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

Geotechncial Engineering Evaluation Garcia Property Additions Everett, Washington NGA Project No. 1040218 June 22, 2018 Page 5 Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the dense glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. Undocumented fill potential for liquefaction is naturally higher due to the unpredicible behavior during seismic events. Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area Washington by the Soil Conservation Service (SCS) classifies the site as Alderwood-Urban land complex, 8- to 15 percent slopes, and ravine areas as Everett very gravelly sandy loam, 15 to 30 percent slopes. The erosion hazards for these soils are listed as slight and moderate, respectively. The erosion hazard on the ravine slopes is considered very high to severe by the City of Everett for the ravine slopes greater than 40 percent and have the geological classification of Qva. It is our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep, northeast -facing ravine slope occupies the eastern portion of the property, where it steps down at gradients up to 33 degrees (65 percent) with total estimated vertical relief of approximately 75 feet. The slope is designated as landslide hazard area as determined by the City of Everett Municipal Code (EMC) 19.37.080.Aig, for slopes with a gradient greater than 40 percent and more than 15 feet of vertical relief. We did not observe evidence of significant slope instability during our investigation, such as deep-seated landsliding, nor did we observe groundwater seepage from the slope within the vicinity of the property. However, we noted the presence of anthropogenic debris, including construction litter and old lawn clippings on the steep ravine slopes. The core of the slopes is inferred to consist primarily of medium dense or better native soils. Relatively shallow sloughing failures as well as surficial erosion are natural processes and should be expected on the slopes during extreme environmental conditions. This is especially true within the loose surficial and undocumented fill soils on the slopes. It is our opinion that while there is potential for erosion, soil creep, and shallow failures within the loose surficial soils on the steep slope, there is not a significant potential for deep-seated slope failures under current site conditions. Proper slope stabilization measures, site grading and drainage, as well as foundation placement as recommended in this report should help maintain and enhance current stability conditions. NELSON GEOTECHNICAL ASSOCIATES, INC.