1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 5 Hydrologic Conditions We did not encounter groundwater seepage in our explorations. However, we anticipate that a perched water condition will likely occur within the undocumented fill and silty soils encountered within our explorations. The previous borings performed by Geotech Consultants, Inc. encountered perched groundwater at a depth of approximately 11.0 feet below the existing ground surface. Perched water occurs when surface water infiltrates through less dense,more permeable soils and accumulates on top of underlying, less permeable soils. Perched water does not represent a regional groundwater"table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of precipitation. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. For this site, perched water seepage was also observed on the slope below the residence. Also, we expect the slope to indicate some seepage even during the dry period of the year. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2015 International Building Code (IBC). Since mostly medium dense or better native glacial soils are interpreted to underlie the site at depth,the site conditions best fit the IBC description for Soil Class D for native soils. 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 competent glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. The medium dense or better native glacial soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However, the loose surficial materials and undocumented fill on the slope have the potential for shallow sloughing failures during seismic events. Such events should not directly affect the proposed wall and existing residence provided our recommendations as described in this report are followed. Landslide Hazard/Slope Stability The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. A steep north-facing slope with gradients up to 35 degrees (70 percent) is located below the existing residence. We did not observe significant evidence of past deep-seated slopes instability, or NELSON GEOTECHNICAL ASSOCIATES, INC.