4330 HOYT AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation—REVISED NGA File No. 1148519 Staniford Residence Remodel June 23,2020 Everett,Washington Page 8 Foundation Underpinning Due to the presence of undocumented fill within the vicinity of the addition foundations, we recommend that the new addition foundations, as well as the existing western foundation line be supported on 2-inch diameter driven steel pipe piles. The piles should consist of 2-inch diameter galvanized extra strong (Schedule 80) steel pipe sections driven using a hand-held, 140-pound jackhammer. For 2-inch diameter pipe piles driven to refusal using a hand-held, 140-pound jackhammer,we recommend a design axial compression capacity of three tons for each pile. The refusal criterion is defined as less than one inch of movement during 60 seconds of continuous driving. Final pile depths should be expected to vary somewhat and will depend on the nature of the underlying soils. The piles should advance through the fill and organic-rich soils, reach a minimum embedment of 5 feet into competent native material, and meet the minimum refusal criteria as described in this report. Piles that do not meet this minimum embedment criterion should be rejected, and replacement piles should be driven after consulting with the structural engineer on the new pile locations. While underpinning the existing residence foundation, structural brackets should be used to attach the pin piles to the existing foundations. The structural engineer should design the new foundation supports and determine the location of the supports based on the recommendations provided in this letter. A qualified contractor specializing in foundation retrofit should be retained to complete the repairs. Extreme care should be taken during the proposed repairs as to not impact existing utilities or foundation elements within the proposed repair area of the residence. Also, the residence and vicinity should be monitored for excessive vibrations and installation methods adjusted to limit any impacts to the residence and vicinity. Due to the relatively small slenderness ratio of pin piles, maintaining pin pile confinement and lateral support is essential to preventing pile buckling. Vertically driven pin piles do not provide meaningful lateral capacity. All lateral loads should be transferred back to the existing structure foundations via grade beams. We recommend that NGA be retained to review final project plans and provide consultation regarding structure placement, site grading, and foundation support. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed differ from those anticipated,and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. NELSON GEOTECHNICAL ASSOCIATES,INC.