12815 19TH AVE SE Geotech Report 2022-10-25

Silver Lake Elementary School Subsurface Exploration, Geologic Hazard, Building Additions, Service Drive, and Water Line Installation and Geotechnical Engineering Report Everett, Washington Project and Site Conditions 3.1 Exploration Borings The exploration borings were completed by advancing a 33/8-inch, inside-diameter, hollow- stem auger with a trailer-mounted drill rig to depths of up to 10 feet below existing grades. During the drilling process, samples were obtained at 2.5-foot or 5-foot-depth intervals thereafter. The borings were continuously observed and logged by a geotechnical engineer from our firm. The exploration logs presented in the Appendix are based on the field logs, drilling action, and inspection of the samples secured. Disturbed, but representative samples were obtained by using the Standard Penetration Test procedure in accordance with American Society for Testing and Materials (ASTM):D 1586. This test and sampling method consists of driving a standard 2-inch, outside-diameter, split- barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free-falling a distance of 30 inches. The number of blows for each 6-inch interval is recorded, and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance ("N") or blow count. If a total of 50 is recorded within one 6-inch interval, the blow count is recorded as the number of blows for the corresponding number of inches of penetration. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are plotted on the attached boring logs. The samples obtained from the split-barrel sampler were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification and laboratory testing, as necessary. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this and our previous study and a visual reconnaissance of the site. As shown on the field logs, the exploration borings generally encountered 2 to 3 inches of asphalt concrete or 4 to 6 inches of grass and topsoil overlying natural, glacially consolidated till deposits consisting of silty sand with gravel and trace cobbles. The upper horizon of the till consisted of medium dense to dense, moist, mottled gray, silty sand with gravel. With increasing depth, these materials typically became denser and were no longer mottled. Till represents the ground moraine of the southward advancing glacier that dominated the landscape during the Vashon Stade of the Frasier Glaciation 14,000 to 17,000 years ago. Several thousand feet of ice consolidated this material during this last glacial advance. This process resulted in a compact soil possessing high strength, low compressibility, and low permeability characteristics. The soils encountered at the site are consistent with the soil types mapped in the publication U.S. Geological Survey (USGS) Open File Report 81-248 by Minard, 1981. The medium dense to November 8, 2006 ASSOCIATED EARTH SCIENCES,INC. SGB/Id-KE02599B2-Projects1200205991KElWP Page 3