202 ALDER ST Geotech Report 2025-07-30 (2)

Proposed View Ridge Subsurface Exploration, Geologic Hazards, Infiltration Potential, Elementary School Replacement and Preliminary Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations 1613.5.2. The 2009 IBC seismic design parameters for short period (Ss) and 1-second period (Si) spectral acceleration values were determined by the latitude and longitude of the project site using the U.S. Geological Survey (USGS) software utility Seismic Hazard Curves and Uniform Response Spectra. The USGS software interpolated ground motions at the project site as follows for periods of 0.2 and 1.0 seconds, respectively, with a 2 percent chance of exceedence in 50 years: Ss = 1.212, Si= 0.430. 7.0 EROSION HAZARDS AND MITIGATIONS The following discussion addresses Washington State Department of Ecology (Ecology) erosion control regulations that will be applicable to the project. The City of Everett definition for Erosion Hazard Areas (Everett Municipal Code Title 19, Chapter 37.080) is linked to published mapping and soil types. Although the site does not appear to contain mapped erosion hazard areas, man-made slopes on the south and east parts of the site might meet the intent for management as Erosion Hazard Areas. The following recommendations are related to compliance with Ecology management of erosion on construction sites. The state requirements are detailed, and in our opinion, addressing the state requirements is likely to also comply with City of Everett requirements that might be applicable. As of October 1, 2008, the Ecology Construction Storm Water General Permit (also known as the National Pollutant Discharge Elimination System [NPDES] permit) requires weekly Temporary Erosion and Sedimentation Control (TESC) inspections and turbidity monitoring for all sites 1 or more acres in size that discharge storm water to surface waters of the state. Because we anticipate that the proposed project will require disturbance of more than 1 acre, we anticipate that these inspection and reporting requirements will be triggered. The following recommendations are related to general erosion potential and mitigation. The erosion potential of the site soils is high. The most effective erosion control measure is the maintenance of adequate ground cover. Maintaining cover measures atop disturbed ground provides the greatest reduction to the potential generation of turbid runoff and sediment transport. During the local wet season (October ls` through March 31.5t), exposed soil should not remain uncovered for more than 2 days unless it is actively being worked. Ground-cover measures can include erosion control matting, plastic sheeting, straw mulch, crushed rock or recycled concrete, or mature hydroseed. Some fine-grained surface soils are the result of natural weathering processes that have broken down parent materials into their mineral components. These mineral components can have an inherent electrical charge. Electrically charged mineral fines will attract oppositely charged particles and can combine (flocculate) to form larger particles that will settle out of suspension. September 22, 2010 ASSOCIATED EARTH SCIENCES, INC. BWG/tb-KE100255A2-Projeccs1201002551KE1WP Page 9