeQ the surface areas for springs and seeps, and observtd our backhoe exploration (test) pits for ground �aater seepage conditions. We did not encounter any evidence of springs or seeps within the site along the northerly or westerly slope. We did observe evidence of water within the swale area to the northeast, off- site. This water appears to be drainage within the swale from upslope. We also noted that a utility line had been constructed within this swale which may contribute to the observed wet conditions . Indications of seasonal wet and seepage conditions observed within the test pits just above and within the basal Whidbey Formation silts gives potential for perched ground water within the area. However, this would be considered as interflow and per�;hed water within the surficial sandy soils and above the relatively impermeable intei•glacial silts (Whidbey) . These conditions are expected to be seasonal in nature, and to lessen during the normally drier periods of the year. Our discussions in the RECOhS7ENDATIONS section present methods of mitigation of any perched or seepage waters encountered during construction. Dewatering and control of the drainage within this site will greatly increase the stability within the more sensitive soils and slope areas . ' Steep Slopes Associated with The Above Elements The ordinance states that steep slopes are those at 25 percent or - greater. Within this site, the steep slopes as defined appear to occur within the northerly and westerly portion of the site. We specifically observed this area, and performed test pits dcwnslope of the proposed residence area, to ascertain our � _ _ '. _" ._,w,,,....—_. __ :...._.:i'�fcF',.3�rirn..