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October 19, 2021 <br />Page z of 12 <br />Geotechnical Evaluation <br />to fine grained sand trace gravel (Vashon Glacial Till), which continued to the termination depths <br />of the explorations. The till was partially to well cemented. <br />Groundwater was not encountered in the explorations; however, the shallow soils were locally <br />mottled between about 3 and 5 feet below grade. Perched groundwater could be present at <br />shallow depths below the site during the winter months. A likely seasonal high level would be 3.5 <br />to 8 feet below grade where groundwater becomes perched. <br />Water table elevations often fluctuate over time. The groundwater level will depend on a variety <br />of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and <br />soil permeability. Water levels at the time of the field investigation may be different from those <br />encountered during the construction phase of the project. <br />Erosion Hazard <br />The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that <br />the site is underlain by Alderwood-Urban land complex (2 to 8 percent slopes). These soils would <br />have a slight to severe erosion potential in a disturbed state depending on the slope magnitude. <br />It is our opinion that soil erosion potential at this project site can be reduced through landscaping <br />and surface water runoff control. Typically, erosion of exposed soils will be most noticeable <br />during periods of rainfall and may be controlled by the use of normal temporary erosion control <br />measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The <br />typical wet weather season, with regard to site grading, is from October 31' to April tat. Erosion <br />control measures should be in place before the onset of wet weather. <br />Seismic Hazard <br />The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the <br />International Building Code (IBC). A Site Class D applies to an overall profile consisting of <br />stiff/medium dense soils within the upper too feet. <br />We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to <br />obtain values for Ss, S,, Pa, and P,,. The USGS website includes the most updated published data <br />on seismic conditions. The following tables provide seismic parameters from the USGS web site <br />with referenced parameters from ASCE 7-10 and 7-16. <br />Seismic Design Parameters (ASCE -10) <br />Site <br />Spectral <br />Spectral <br />Site <br />Design Spectral <br />Design <br />Class <br />Acceleration <br />Acceleration <br />Coefficients <br />Response Parameters <br />PGA <br />at 0.2 sec. (g) <br />at t.o sec. (g) <br />Fa <br />F <br />SDS <br />SD, <br />D <br />1.436 <br />0.554 <br />t.o <br />t.5 <br />0.957 <br />0.554 <br />o.61 <br />www.cobaltgeo.com (2o6) 331-1097 <br />