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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 6 Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. 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 rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE/CRITICAL AREA EVALUATION Landslide hazards areas within the site are subject to the definitions set forth in Everett Municipal Code (EMC) Section 19.37.040. In general, it is our opinion that the steep slope below and to the south of the property meets the definition of a landslide hazard area. Based on our site investigation, this is primarily due to slope gradients equal to or greater than 33 percent which intersect geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment which contain springs or ground water seeps. Additionally, the recent landslide activity and surficial expression of slope movement further classify the steep slope area as a landslide hazard area. In general, development within a landslide hazard area and its buffer is restricted. Given the recent severe landslide activity and impact to residential properties, it is our opinion that slope stabilization as described in this report should be considered for exemption of certain development regulations within the landslide hazard area per 19.37.050(1)(a), which states "Emergencies that threaten the public health, safety and welfare, as verified by the city. Emergency actions that create an impact to a critical area or its buffer shall use reasonable methods to address the emergency; in addition, they must have the least possible impact to the critical area and/or its buffer." The crown of the slide extends within approximately five feet of the Miller residence along the southeastern side and has the potential for severely undermining the residence, as well as propagation to other adjacent properties. In our opinion, the slide area should be stabilized as soon as feasible. It is also our opinion that the recommended subsurface pile wall will provide the least impact to the critical area while adequately mitigating slide risk. Seismic Related Hazards We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since dense soils were found to underlie the site at depth, the site best fits the IBC description for Site Class D. Table 1 below provides seismic design parameters for the site that are in conformance with the 2018 IBC, which specifies a design earthquake having a two percent probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. NELSON GEOTECHNICAL ASSOCIATES, INC.