10901 27TH AVE SE Geotech Report 2025-07-29

1 I Proposed James Monroe Elementary School Replacement Subsurface Exploration, Geologic Hazards, and Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations 1.0 seconds, respectively, with a 2 percent chance of exceedence in 50 years: Ss = 1.191, Si= 0.414. 1 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 northwest part of the site might meet the intent for management as an Erosion Hazard Area. 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. I The erosionenda potential of the site soils se 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 1st through March 3151), 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. The sediments produced during the recent glaciation of Puget Sound are, however, most commonly the suspended soils that are carried by site storm water. The fine-grained fraction of the glacially derived soil is referred to as "rock flour," which is primarily a silt-sized particle with no electrical charge. These particles, once suspended in water, may have settling times in periods of months, not hours. i October 5, 2009 ASSOCIATED EARTH SCIENCES,INC. BWG/Id-KE090312A3-Projects12O090312IKEIWP Page 8 1