801 E CASINO RD Geotech Report 2022-10-25

Cascade High School Subsurface Exploration and Synthetic Turf Field Geotechnical Engineering Report Everett, Washington Design Recommendations 11. DESIGN RECOMMENDATIONS 5.0 INTRODUCTION In our opinion the proposed field improvements are feasible from a geotechnical engineering standpoint provided that the recommendations contained herein are properly followed. The existing fill is relatively loose and presents some risk of greater than normal post -construction settlement if it is left in place below the new sports field. The recommendations contained in this report are intended to reduce the potential for post -construction settlement if existing fill is left below the new field, but not eliminate it. The settlement risk mitigation measures recommended in this report are expected to be reasonable to implement and economically viable, but more elaborate settlement risk mitigation measures are possible. We are available to discuss more aggressive settlement risk mitigation strategies and costs on request. The risk of post -construction settlement at this site is difficult to quantify with the existing data, but is expected to be relatively small, on the order of a few inches or less. Existing fill and native sediments at the site are anticipated to drain slowly, and an underdrain system below the planned new field is recommended. 6.0 EROSION HAZARDS AND MITIGATION The following discussion addresses Washington State Department of Ecology (Ecology) erosion control regulations that will be applicable to the project. We anticipate that if the project complies with Washington State requirements, it will also be acceptable with respect to City of Everett requirements. 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. Maintaining cover measures atop disturbed ground typically provides the greatest reduction to the potential generation of turbid runoff and sediment transport. During the local wet season (October Vt through March 315t), 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. March 20, 2017 ASSOCIATED EARTH SCIENCES, INC. DDV/pc-170081E001-2—Pro/ects�201700811KEJWP Page 5