417 TAMARACK AVE Geotech Report 2025-08-19

September 23,2022 Page 2 of io Geotechnical Evaluation Groundwater was not encountered in the explorations; however, the shallow soils were mottled. The underlying till was cemented and it appears likely that groundwater may develop on the denser soils during the wet season. The depth to groundwater could be 3.5 to 8 feet if present. It would be necessary to install monitoring wells to fully determine the depth to any seasonal groundwater/interflow. Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is underlain by Alderwood-Urban land complex(2 to 8 percent slopes). These soils would have a slight to moderate erosion potential in a disturbed state depending on the slope magnitude. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season,with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the International Building Code (IBC). A Site Class D applies to an overall profile consisting of medium dense to very dense soils within the upper 10o feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss,Si,Fa, and Fa. The USGS website includes the most updated published data on seismic conditions. The following tables provide seismic parameters from the USGS web site with referenced parameters from 7-16. Seismic Design Parameters(ASCE 7-16) Site Spectral Spectral Site Design Spectral Design Class Acceleration Acceleration Coefficients Response Parameters PGA at 0.2 sec.(g) at 1.o sec.(g) Fa F Sns SDI D 1.332 0.472 i.o Null 0.888 Null 0.575 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The site has a low likelihood of liquefaction. For items listed as "Null" see Section 11.4.8 of the ASCE. www.cobaltgeo.com (206)331-1097