909 51ST PL SW Geotech Report 2022-10-25

Geotechncial Engineering Evaluation Garcia Property Additions Everett, Washington CONCLUSIONS AND RECOMMENDATIONS NGA Project No. 1040218 June 22, 2018 Page 6 General It is our opinion from a geotechnical standpoint that the site is compatible with the proposed residence addition and deck development provided all recommendations provided in this report are incorporated into the development plans and strictly implemented during construction. It is also our opinion that the soils that underlie the site and form the core of the site slope should be stable with respect to deep-seated earth movements, due to their inherent strength and slope geometry. However, shallow failures could occur on the slopes in the loose surficial and undocumented fill soils, especially during adverse weather or a significant seismic event. Top -of -Slope buffers of 25 feet for the ravine slopes are applicable to the project, as outlined by the 2018 IBC section 1808.7, and by EMC 19.37.080.132. As a modification to the buffer given the conditions observed on the slopes, we recommend that development be set back at least 15 feet from the top of the steep ravine slopes, such that a 25-foot effective setback can be maintained. We have provided recommendations for structure setback and foundation embedment to lessen the impacts of the proposed development on the slope and to allow for some slope recession during a reasonable life span of the structure. This is further discussed in the Structure Setback subsection of this report. Our explorations indicated that the site is underlain by surficial undocumented fill soils with medium dense or better native glacial soils at depths between 1.5 and 8.0 feet within the proposed development area. It is our opinion that while the residence foundations could be designed using shallow foundations, footings would need to extend through any loose surficial soil and key into the underlying competent native soils interpreted to underlie the site at depth. We would anticipate excavations of up to 8.0 feet or greater within the site to support the foundations on competent native soils, based on our explorations. It is our opinion that the undocumented fill soils are considered unsuitable for foundation support of the planned structures. We should note that localized areas of deeper unsuitable soils and/or undocumented fill could be encountered at this site. It is our opinion that removal and replacement of the substantial amount of undocumented material below the proposed addition will result in additional costs and pose issues for slope stability along the top of the slope. To reduce the risk of potential slope failures of affecting the structure, the structure should be supported on pin pile foundations extending deep enough to provide a 25-foot effective setback from the face of the slope to the bottom of the pin piles. We also recommend that the new residence foundations be supported on a deep foundation system consisting of 4-inch diameter driven steel pin piles extending through the fill and terminating within the underlying competent native glacial soils outlined in the Foundations subsection of this report. NELSON GEOTECHNICAL ASSOCIATES, INC.